Ionisation Energy

Energy required to remove one electron / one mole of electrons; from each atom in one mole of gaseous atoms; to form one mole of gaseous +1 ions

Ca⁺(g) → Ca²⁺(g) + e⁻; correct state symbols (g) on both species

Outer electron in Al is in a 3p orbital / Mg's outer electron is in 3s; 3p is higher in energy / further from nucleus than 3s; 3p electron in Al is shielded by the filled 3s sub-shell, so less energy is needed to remove it

In P, each 3p orbital contains one electron / 3p electrons are unpaired; in S, two electrons must pair in one 3p orbital; spin-pair repulsion between paired electrons makes one electron easier to remove

Large jump occurs between IE₂ and IE₃; two electrons are removed before the jump; outer shell contains two electrons, so Y is in Group 2; Period 3 Group 2 element is magnesium / Mg; the third electron is removed from an inner shell (2p) which is closer to the nucleus / less shielded; this requires much more energy, producing the large jump

Atomic radius increases down the group; shielding by inner shells increases; attraction between nucleus and outer electron decreases / outweighs the increase in nuclear charge

Successive IE values increase as each electron is removed from an increasingly positive ion; small gradual rises indicate electrons removed from the same shell or sub-shell; large jumps indicate a new shell closer to the nucleus; for Si, IE₁ to IE₄ rise gradually showing four outer-shell electrons; large jump between IE₄ and IE₅ shows the next electron comes from the inner shell (n=2); a smaller increase in slope can also indicate a sub-shell change (3p to 3s); the pattern matches the configuration 1s² 2s² 2p⁶ 3s² 3p²; this provides direct experimental evidence for shells and sub-shells