A) arrange the following in order of increasing first ionization energy: Na, Cl,

Question

A) arrange the following in order of increasing first ionization energy: Na, Cl, Al, S, and Cs. B) the first and second ionization energies of K are 419 kJ/mol and 3052 kJ/mol, and those of Ca are 590 kJ/mol and 1145 kJ/mol, respectively. Compare their values and comment on the differences. A) arrange the following in order of increasing first ionization energy: Na, Cl, Al, S, and Cs. B) the first and second ionization energies of K are 419 kJ/mol and 3052 kJ/mol, and those of Ca are 590 kJ/mol and 1145 kJ/mol, respectively. Compare their values and comment on the differences. B) the first and second ionization energies of K are 419 kJ/mol and 3052 kJ/mol, and those of Ca are 590 kJ/mol and 1145 kJ/mol, respectively. Compare their values and comment on the differences.

Explanation / Answer

A) Na, Cl, S, Al all are in the same period. In the period as we go for left ot right the first ionization energy increases, due to the increase in effective nuclear charge and decrease in size of atoms.

Cs is in the 6 th period and much larger in size than all these. Thus it is the one which has lowest first ionization energy.

Thus the order is Cs < Na < Al < S <Cl

B) The first I.E. of K is relatively les due to the 4s1 configuration.After losing the first electron the outer configuration of K becomes

K+ (18) = 1s2 2s2 2p63s2 3p6

which is a stable octet configuration. Thus to remove an electron from this already positivley charged stable configuration is very difficult , whichis shown by high value of second ionization energy.

For Ca (20) the outer configuration is 4S2.

This itself is stable configuration , thus first I.E. of Ca is greater than that of K.

After losing first electron Ca + has 4s1 configuration which is less stable than 3s2 3p6 configuration. Thus it can be easily removed by using less energy to gain the stable octet configuration.

Hence second I.E. of Ca is less than that of K.

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