The first ionization energies of the main group elements are given in Figure 7.3
ID: 1029608 • Letter: T
Question
The first ionization energies of the main group elements are given in Figure 7.39. Explain the differences in the first ionization energy between (a) He and Li; (b) Li and Be; (c) Be and B; (d) N and O.
Period number 1 2500 2500 He 2000 2000 1500 1500 Kr 1000 HB Cd 500 In 5 500 10 18 54 Atomic number (Z) 18 17 15 14 13 FIGURE 7.39 The first ionization energies of the elements generally increase from left to right in a period and decrease from top to bottom in a group: (a) IE1 values for the elements in the first five periods; (b) IE1 values for all the main group elementsExplanation / Answer
Solution:
(a) Consider the electronic configurations of the elements.
He - 1s2
Li- 1s2 2s1
There is a drop in ionization energy from He to Li because the outer electron in Li is in the 2nd shell, whereas He has electrons in the 1st shell. Li has the 1s shell sheilding the outer electron from the nucleus, therefore it is easier for it to be removed.
(b) The increase from Li to Be is due to the increase in the number of protons by one. Therefore, the outer electron in Be feels more attraction than the outer electron of Li, which makes it a bit harder to remove.
Li- 1s2 2s1
Be- 1s2 2s2
The outer electrons of Li and Be are in the same shell, but Be has one extra proton.
(c) Be- 1s2 2s2
B- 1s2 2s2 2p1
The outer electron of B is added into a new sub-shell, 2p, which makes it easier to ionize than Be as it is shielded by both 1s and 2s sub shells.
(d) N-1s2 2s2 2p3
O-1s2 2s2 2p4
The 2p level of nitrogen is half filled.The half filled configuration is stable, when compared to oxygen which has 4 electrons in the 2p level. Thus when moving from N to O, the ionization energy drops.
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