JEE Main · 2021 · 17 Mar 2021 · Shift 2 · d and f Block Elements

Atomic number $24$ is chromium, with the ground state configuration $[\mathrm{Ar}]3d^{5}4s^1$. The $3d$ and $4s$ levels lie close enough in energy that all six of these electrons are available for bonding, which fixes $+6$ as the highest state, reached in $\mathrm{Cr}{\mathrm{O}}_4^{2-}$ and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$. Loss of the single $4s$ electron alone would give $\mathrm{C}{\mathrm{r}}^{+}$, which is not stable enough to appear in ordinary compounds, so the series starts at $+2$, where the $4s$ electron and one $3d$ electron are lost. Between these limits $+3$ and $+6$ are the most stable, with $+4$ and $+5$ met in oxides and oxo-anions. The common positive oxidation states run from $+2$ to $+6$.