Past exam of the chemistry B course of the University of Cambridge / 2021 / Paper 2 / B9 / a / Solution

For the two-atom inorganic backbone unit, $Z_{\mathrm{Si}}+Z_{\mathrm{O}}=14+8=22$ and $Z_{\mathrm{P}}+Z_{\mathrm{N}}=15+7=22$. Their valence-electron sums also agree: $4+6=5+5=10$. Since each unit bears the same two organic groups, ${\mathrm{R}}_2\mathrm{SiO}$ and ${\mathrm{R}}_2\mathrm{PN}$ are isoelectronic species. This electron-count relationship does not require identical bond localization, bond angles or chemical reactivity.