Describe variation of first ionization energy across a period

Variation of first ionization energy across period II and period III

From lithium to neon there is a pronounced increase in ionizations energies with slight breaks occurring at positions occupied by boron and oxygen.  An exactly similar trend occurs in the portion of the graph from sodium to argon.

Reasons for the two trends

1. The increase in the first ionizations energies across the period is due to increase in the effective nuclear charge on the valence electrons as the number of electrons on the outer shell increases.

2. Half filled (ns¹, np³ nd⁵) or completely filled (ns²np⁶nd¹⁰) orbitals are comparatively more stable and hence more energy is needed to remove an electron from them. This explains why the first ionization energies of beryllium (full orbital, 1s² 2s²) and nitrogen (half full orbital, 1s² 2s² 2p³) are higher than those of boron and oxygen respectively.

3. The sharp decrease in ionization energies from helium to lithium is explained by the fact that the outer electron in lithium is farther from the nucleus than in helium. Also the removal of an electron from helium leads to a less stable half full orbital while removal of an electron from lithium leads to formation of a stable full orbital 1s².

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