NCERT Solutions for Class 9 Science Chapter 5 The Fundamental Unit Of Life

Compare the properties of electron, proton and neutron.

What are the limitations of J.J. Thomson’s model of atom?

What are the limitations of Rutherford model of the atom?

Describe Bohr's model of the atom ?

Compare all the proposed models of an atom given in this chapter.

Summarize the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Define valency by taking examples of silicon and oxygen.

Explain with examples, (i) Atomic number, (ii) Mass number, (iii) Isotopes, and (iv) Isobars. Give any two uses of isotopes also.

Na⁺ has completely filled K and L-shells. Explain.

If bromine atom is available in the form of, say, two isotopes ${}_{35}{}^{79}\mathrm{Br}$ (49.7%) and ${}_{35}{}^{81}\mathrm{Br}$ (50.3%), calculate the average atomic mass of bromine atom.

If Z = 3, what would be the valency of the element? Also, name the element.

Composition of the nuclei of two atomic species X and Y are given as under


                              X        Y
Protons  =                6        6
Neutrons=                6        6

Give the mass numbers of X and Y. What is the relation between the two species?

For the following statements, write T for True and F for False.

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

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

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

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

Rutherford’s α-particle scattering experiment was responsible for the discovery of

A.

Atomic nucleus

B.

Electron

C.

Proton

D.

Neutron

Isotope of an element has

A.

Same physical properties

B.

Different chemical properties

C.

Different number of neutrons

D.

Different atomic numbers.

Number of valence electrons in Cl^- ion are

• 16

• 8

• 17

• 18

Which one of the following is a correct ? Electronic configuration of sodium

(a) 2, 8,2 (b) 8, 2, 1 (c) 2, 1, 8 (d) 2, 8, 1.

Complete the following Table:

How do you establish that matter is electrical in nature?

What is a discharge tube?

What are the cathode rays? How are they produced?

What are important properties of cathode rays?

                            Or

Give four characteristics of cathode rays.

State the observations made, while conducting a discharge experiment, which showed that

(i) Cathode rays travel in straight lines.

(ii) Cathode rays are made up of material particles having mass and kinetic energy.

(iii) Cathode rays are negatively charged.

How is the flow of current in a cathode ray experiment explained knowing that gases are bad conductors of electricity?

Why does a part of the glass glow in the discharge tube experiment ? Name the scientist who first performed the discharge tube experiment.

What are X-rays?

Give the properties and use of X-rays.

Describe how X-rays are used to locate cracks in fractured bones.

Give one use of X-rays stating the property which makes this use possible.

Give the mass and charge of an electron.

Draw a labelled diagram to show the presence of charge on cathode rays in the discharge tube.

Comment on the statement: “Electrons are common constituents of all matter”.

What were the observations that led to the conclusion that cathode rays are negatively charged particles?

Describe an experiment to show that cathode rays travel in straight lines.

What is phosphorescence?

What are the different stages observed when a discharge tube is connected to a high voltage source and the pressure of the air in it is reduced gradually?

What happens when the cathode rays are passed through an electric field between two parallel plates ? Can one determine the nature of charge of the particles constituting the cathode rays from this experiment ? If so how?

A student weighs 30 kg. Suppose his entire body is made up of electrons. How many electrons are there in his body? Compare the total number of electrons in his body with the population of India.

What are anode or positive rays? How are they produced?

What are the important properties of anode rays?

What observation led to the conclusion that the nature of the anode rays depend upon the gas used in the cathode rays tube?

Can you predict the mass of the particles of anode rays from the nature of the gas used hydrogen?

Explain the conduction of electricity through gases at very low pressures.

What is a proton ? What are its characteristics?

In what respects cathode rays differ from positive or anode rays?

                            Or

What are cathode rays and positive rays?

Comment:All substances contain protons in addition to electrons.

What is the nature of charge on different rays:

(i) X-rays
(ii) cathode rays
(iii) gamma rays
(iv) anode rays?

What are the differences in the discharge tubes used to study cathode rays and the positive rays?

What are canal rays?

How were the results of discharge tube experiments interpreted by J. J. Thomson in terms of model of an atom?

Give an evidence for the existence of nucleus in an atom.
                           Or
What important informations are furnished about the nucleus of an atom by α-particle scattering experiment of Rutherford?

Who discovered neutrons? What observation lead to the discovery of neutrons?

What is a nucleus?

What determines the number of positive charges on the nucleus?

In Rutherford's experiment, some of the α-particles when bombarded against a gold leaf, were repelled. Give the reasons for this observation.

                                               Or

Give experimental evidence to show that the nucleus of an atom is positively charged.

Why is an atom neutral in spite of the presence of charged particles in it?

How was it shown that an atom has a lot of empty space within it?

How was it shown that atomic nuclei are positively charged?

                                 Or

Describe an experiment to show that the nucleus of an atom contains positively charged particles.

What observations led Rutherford to believe that central part of the atom is positively charged?

Describe the magnitude of the size of an atom in comparison to its nuclear size.

What are the characteristic features of the Thomson model of atom?

Describe the essential properties of the atomic nucleus. Compare these with the properties of the electron.

What was the model of atom as proposed by Rutherford?

What was the main objection to Rutherford’s model of atom?

Describe the nuclear (Bohr’s) model of an atom.

In Rutherford experiment of scattering of alpha particles, can we take foil of any other metal instead of gold?

When an electron jumps from energy level K to energy level L, why does the energy of the atom increase?

Why are the shells in which the electrons revolve around the nucleus of an atom called energy levels?

How does Bohr’s model of atom explain characteristic spectra of different atoms and ionisation of gases in the discharge tube experiment?

What is an orbit?

Describe the essential features of the model of atom proposed by E. Rutherford. How is it different from that proposed by J.J. Thomson?

What are the fundamental contributions of E. Rutherford in understanding the structure of atom?

What was the concept incorporated by Neils Bohr in the model of atom proposed by him?

What is a neutron? What are its characteristics?

Compare an electron, a proton and a neutron in respect of their symbol, mass and charge.

State the fundamental particles present in an atom of any element.

State the similarities and dissimilarities between protons and neutrons.

What will the addition of a neutron to the nucleus of an atom do?

Which constituent particles of the atom determine the following:

(i) Mass of the atom. 

(ii) Size of the atom,

(iii) Charge on the nucleus.

Can you explain why scattering experiment cannot be used to prove the existence of neutrons?

Nucleus does not contain any electrons. Even then the β-particle emission has been described as the ejection of an electron from the nucleus. Comment.

How does the emission of γ-rays affect the nucleus?

On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole?

On the basis of Rutherford's model of an atom, which sub-atomic particle is present in the nucleus of an atom.

Draw a sketch of Bohr's model of an atom with three shells.

What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?

Name the three sub-atomic particles of an atom.

Helium atom has an atomic mass of 4 u and has two protons in its nucleus. How many neutrons it have?

State the suggestions made by Bohr and Bury with regard to distribution of electrons in different energy levels in the atoms of elements.

                                  Or

Explain the arrangement of electrons outside the nucleus.

What is meant by electronic configuration of elements?

If both K and L-shells of an atom are full, what is the total number of electrons contained in them?

Can electrons be arranged in second shell or L-shell without filling first shell or K-shell?

Fluorine atom has 9 electrons and 9 protons. How many energy shells it has?

Magnesium atoms has 12 electrons. Which energy shell is incomplete?

Argon atoms has 18 electrons. How many energy shells or orbits are incomplete ? How many energy shell it contains?

What are the similarities in the electronic structure of the following sets of atoms:No. of electrons in each atom is given in brackets:

i) lithium (3), sodium (11) and potassium (19).
ii) helium (2), neon (10) and argon (18).
iii)beryllium (4), magnesium (12), and calcium (20).

Write the distribution of electrons in carbon and sodium atoms.

If K and L shells of an atom are full then what would be the total number of electrons in the atom?

What are valence electrons?

Give two examples to illustrate the answer.

Which electrons of an atom decide the chemical properties of the element?

How do you define valency of an atom?

An element has 16 protons. How many electrons will be present in K, L and M shell of its atom. What will its electrovalency be?

How will you find the valency of nitrogen, oxygen and fluorine:
                    K-Shell           L-shell
   Oxygen         2                    6
   Nitrogen        2                    5
   Fluorine         2                    7

What is atomic number? How has this concept improved the definition of an element?

Why is atomic number is the fundamental property of elements?

Define the terms:
(i) atomic number
(ii) mass number, of an atom in an element.

Calculate the atomic number of an element whose atomic nucleus has mass number 23 and neutron number 12. What is the symbol of this element?

The mass number of an element is 18. It contains 9 electrons. What is the no. of protons, neutrons? What is the atomic no. of the element

Explain the method to write the electronic configuration of the atom of an element whose mass number (A) is 35 and atomic number 16. What are the no. of electrons, protons and neutrons?

An element has an atomic number of 19 and atomic mass of 39.

(i) How many electrons are there in each atom of the element?

(ii) How are these electrons arranged in the atom?

                         Or

Write electronic distribution in an atom of potassium.

Potassium with 19 electrons has the electronic configuration as K = 2, L = 8, M = 8, N = 1. Why cannot you think of 9 electrons in M-shell when it can contain 18 electrons (2n² = 18)?

Write the electronic configuration of an element X, whose atomic number is 12.

If an element M has atomic weight 24 and atomic number 12, how many neutrons does its atom contain? How many electrons will be present in K, L and M energy shells of its atom?

Give the nuclear composition and electronic configuration of 18 elements with atomic number 1 to 20.

Is atom an electrically neutral particle? If yes, why?

How will you find the atomic mass of an element sodium and carbon? Illustrate your answer with examples.

An element has atomic number 15. Write the electronic configuration of the atom and indicate the number of shells it will occupy.

An atom contains three shells. What does it mean? It is possible that the same atom may have more shells?

Complete the following table:

Atomic numbers and atomic mass numbers of the elements are given below. Complete the following information in the blanks:

The mass number of chlorine atom is 35 and its atomic number is 17. How will this chlorine atom be represented?

How does an atom become a cation?

What is an anion?

Substances from A to E have in them the distribution of electrons, neutrons and protons as follows:

Making use of these data find—
(i) a cation,
(ii) an anion,
(iii) atom of noble gas.

What is meant by atomic no. of an element ? Does the atomic number of an element change when its atoms form ions ? Give one example each of diatomic and triatomic molecules.

What is the atomic number of an element which has 7 electrons in the M shell?

Give answers to each part in one or two words or a sentence :

(i) The atomic number of chlorine is 17. What is the number of electrons in Cl^-?

(ii) An element has two electrons in the M-shell. What is the atomic number of the element?

Is there any change in atomic number when oxygen atoms form oxide ions?

The atomic numbers of aluminium and chlorine are 13 and 17 respectively. How many electrons are present in Al⁺³ and Cl^- respectively?

Why is that certain atoms are radioactive while many others are not?

What kind of electronic configuration is attained by chemically inert substances?

If number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of the atom ?
(ii) What is the charge on the atom?

Define the term isotope with a suitable example.

Two atoms of chlorine, A and B have the same nuclear charge. Their mass numbers are 35 and 37 respectively. What are the atoms collectively known as? What is the difference between the two ? Do they differ in their chemical properties?

State the similar properties of isotopes.

What are the uses of isotopes?

Atomic mass of an element is the sum of no. of protons and neutrons which are whole numbers. How do you account for the fractional atomic masses of elements?

Composition of the nuclei of two atomic species A and B is given as under. Give the mass numbers of A and B. What is the relation between the two species and which element or elements do they represent

The nuclear composition of two atomic species X and Y is



What is the relation between these two atomic species ? Name the element or elements they represent.

A and B are two atoms in whose nuclei, the number of protons and neutrons are:

Which element/elements do they represent and what is the relationship between them?

Why do isotopes of an element differ in atomic masses and not in atomic number?

The number of protons, neutrons and electrons in species from A to E are given in the following table.

Indicate from the above table the species that represent:

(i) A cation
(ii) An anion
(iii) An atom of inert gas 
(iv) A pair of isotopes.

A, B and C are three elements. The nuclei of the atoms of these elements have protons and neutrons as under:

Isotopes of elements have the same chemical properties but different physical properties. Give reasons.

Sulphur exists as four naturally occurring isotopes with mass numbers 32, 33, 34, 35 and atomic number 16.

(i) Give atomic symbol for each indicating mass number and atomic number.

(ii) Would these have similar physical or chemical properties?

What are nucleons?

What are isobars. Give examples.

What do you say about physical and chemical properties of isobars?

Describe the essential features of the experiment that led to the discovery of isotopes.

For the symbol H, D and T tabulate three fundamental particles found in each of them.

Substances having similar chemical properties, but different physical properties are called _________.

Isotopes contain the same number of ___________ but different number of _________.

The negative particle discovered by J.J. Thomson was given the name ________.

E. Goldstein reported the presence of _________ inside the atom.

E. Rutherford used ____________ particles in his scattering experiments.

According to Rutherford, the force of attraction between the moving electrons and __________ is counter balanced by the centrifugal force of electrons.

Almost all the mass of an atom is concentrated in a small region of space called ____________.

Most of the space in an atom is occupied by _________.

A proton is __________ times heavier than an electron.

Electronic configuration of helium atom is ____________.

The nucleus of an atom consists of _________ and __________.

When α-particles are sent through a thin metal foil, most of them go straight through the foil because α-particles are _________.

The atomic number of an element signifies the number of ________.

Substances formed from more than one kind of atoms, combined together are called _________.

The extranuclear part of the atom in which electrons are revolving are called _________.

If the nucleus of an atom has too many __________ it becomes unstable.

The total number of protons and neutrons in the nucleus of an atom is called its _________.

The maximum number of electrons that can go into the M-shell is _________.

The neutron has _________ charge.

The fundamental particle not present in a hydrogen atom is __________.

Volume of nucleus of an atom is ___________ as compared to the volume of the extranuclear part of the atom.

Neutron was discovered by __________.

Charged bodies when move in circular motion, emit _________.

An electron revolving in a particular orbit has _________ energy.

The combining capacity of an element is called its ___________.

The number written on upper left corner of symbol of an element gives its __________ number.

Deuterium and Tritium are _______ of hydrogen.

An alpha particle is ________ charged helium atom.

K-shell of an atom is full when it contains _____ electrons.

In a neutral atom, the atomic number is equal to the number of ___________.

Who discovered electron?

Write the units of which the cathode rays are composed.

What happens when cathode rays strike a solid under suitable conditions?

Cathode rays cast shadow of the object placed in their path. What conclusion can be drawn from this observation regarding nature of cathode rays?

What is the nature of charge on cathode rays?

What are cathode rays?

Is the nature of anode rays independent of the gas used in the cathode rays tube like cathode rays?

What is the name given to the particles of anode rays obtained using hydrogen gas in the discharge tube?

Name the gas which produces positive rays consisting of protons in the discharge tube experiment.

Name the fundamental particle that is not present in an atom of hydrogen.

Name the fundamental particle that determines size of an atom.

What is the central part of the atom called where neutrons and protons are held together?