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Classification of Elements and Periodicity in Properties
Question
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Why is the radius of cation less than that of parent atom?

Answer

A cation is formed by the loss of one or more electrons from the neutral gaseous atom. With the removal of one or more electrons from an atom, a magnitude of nuclear charge remains the same as that of the parent atom while the number of electron decreases. As a result, the same nuclear charge acts on the lesser number of electrons. In other words, the effective nuclear charge per electron increases and therefore, electrons are more strongly attracted and pulled towards the nucleus. This causes the decrease in the size of positive ion.
For example.
Atomic radius of Na = 154 pm (Nuclear charge = + 11 ; e = 11)
Ionic radius of Na++ = 95 pm (Nuclear charge = + 11 ; e = 10)

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Some More Questions From Classification of Elements and Periodicity in Properties Chapter

How many elements are present in:
(i) second period    
(ii) fourth period    
(iii) sixth period?

What is the name given to the s-block elements?

To which block the element with outer electronic configuration 4s23d10 belongs?

Why are there 10 elements in each series of d-block?

In terms of electronic configuration,
what is common in a given period and group?

Give the general electronic configuration of
(i) Transition elements     (ii) Inner transition elements.

Name the group and period to which an element with Z = 15 belongs ?

What are the advantages of periodic classification of elements?

In the modern periodic table, the period indicates the value of:
(a) atomic number
(b) atomic mass
(c) principal quantum number
(d) azimuthal quantum number.

Which of the following statements related to the modern periodic table is incorrect?
(a) The p-block has 6 columns, because a maximum of 6 electrons can occupy all the orbitals in a p-shell.

(b) The d-block has 8 columns, because a maximum of 8 electrons can occupy all the orbitals in a d-subshell.

(c) Each block contains a number of columns equal to the number of electrons that can occupy that subshell.

(d) The block indicates value of azimuthal quantum number (l) for the last subshell that received electrons in building up the electronic configuration.