(a) Periodicity of Valence or Oxidation States

Periodicity of Valence or Oxidation States

Valence is the most characteristic property of an element. For representative (s- and p-block) elements, the valence is usually equal to either the number of electrons in the outermost shell or eight minus that number. This dual possibility exists because an atom can either lose/gain electrons to achieve a noble gas configuration or share electrons to complete its octet.

Defining Oxidation State

Consider two oxygen-containing compounds: $\mathrm{OF_2}$ and $\mathrm{Na_2O}$. The electronegativity order of the elements involved is $\mathrm{F} > \mathrm{O} > \mathrm{Na}$.

In $\mathrm{OF_2}$: Fluorine has the outer configuration $2s^2 2p^5$. Being the most electronegative element, each fluorine atom is assigned oxidation state $-1$. Oxygen ($2s^2 2p^4$) shares two electrons with the two fluorine atoms, so it exhibits oxidation state $+2$.

In $\mathrm{Na_2O}$: Oxygen is more electronegative than sodium. It accepts two electrons (one from each sodium atom) and shows oxidation state $-2$. Sodium ($3s^1$) loses one electron to oxygen and is assigned oxidation state $+1$.

Problem 3.8 in Section 3.7.2's Problems group below predicts the formulas of silicon–bromine and aluminium–sulphur compounds from these group valences.

Periodic Trends in Valence: Hydrides and Oxides

Table 3.9 shows the periodic trends in valence as demonstrated by the formulas of hydrides and oxides across groups 1, 2, and 13–18.

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