Explain the following:
$(1)$ Gallium has higher ionisation enthalpy than aluminum.
$(2)$ Boron does not exist as $B^{3+}$ ion.

(N/A) $(1)$ The first ionization enthalpy of $Ga$ $(579 \ kJ \ mol^{-1})$ is higher than that of $Al$ $(577 \ kJ \ mol^{-1})$ because the $d$-electrons present in $Ga$ have a poor shielding effect. This leads to a higher effective nuclear charge,which holds the valence electrons more tightly.
$(2)$ Boron has a very small atomic size and a very high sum of the first three ionization enthalpies $(\Delta_{i}H_{1} + \Delta_{i}H_{2} + \Delta_{i}H_{3})$. The energy required to remove three electrons is too high to be compensated by the lattice energy or hydration energy,so boron does not form $B^{3+}$ ions and instead forms covalent compounds.