Q.(a)

(a) $CO_2$ is sp, $NO_2^-$ and $ClO_2^-$/$XeF_2$ show increasing lone pairs giving $sp^2$, $sp^3$, $sp^3d$ respectively, and Heisenberg's principle bars simultaneous exact knowledge of position and momentum. (b) The inert pair effect explains why heavier p-block elements favour a lower oxidation state, and silicones serve as waterproofing agents, lubricants and insulators.

(a)(i) Hybridisation (using total electron-pair count around the central atom = bond pairs + lone pairs):

• (A) $CO_2$: central C is doubly bonded to each O ($O=C=O$), 2 sigma bond pairs, 0 lone pairs on C $\Rightarrow$ 2 electron domains $\Rightarrow$ sp hybridised, linear shape.
• (B) $NO_2^-$ (nitrite ion): central N has 2 sigma bonds (to the two O atoms) and 1 lone pair $\Rightarrow$ 3 electron domains $\Rightarrow$ $sp^2$ hybridised, bent (angular) shape.
• (C) $ClO_2^-$ (chlorite ion): central Cl has 2 sigma bonds (to the two O atoms) and 2 lone pairs $\Rightarrow$ 4 electron domains $\Rightarrow$ $sp^3$ hybridised, bent shape.
• (D) $XeF_2$: central Xe has 2 sigma bonds (to the two F atoms) and 3 lone pairs $\Rightarrow$ 5 electron domains $\Rightarrow$ $sp^3d$ hybridised, linear shape.

(a)(ii) Heisenberg's uncertainty principle: it is impossible to determine, simultaneously and with unlimited precision, both the exact position and the exact momentum of a microscopic (sub-atomic) particle such as an electron. If $\Delta x$ is the uncertainty in position and $\Delta p$ the uncertainty in momentum:

$\Delta x \cdot \Delta p \geq \frac{h}{4\pi}$

where $h$ is Planck's constant. This is a fundamental limitation of nature (not of instruments), and it is why the Bohr model's idea of a fixed electron orbit was replaced by the quantum-mechanical idea of an orbital (a probability region).

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