Science Chapter 5 The Fundamental Unit Of Life

According to J.J Thomson's model of an atom, an atom consists of a positively charged sphere with electrons embedded in it. However, it was later found by the Rutherford that the positively charged particles reside at the centre of atom called the nucleus, and the electron revolve around the nucleus.

According to Rutherford model, there is a positively charged centre in an atom called the nucleus and the electron present in nucleus revolve aroun it, in well- defined orbits. But, since an electron is charged particle. Thus, while revolving around the nucleus it lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.

According to Bohr's theory:
(i) The atom consists of a small (positively charged) nucleus at its centre.
(ii) The whole mass of the atom is concentrated in the nucleus and the volume of the nucleus is smaller than the volume of the atom by a ratio of about 1 : 10⁵.
(iii) All the protons and neutrons of the atom are contained in the nucleus.

(iv) The electrons of the atom revolve around the nucleus in definite circular paths known as orbits or which are designated as K, L, M, N etc. or numbered as (n) = 1, 2, 3, 4 etc. outward from the nucleus.
(v) Each orbit is associated with a fixed amount of energy. Therefore, these orbits are also known as energy levels or energy shells.
(vi) The energy of the atom changes when an electron jumps from one state (energy level) to another state (energy level). As long as an electron remains in a particular orbit, it does not lose or gain energy.

1).Thomson proposed the model of an atom to be similar to that of a watermelon. In which the positive charge in atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon.

On the base of this model; Thomson proposed that:

i)An atom consists of a positively charged sphere and the electrons are embedded in it.

ii)The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

2.)Rutherford designed a model in which fast moving alpha (a)-particles were made to fall on a thin gold foil. On the basis of above experiment Rutherford concluded that:

i)Most of the space inside the atom is empty because most of the (a)-particles passed through the gold foil without getting deflected.

ii)Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

iii)A very small fraction of (a)-particles were deflected by 180⁰, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume with in the atom.

3.)Neils Bohr. Since charged bodies moving in circular motion emit radiations. This will lead to loss of energy of the moving electron and ultimately giving unstable model of atom. To explain stability of atom and atomic spectra, Bohr suggested that electrons are moving round the nucleus in orbits which have fixed energy shells. There is a loss or gain in energy of electron when it moves from one orbit to the other.

Following rules are followed to fill electrons in different energy levels,

(i) If n gives the number of orbit or energy level, then 2n² gives the maximum number of electrons possible in a given orbit or energy level. Thus

First orbit or K-shell will have 2n² =2 x 1 = 2 electrons

Second orbit or L-shell will have 2n² = 2 x 2² =8 electrons

Third orbit or M-shell will have 2n² = 2 x 2³ =18 electrons

(ii) If it is the outermost orbit, then it should have not more than 8 electrons.

(iii) There should be stepwise filling of electrons in different orbits, i.e., electrons are not accommodated in a given orbit if the earlier orbits or shells are incompletely filled.

The combining capacity of an element is called its valency. The number of electrons present in the outermost shell of an atom gives its valency. But, if the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the number of valence electrons. For example, the atom of silicon has four valence electrons. Thus, the valency of silicon is four. 
On the other hand, if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is obtained by subtracting the number of valence electrons from eight. For example, the atom of oxygen has six valence electrons. Thus, the valency of oxygen is (8 − 6) i.e., two.   

(i) Atomic number is defined as number of protons present in the nucleus of an atom. For example, there are 6 protons in carbon, so the atomic number of carbon is 6. All atoms are characterized by their atomic numbers.

(ii) Mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. For example there are 6 protons and 6 neutrons in the nucleus of carbon, so its mass number is 12.

(iii) Isotopes are atoms of the same element thus having same atomic number but different mass number. For example, chlorine has two isotopes with atomic number 17 but mass numbers as 35 and 37.

(iv) Isobars are such atoms which have same mass number but different atomic numbers. Thus isobars are different elements. For example, Ne has atomic number as 10 and sodium has atomic number as 11 but both of these have mass numbers as 22.

Uses of Isotopes

(i) Isotope of cobalt, ( Co), is used in the treatment of cancer.

An atom of Na has a total of 11 electrons. Its electronic configuration is 2, 8, 1. But, Na⁺ ion has one electron less than Na atom i.e., it has 10 electrons. Therefore, K-shell contains 2 electrons and L-shell 8 electrons. Thus, Na⁺ has completely filled K and L-shells.

Isotope of bromine with atomic mass 79 u = 49.7%

So, Contribution of 79 Br to atomic mass of bromine =$79 \mathrm{x} \frac{49.7}{100} =39.26\mathrm{u}$

Isotope of bromine with atomic mass 81 u = 50.3%
therefore, Contribution of 81  Br to atomic mass of bromine = $81 \mathrm{x} \frac{50.3}{100 } =40.74\mathrm{u}$

Hence, atomic mass of bromine atom = 39.26 + 40.74 = 80.0 u

If z = 3, i.e., atomic number is 3. Its electronic configuration is 2, 1. Hence, the valency of the element is 1 (since the outermost shell has only one electron). Then element is lithium. It has distribution of electrons as 2, 1.

Mass number of  X = 6+6 = 12
Mass number of  Y = 6+8 = 14

Since X and Y both have atomic numbers as 6 but mass numbers are different, therefore, these are isotopes to each other.

(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons. (F)

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral. (F)

(c) The mass of an electron is about 1/2000 times that of proton. (T)

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)

B.

8

(d) 2, 8, 1.

Matter is made of atom.The protons and electrons create attraction and repulsion forces as these atomic particles have an electric charge. Thus, it indicate that matter is electrical in nature. For example,

(i) When an ebonite rod is rubbed with a cat skin, ebonite rod gets negatively charged and cat skin positively charged.

(ii) A glass rod on rubbing with a silk cloth gets a positive charge.

(iii) A nylon comb rubbed with hair attracts bits of paper to it.

The discharge tube is a glass tube which is 70 cm long and having a diameter of 5 cm. Two metal electrodes are sealed at the two ends, one of which is connected to the negative terminal of a battery and the other to the positive terminal and a gas at low pressure inside the tube. A side tube is fused at the centre of the glass tube which serves to pump out air from it, using a suction pump.

When the pressure in the discharge tube (Fig) falls below 0.001 mm Hg and a potential difference of about 10,000 volts is applied across the electrodes, the walls of the discharge tube opposite to the cathode starts glowing with a faint greenish light. This is due to the bombardment of walls by some rays emerging from the cathode or negative electrode. These rays are known as cathode rays. These observations lead to the conclusion that cathode rays consist of rapidly moving negatively charged particles. These are called electrons and are shot out from the cathode of a discharge tube when an electric current is passed at high voltage through a gas at very low pressure.

Important properties of cathode rays:

(i) Cathode rays travel in straight lines and thus cast shadows of objects placed in their path.

(ii) Cathode rays possess material particles because they can rotate a light paddle wheel placed in their path.

(iii) They are deflected towards positive plate thus showing that these are negatively charged particles known as electrons.

(iv) They ionise gas through which they pass.

(v) They are deflected by magnetic fields.

(vi) The nature of cathode rays is independent of the material of cathode. Hence they are common constituents of all matter.

(vii) They can penetrate through thin metallic sheet.

(viii)They can produce X-rays.

(ix) The mass of a cathode ray particle is very-very small as compared to the mass of the atom from which it is formed.

(i) Cathode rays cast shadow of the objects placed in their path. This observation shows that cathode rays travel in straight lines.

(ii) It was observed that cathode rays could rotate the paddle of light wheel. This suggested that cathode rays possess mass and hence kinetic energy.

(iii) The cathode rays were deflected towards positive plate of the electric field applied in their path. This shows that cathode rays are negatively charged particles because only opposite charge attract each other.

The gas in the discharge tube experiment (at low pressure and high voltage) gets decomposed. Negatively charged particles are produced which travel from cathode to anode, thus facilitating the flow of current. Under other conditions, e.g., 1-atmosphere pressure, no dissociation of gas occurs and no current flows.

Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode.William Crooks in 1879 first performed this experiment.

X-rays are electro-magnetic waves with very short wavelengths. These are formed when fast moving electrons are suddenly stopped by putting some obstructions like a block of tungsten metal. For example, in the discharge tube, X-rays can be produced if the electrons/ cathode rays are made to strike against the metal target.

The important feature of  X rays is that it can penetrate or pass through the human body and produce shadow-like images of structures such as bones, some of the organs, and signs of disease and injury.

Properties of X-rays:

(i) X-rays travel in straight lines with speed of light.

(ii) X-rays are not deflected by electrical and magnetic fields.

(iii) They are more penetrating than cathode rays.

(iv) They can pass through opaque materials such as black paper and affect a photographic plate wrapped in it.

(v) They cause fluorescence in several materials. A plate coated with barium platinocyanide, zinc sulphide etc. becomes luminous when exposed to X-rays.

Uses of X-rays :

(i) They are useful to locate the broken parts of the body such as bones because solid bone stops X-rays from passing through it and are seen as an opaque image in the X-rays film.

(ii) They are used by detective departments to examine the contents of a parcel without opening it and to detect diamonds/gems concealed by smugglers within their body.

(iii) They are used in the treatment of tumours and cancer in the animals and the human beings.

(iv) They are quite helpful in the study of the nature and structure of the materials.

X-rays are used to locate cracks in the fractured bones because these can penetrate the flesh easily but are stopped by the bones which are quite hard. When X-rays are focussed on the fractured part of the body, then a clear image of the bones in that part of the body is formed on a photographic film placed behind the body part. The cracks in the bones can be seen clearly in the X-ray photograph.

X-rays are used to locate cracks in the fractured bones. This is because X-rays can penetrate the flesh easily but are stopped by the bones. Thus, when X-rays are focussed on the body, a clear image of the bones is formed on the photographic plate placed behind the body. A study of the photographic plate can easily reveal the cracks, if any, in the bones.

(i) The mass of an electron is about 1/1840 that of an hydrogen atom, i.e., about 9.0 x 10^-31 kg.

(ii) An electron is negatively charged particle and has a charge of 1.6 x 10^-19 coulomb. This is one unit of charge.

The statement is true because :

(i) Whatever be the nature of the gas or the material of the cathode, the electrons have the same charge to mass (e/ m) ratio i.e., 1.7589 x 10¹¹ coulomb per kg.

(ii) The electrons obtained by different methods are identical.

When an electric discharge at a very high voltage was passed through discharged tube containing air at a very low pressure, it was observed that cathode rays emanated from cathode and proceeded towards positive plate. This led to the conclusion that cathode rays are negatively charged particles.

William Crookes in 1879 demonstrated that cathode rays travel in straight lines. He placed an opaque object like metal cross in the path of cathode rays in a discharge tube. It was observed that a sharp shadow of the cross was cast on the glass behind the cross. A shadow of metal cross under the circumstances can be formed only if cathode rays travel in straight lines and cannot bend round the corners of the metal cross.

Phosphorescence is luminescence that occurs when energy is supplied by electromagnetic radiation, such as ultraviolet light. The energy source excite an atom from a lower energy state into an excited higher energy state; then the electron releases the energy in the form of light when it falls back to a lower energy state. The light emission continues even after the radiation, responsible for producing phosphorescence is removed.

The following observations are made in the discharge tube under high voltage when the air pressure is gradually reduced.

(i) At normal air pressure — No change

(ii) At slightly reduced air pressure — Unsteady light emission

(iii) At still lower pressure — Tube fills up with uniform glow with magneta-red colour

(iv) At pressure below 0.001 mm Hg — Tube appears dark and the cathode starts glowing with a faint greenish light.

When the cathode rays are passed through an electric field between two parallel plates, cathode rays are deflected towards positive plate. This determines the charge of the particles constituting the cathode rays. As these are deflected towards positive plates, the particles of cathode rays are negatively charged.

Mass of electron = 9.1 x 10^-31kg.

Number of electron in the body of student= total mass/ mass of each electrons=30kg/9.1 x 10^-31 kg

Therefore,

The student is made up of approximately 3.29 x10³¹ electrons.

Population of India is about 100 crores

3.29 x 10³¹/10⁹ =3.29 x 10²²

Therefore,

Number of electrons in the body of the student is 3.29 x 10²² times the population of India.

E. Goldstein found that in the discharged tube if the cathode used is perforated and high voltage is applied between the electrodes when the pressure within the tube is below 0.001 mm Hg (Fig. 4.4), it will be observed that new type of rays come through this experiment.

Fig. Production of positive rays in a discharge 
perforations in the cathode. Since these rays come through perforations in the cathode, these were initially called canal rays. These are also called positive or anode rays, as they move away from the anode side through the gas. In a discharge tube, when the gas atoms lose electrons, they acquire a positive charge and thus move away from the anode. Thus, anode rays are positively charged particles which are emanated from the anode of a discharge tube when a current of high density is passed through a gas at very low pressure. The mass and charge of the anode rays depends on the nature of the gas taken in the discharge tube.

These are Properties of anode rays as under:

(i) They consist of Positively charged particles. Their mass is virtually the same as that of the atoms from which they are derived and is found to be equal to the atomic mass of the gas in the discharge tube.

(ii)They travel in straight lines.

(iii)They are capable of producing physical and chemical changes.

(iv) They can penetrate thin metal foils.

(v)They can produce ionization in gases.

(vi)They are deflected by electrical and magnetic fields just as the cathode rays but in opposite directions showing that they are oppositely charged, i.e., they carry positive charge. Since their deflection is very little, they consist of very heavy particles.

It was found that e/ m value for positive rays are different when the enclosed gas in the cathode rays tube is different. This is possible when different gases give different types of positive rays which contain particles having different masses and different charges. Thus, the mass and charge of positive ray particles depends upon the gas which is taken in the discharge tube. The positive particles obtained from hydrogen gas are the lightest and have the highest charge to mass ratio.

In the case of hydrogen gas the anode rays consist of proton. A proton possesses one unit of positive charge and one unit of mass.

When we apply high electric voltage, the electric energy splits the gasesous molecules into positively charged particles and negatively charged particles. The negatively charged particles form cathode rays and positively charged particles form anode rays. These charged particles can conduct electric current through the discharge tube. However, if the gas in the discharge tube is at the atmospheric pressure, the large number of gas atoms collide with the electrons and prevent them from reaching the anode. In such circumstances no current flows through the discharge tube. When the gas pressure is very low, there are a few gas atoms in the discharge tube. In this situation the movement of electrons towards the anode is not hindered and thus the gas conducts electricity.

A proton is the lightest positive particle. It is obtained when the gas inside the cathode rays tube is hydrogen.

Characteristics of Proton :

(i) A proton is one of the fundamental particles of the atom and is present in the nucleus of all atoms.

(ii) Charge: The charge on a proton is equal in magnitude but opposite in sign to that of an electron. Thus, it possesses a unit positive charge or + 1.6 x 10^-19 coulombs.

(iii) The mass of a proton is equal to that of a hydrogen atom, i.e., 1.672 x 10^-27 kg which is about 1837 times the mass of the electron.

Cathode rays are made up of negatively charged particles called electrons. The nature of cathode rays does not depend on the nature of the gas from which these are produced. The mass of a cathode rays particle is very very small compared to the mass of the atom from which it is formed. These particles were found to be about 2000 times smaller in mass than the hydrogen atom.

Anode rays are a stream of positively charged particles. The mass of an anode rays particle is equal to the mass of the atom from which it is formed. The nature of the anode rays depends on the gas from which these are produced.

There are numerous experiments to show that protons are present along with electrons in all substances. Protons are emitted during radioactive decay of various radioactive substances. Nitrogen present in the upper layer of atmosphere is converted into radioactive carbon by emission of a proton when the cosmic rays coming from the sun fall on it.

It is thus concluded that protons like electrons are the constituent particles of all matter.

The nature of charge ion on the rays are:

(i) X-rays—no charge 
(ii) Cathode rays—negative charge
(iii) Gamma rays—no charge 
(iv) Anode rays—positive charge.

A cathode ray discharge tube is essentially a glass tube with closed ends. Two metal plates which serve as cathode and anode are sealed at the two ends of the glass tube.

The discharge tube used to study anode rays has the perforated plate serving as cathode and is sealed in the middle of the glass tube.

The Canal rays are positively charged radiations consisting of particles which have a charge equal in magnitude but opposite in sign to that of the electron. The mass of a canal ray particle is 2000 times as that of the electron. This particle is known as proton.

The identification of cathode rays constituting charged electrons led to the conclusion that
(i) Electrons are present in an atom.
(ii) These electrons are embedded in a sphere of positive charge.
(iii) The comparison of e/m values for electron and H⁺, Na⁺ or other ions showed that the remaining part should be 2000 times heavier than the electron.
On this basis J.J. Thomson gave a model of the atom in which electrons and mass were distributed throughout the full size of atom. The size of the atom was estimated to be about 10^-10 m. fig. gives model of three atoms containing one, two and three electrons. This model was considered to be very unstable.

Thomson proposed the model of an atom to be similar to that of watermelon. the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon. 

When Rutherford bombarded thin sheet of gold foil with α -particles (α-particles are doubly charged helium ions, He²⁺), he found that:
(i) Most of the α-particles passes through the foil without any deflection. He calculated that only one particle in 10⁵ bounced back. This shows that most of the space inside the atom is empty and hollow.

(ii) Some of the α-particles were deflected through various angles while a very small number were actually deflected by as much as 180°.
This shows that:
(a) There is a heavy positively charged centre inside the atom. This centre is known as nucleus.

(b) Since only a very small fraction of α-particles were deflected through large angles, the nucleus is situated in a very small volume of the atom and is positively charged. The nucleus was found to be about 10⁵ times smaller than the total area occupied by the atom as a whole.

(c) Since α-particles deflected by the nucleus have an appreciable mass, it means that the entire mass of the atom lies inside the nucleus.

Chadwick discovered neutrons. The observation that the atomic masses of elements other than hydrogen could not be explained on the basis of the presence of only protons and electrons in an atom, and the atomic masses of all elements are much higher than that expected on the basis of the presence of only protons and electrons in an atom led to the discovery of neutron.

The small positively charged central part of an atom is called nucleus. All the protons and neutrons are present in this region.

The Number of protons an atom contains determines the number of positive charges on the nucleus.

α-particles are positively charged helium ions containing two protons and two neutrons each. In Rutherford's experiment only those α-particles, which bombarded the central part i.e., positively charged nucleus, were repelled. As same charges repel each other, therefore, it was concluded that the nucleus of an atom is positively charged.

The number of negatively charged electrons (total negative charge) is the same as the number of positively charged protons (total positive charge) in the atom of an element. 

When α-particles are allowed to strike a very thin gold foil, it is found that most of these particles pass through the foil without any deflection. It is observe that one particle in 12000 bounces back.
Thus, Rutherford calculated that the radius of the nucleus is about 10⁵ time less than the radius of the atom. hence, atom has a lot of empty space within it.

The fact that atomic nuclei are positively charged can be shown by performing Rutherford’s α-particle scattering experiment. Take a thin sheet of metal foil. Allow α-particles to bombard over it. It will be observed that only a small fraction of α-particles (positively charged) are deflected through large angles and the rest pass through the foil without any deflection. This shows that positive charge of the atom is concentrated at the centre or the nucleus.

Rutherford designed a model in which fast moving alpha (a)-particles were made to fall on a thin gold foil. On the basis of above experiment Rutherford concluded that:

i) Most of the space inside the atom is empty because most of the (a)-particles passed through the gold foil without getting deflected.

ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

iii) A very small fraction of (a)-particles were deflected by 180⁰, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume with in the atom.

The radius of an atom is 10⁵ times greater than the radius of the nucleus.

Characteristic features of Thomson model of an atom are:

(i) An atom consists of a positively charged sphere and the electrons are embedded in it.

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

The nucleus is a heavily charged positive centre located in a very small space of around 10^-15 m. An electron is a very small negatively charged particle with well established charge to mass ratio. The charge on electron forms the smallest unit of charge on atomic particles.

The main features of Rutherford's model of an atom are:

(i) The atom consists of a positively charged centre called the nucleus.

(ii) Most of the mass is concentrated in the nucleus.

(iii) The volume of the nucleus is very small compared to the total volume of the atom.

(iv) The nucleus is surrounded by the negatively charged electrons which are revolving round the nucleus at very high speeds like the planets revolving round the sun.

Rutherford proposed that the electrons revolve around the nucleus in well- defined orbits.But, electron is charged particle any charge particles during acceration radiate energy circular orbit would undergo acceleration. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.

According to Bohr’s theory:
(i) The atom-consists of a small (positively charged) nucleus at its centre.
(ii) The whole mass of the atom is concentrated at the nucleus and the volume of nucleus is smaller than the volume of the atom by a ratio of about 1 : 10⁵.
(iii) All the protons and neutrons of the atom are contained in the nucleus.
(iv) The electrons of the atom revolve round the nucleus in definite circular paths known as orbits or shells which are designated as K, L, M, N etc. or numbered as (n) = 1, 2, 3, 4 etc. outward from the nucleus.
(v) Each orbit is associated with a fixed amount of energy. Therefore, these orbits are also known as energy levels or energy shells.
(vi) The energy of the atom changes when an electron jumps from one state (energy level) to another state (energy level). As long as an electron remains in a particular orbit, it does not lose or gain energy.

In Rutherford experiment of scattering of alpha particles, we can take foil of any other metal if it is highly malleable because for this experiment we need a very thin foil. If the observation is taken by heavy metal like gold or Platinium, silver than the observation would be same but if it would be taken by light metal like lithium than the massive alpha particles will push the nucleus and may not be deflected back.

An electron revolving in an orbit (energy level) such as K, has a fixed amount of energy. When it jumps from K to L, it acquires energy of level L which is higher and thus the electron acquires more energy than it previously had. This leads to the overall increase in the energy of the atom.

An electron revolving in a cell is associated with a definite amount of energy. Its energy changes when it jumps to another cell, that is to say when it goes from one level of energy to another level of energy. Thus, a shell also gives the energy of an electron besides its location and are, therefore, called energy cells or levels.

In Bohr's model of atom, the electrons can occupy orbits with discrete energy levels only. Thus, when an electron falls from a higher energy level to a lower one, the difference in energy is radiated in the form of electromagnetic radiation of a fixed wavelength only. Since each atom has its specific energy levels, it can emit radiations of specific wavelength. This explains why different atoms give different or characteristic atomic spectra.

When the electron is so excited due to external energy that it is able to overcome force between it and the positively charged nucleus, it comes out of the atom. This explains the formation of cathode rays in the discharge tube. This is also known as ionization of gases.

Orbit is the path of the electron around the nucleus. At which electron revolve around the nucleus.

E. Rutherford proposed that each atom consists of a positive nucleus around which negatively charged electrons are revolving round the nucleus just like the planets move round the sun in fixed orbits in our solar system. According to this model most of the mass is concentrated in a small central part of the atom whereas J.J. Thomson proposed that , an atom consists of a positively charged sphere and the electrons are embedded in it. He also proposed that atom is electrically neutral.

Fundamental contributions of E. Rutherford in understanding the structure of the atom are:

(i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.

(ii) Most of the mass is concentrated at the nucleus. The electrons revolve around the nucleus in circular paths.

(iii) The size of nucleus is 10⁵ times smaller than the atom.

He gave a new concept that particles at atomic level would behave differently from the macroscopic objects. He suggested that electrons could revolve in stable orbits without continuously radiating or losing energy. According to Bohr's model, an electron revolves in an orbit with well-defined orbit.

Neutrons are present in the nucleus of an atom.
Charge: It is a neutral particle because it has no charge.
Mass of neutron: Mass of neutron is 1.0086654 a.m.u. or 1.6749 x 10^-27 kg.

The fundamental particles in an atom of any element are:

(i) Electron (₀e^-)

(ii) Proton (₁p⁺¹)

(iii) Neutron (₁e⁰).

Similarities of protons and neutrons:

(i) Both protons and neutrons are present in the nucleus of an atom.
(ii) The mass of a neutron is approximately equal to the mass of a proton, i.e., 1.67 x 10^-27 kg.

Dissimilarities between protons and neutrons:
Protons possess a unit positive charge (1.6 x 10^-19 coulomb).
Neutron carries no charge, i.e., it is electrically neutral particle.

The addition of a neutron to the nucleus of an atom, will increase the atomic mass of the atom.

(i) Protons and neutrons.

(ii) Electrons

(iii) Protons.

We cannot use scattering experiment to prove the existance of neutrons because the neutrons are neutral particles and so there will be no repulsion between positively charged α-particles and neutrons.

It is believed that electrons are produced as a result of decay of neutrons as indicated below:

The electron produced escapes as a (β-particle leaving the proton behind in the nucleus.

γ-rays are emitted as a result of rearrangement of neutrons and protons in a nucleus. Thus, the nucleus is not affected in terms of number of protons and neutrons due to emission of γ-rays.

According to Thomson's model of an atom, the negative and positive charges are equal in number and magnitude. therefore that the atom as a whole is electrically neutral.

Protons reside in the nucleus of an atom on the basis of Rutherford's model of an atom.

Bohr’s model with three shells.

If α-particle scattering experiment is carried out using a foil of any metal as thin as gold foil used by Rutherford, there would be no change in observations if heavy metal like gold use for example Platinium, sliver. but if it would be taken by light metal like lithium than the massive alpha particles will push the nucleus and may not be deflected back.

The sub-atomic particle of atom are:

(i) electron (₀e^-1)

(ii) proton (₁p⁺¹)

(iii) neutron (₁n⁰)

The mass of an atom is given by the sum of the masses of protons and neutrons present in the nucleus.

Mass of atom =Protons +Neutrons

Since helium atom has a atomic mass of 4 u and it has two protons. Two protons contribute 2 u to atomic mass.
 
4u =2u +Neutrons

Neutrons =4-2 =2

Hence, it must contain 2 neutrons.

The systematic distribution of electrons in various orbits or energy shells of an atom is called the electronic configuration of elements.

Since the both of K and L- shell of an atom are full thus, the total number of electron present in atom is 10 (K = 2, L = 8).

No, electron cannot be arranged in second shell without filling first shell because electrons first fill in lower value of energy and the energy of first shell is lower than second shell. So electrons will be arranged first in the K shell.

Fluorine has 9 electrons, 2 electrons can be accommodated in the first shell, the remaining seven electrons are accommodated in the second shell or L-shell which has a maximum capacity of 8 electrons. So, fluorine can have two energy shells.

There are 12 electrons in an atom of magnesium. K-shell contains 2 electrons and L-shell contains 8 electrons. Thus K and L-shell are full. Remaining 2 electrons (12-2-8 = 2) are accommodated in third or M-shell. M-shell can accommodate 18 electrons. So M-shell is incomplete.

Distribution of 18 electrons in argon atom is as follows:

K-shell = 2 electrons

L-shell = 8 electrons

M-shell = 8 electrons

K-shell and L-shell have maximum capacity of 2 and 8 electrons respectively. M-shell has a capacity of 18 electrons. Since, it is the outermost orbit and there is a rule that ‘the maximum number of electrons that can be accommodated in the outermost orbit is 8’. In the present case, M-shell is also complete. Thus, in argon atom, no shell is incomplete. Argon atom has three energy shells.

i) Lithium, sodium and potassium all have one electron in outermost shell. .

ii) Helium, neon and argon all have eight electrons in outermost shell.

iii) Beryllium, magnesium and calcium all have two electrons in outermost shell.

Carbon has 6 electrons. So its electronic configuration is:

Sodium has 11 electrons. Its electronic configuration is:
K-shell - 2 electrons
L-shell - 8 electrons or 2, 8, 1
M-shell - 1 electron

Distribution of electrons in shell is as follows:

K-shell = 2 electrons
L-shell = 8 electrons

If K-shell and L-shell are full then, the total number of electrons are 10.

The number of electrons present in the outermost shell of an atom are known as valence electrons.

Examples:

Valence electrons of an atom decide the chemical properties of the element.

The number of electrons determining the combining capacity of an atom is known as its valency.
For example, oxygen has six valence electrons but to combine with other atoms it must accept two electrons. So its valency is -2. The valency of lithium is one as these lose one electron, when combine with other atoms.
i) If the atom contains 1, 2 or 3 electrons in its outermost shell, then valency is equal to the valence electrons.
ii) If the number of electrons in the outermost shell of an atom is more than four then the valency is equal 8 minus number of valence electron present in outer most shell.

No. of electrons = no. of protons = 16

Distribution of 16 electrons in atom is as follows:

K-shell = 2 electrons

L-shell = 8 electrons

M-shell = 6 electrons

Valence electrons in the element are 6.

∴
Valency or electrovalency is 8 - 6 = 2.

Valency  of oxygen  = 8 - 6  = 2
Valency of nitrogen  = 8 - 5  = 3
Valency of fluorine = 8 - 7  = 1 
Note : When the outermost shell of an atom contains 4 or more electrons, its valency is equal to 8 minus the number of electrons in the outermost shell.

(i) Atomic number of an element is equal to the number of protons present in the nucleus of its atom.

∴
Atomic number = Number of unit positive charges on the nucleus.

Since the atom as a whole is electrically neutral, therefore, atomic number is the same as the number of electrons around the nucleus of an atom.

(ii) Atomic number gives the position of the element in the periodic table. In terms of atomic number an element can now be defined as a substance comprising of atoms all of which have the same atomic number.

All the elements are characterised by their atomic numbers. When elements react, their atoms either lose or gain electrons but their atomic number remains the same. Thus, atomic number is the fundamental property of the elements. Also, elements in periodic table are arranged in increasing atomic number.

(i) The atomic number (z) is the number of protons present in the nucleus of an atom.

(ii) Mass number (A) is sum of the number of protons (p) and the number of neutrons (n) in the nucleus of an atom.

Thus, A = n + p or = n + z. (z is atomic number.) For example the  mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u

We have given the mass number of atom and Mass number is given by the formula,

Mass no. = atomic number + no. of neutrons

∴
atomic number = mass no. - no. of neutrons = 23 - 12 = 11

The element is sodium (Na).

The mass number of the atom is given by the formula,
mass number = Total number of protons + Total number of neutrons.

(i) Number of protons = no. of electrons = 9

(ii) Number of neutrons = mass no. - no. of protons = 18 - 9 = 9

∴
(iii) Atomic number = no. of protons = 9.

The atomic number of electron is 9.  Element is Fluorine.

Number of protons = atomic number = 16

∴ No. of neutrons = mass no. - no. of protons = 35 - 16 = 19

∴ No. of electrons = no. of protons = 16.

Writing of electronic configuration

Distribution of 16 electrons in argon atom is as follows:

K-shell = 2 electrons

L-shell = 8 electrons

M-shell = 6 electrons

K-shell and L-shell have maximum capacity of 2 and 8 electrons respectively. M-shell has a capacity of 18 electrons. Since, it is the outermost orbit and there is a rule that ‘the maximum number of electrons that can be accommodated in the outermost orbit is 8’. In the present case, M-shell has  incomplete shell.

(i) 19 electrons = atomic number.

(ii) The electronic configuration of the given element is, 
K = 2, L = 8, M = 8, N = 1.

L-shell can contain 18 electrons but Bohr and Bury scheme also says that the outermost orbit in an element cannot contain more than 8 electrons. Therefore, in accordance with this scheme, the nineteenth electron finds a place in the next shell or in N-shell.

No. of electrons = atomic no. = 12.

Thus, K-shell contains = 2 electrons

L-shell contains = 8 electrons

M-shell contains = 2 electrons

Thus, electronic configuration of X becomes 2, 8, 2.

Number of electron = number of protons =12
No. of neutrons = Atomic mass - No. of protons = 24 - 12 =12
Thus, 
total number of neutrons =12
So,
Electronic configuration is

K - cell 2 electrons
L - cell 8 electrons
M - cell 2 electrons.

Atom is electrically neutral particle because in the nucleus of each atom the no. of protons present in the nucleus is equal to the no. of electrons revolving in its extra nuclear part. Each proton carries unit positive charge and each electron carries unit negative charge. Therefore, the no. of positive and negative charges in the atom are equal. This is why atom as a whole is electrically neutral particle.

Mass of an element can be obtained by adding the number of protons and neutrons. For examples:

The atomic no. of the element is 15. So it contains 15 protons. Hence the atom contains 15 electrons. It will have the following electronic configuration
 K                L             M
 2                8             5
∴  The atom occupies three shells.

An atom contains 3 shells, it means that in that atom the electrons are distributed broadly in three different energy levels. In the excited state the same atom may have the same electrons distributed in more than three energy levels and thus may have more shells.

The mass number of chlorine and atomic number can be reprented as ,  

On losing one or more electrons from its energy shell, an atom acquires positive charge and is called a cation. For example electronic configuration of Magnesium is K=2, L=8,M=2. On loosing 2 electron it become cation Mg²⁺.

When an atom gains one or more electrons, it becomes negatively charged and called by the name of anion.