The table shows the first ionisation energies of elements in Period 3.
Element: Na Mg Al Si P S Cl Ar
IE₁ /kJ/mol: 496 738 577 786 1012 1000 1251 1521
(a) Define the term first ionisation energy.
(b) Explain the general increasing trend in first ionisation energies across Period 3.
(c) Explain why the first ionisation energy of Al is lower than that of Mg.
(d) Explain why the first ionisation energy of S is lower than that of P.