Class 11th

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.

3.9. Atomic radius: It is the distance from the centre of nucleus to the outer most shell of electrons in the atom of any element. It incorporates both the covalent, metallic radius or van there Waal's radius depending on whether the element is a non-metal or a metal.

Ionic radius: It is the distance between the centre of the nucleus of an ion up to the point where it exerts its influence on the electron cloud of a canton or anion.

3.8. The elements in a group have same number of valence electrons and hence have similar physical and chemical properties.

3.7.  (i) Lawrencium (Lr) with atomic number (z) = 103

(ii) Seaborgium (Sg) with atomic number (z) = 106.

3.6. The element is chlorine (Cl) with atomic number (Z) = 17.