Chemistry

3.19. On moving down the group, the ionization enthalpy decreases. This is true for B and Al due to the bigger size of Al.

The ionization enthalpy of Ga is unexpectedly higher than Al because Ga contains 10d electrons in inner shell whose shielding is less effective than that of s and p electrons.

The outer electron is held fairly strongly by the nucleus. The ionization enthalpy increases slightly. A similar increase is observed from In to Tl due to presence of 14f electrons in the inner shell of Tl which have poor screening effect.

Atomic size: With the increase in atomic size, the number of electron shells increase. Therefore, the force that binds the electrons with the nucleus decreases. The ionization enthalpy thus decreases with the increase in atomic size.

Screening or shielding effect of inner shell electron: With the addition of new shells, the number of inner electron shells which shield the valence electrons increases. As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionization enthalpy decreases.

3.15. The ionisation enthalpy is for 1 mole atoms.

Therefore, ground state energy of theatoms may be expressed as

E_{ground state} = (– 2.18 x 10^-18 J) x (6.022 x 10²³ mol^-1)

= –1.312 x 10⁶ J mol^-1

Ionisation enthalpy =E_? –E_{ground state}

= 0– (–1.312 x 10⁶mol^-1)

= 1.312 x 10⁶ J mol^-1.

3.13. A cation is obtained by removing an electron from the outermost shell. The removal of electron (s) results in decrease of the size of the resulting ion than the parent atom because it has fewer electrons while its nuclearcharge remains the same.

Anions are obtained by addition of electron (s) in the outermost shell. This results in increased repulsion among the electrons and a decrease in effective nuclear charge.

3.12.  (a) All of them have 10 electrons each and are isoelectronic in nature.

(b) In isoelectronic species, higher the nuclear charge, smaller will be the atomic or ionic radius.

Al³⁺< Mg²⁺< Na⁺< F^–< O^2-< N^3-

3.11. Species (atoms/ions) which have same number of electrons are called isoelectronic species. The isoelectronic species out of the given atoms/ions are:

(i) Na⁺ is isoelectronic to F^- 

(ii) K⁺ is isoelectronic to Ar

(iii) Na⁺ is isoelectronic to Mg²⁺

(iv) Sr²⁺ is isoelectronic to Rb⁺

3.10. Across a period, the atomic radii decrease from left to right due to increase in effective nuclear charge from left to right across a period

Within a group, atomic radius increases down the group due to continuous increases in the number of electronic shells or orbit numbers in the structure of atoms of the elements down a group.