Periodicity in ionisation potential

i) For normal elements: On moving from left to right in a period, value of ionisation potential of elements increases because effective nuclear charge also increases.

Exceptions

a) In a period, the ionisation energy of IIA group elements is more than the elements of IIIA because penetration power of s-orbitals electrons. The value of ionisation energy of Be(1s22s2) is more than B (1s22s2px1) because the penetration power of 2s-electrons of Be is more than the 2px- electrons of B.

b) In a period, the ionisation energy of VA elements is more than the elements of VIA because the half filled p3 configuration of VA elements is comparatively of higher stability. VIA group elements (p4) have the tendency to acquire comparatively more stable (p3) configuration by the loss of one electron.

Ionisation energy N(Is2 2s2 2p1x py1 pz1) > O Is2 2s2 2p2x py1 pz1

Thus P>S, As>Se.

But the value of I.P. of Sb (VA) & Te (VIA) and Bi (VA) & Po(VIA) are according to general rule i.e.

Sb (VA) < Te (VIA)

Bi (VA) < Po (VIA)

On moving from top to bottom in a group the value of I.P decreases because the atomic size increases.

Exceptions

a) In group IIIA the ionisation potential of Al (13) is equal to the ionisation potential of Ga(31). Before Ga (31) the electrons are filled in 3d – orbitals of ten transition elements. These 3d orbital electrons do not completely shield the 4P electron.
So the increase of +18 units in nuclear charge results in the greater increase of effective nuclear charge. Due to increase in nuclear charge the I.P. increases which counter balance the decrease in I.P. due to the increase in number of shells.

b) The values of I.P. of Tl (81) and Pb (82) of sixth period is more than that I.P. values of In (49) and Sn (50) of same groups in period fifth. This is because of the electrons are filled in 4f -orbitals before Tl (81) and Pb (82) which do not completely shield the outer electrons. Thus increase in + 32 units in nuclear charge results in the increase of ionisation potential values.

ii) For transition elements: On moving from left to right in a transition series (a) As the atomic number increases the effective nuclear charge also increases. Hence the I.P. increases.

(b) The shielding effect of (n – 1)d electrons over ns electrons increases with the addition of electron in (n-1)d orbitals. Hence the I.P. decreases.

(c) The increased values of I.P due to the increase of effective nuclear charge almost balances the decreased value of I.P. due to increase in shielding effect. There is a very small increase in the values of I.P. or it may be said that I.P. almost remains the same.

d) In first transition series from Sc to Cr the value of I.P. increases because effect of increase in effective nuclear charge is more than the shielding effect. I.P. values of Fe, Co, Ni and Cu are almost same. Due to d10s2 configuration of Zn, the first I.P. increases.

On moving from top to bottom in a group in transition series
a) In a group on moving from first to second transition series, the values of I.P. decreases because atomic size increases.

b) In moving from second to third transition series the value of I.P some what increases except IIIB group [Y(39) –> La(57)].
This is because of 14 electrons are filled in 4f -orbitals of lanthanides which do not shield the 5d electrons effectively. Thus the increase in +32 units in nuclear charge results in the increase of I.P., on moving from left to right this effect decreases and becomes negligible in the later part.

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