Chemistry · Ch 9 — Elements of Group 13, 14 and 15
Reaction with halogens
Reaction with halogens
All group 13 elements react directly with halogens to form trihalides, EX3 (2E(s) + 3X2(g) → 2EX3(s)); thallium is the one exception, forming monohalides (TlX) instead, consistent with its preference for the lower, +1 oxidation state. All group 14 elements except carbon react directly with halogens to form tetrahalides, EX4; the heavier elements germanium and lead can additionally form dihalides, and the stability of these dihalides increases down the group because of the inert-pair effect (see Problem 9.5: PbCl2 is more stable than PbCl4, while GeCl4 is more stable than GeCl2). The ionic character of these halides also increases steadily down the group. Group 15 elements form two series of halides, EX3 and EX5; the pentahalides are more covalent because forming them relies on the availability of vacant d-orbitals in the valence shell for the extra bonds. Nitrogen, being a second-period element without valence d-orbitals, cannot form a pentahalide a …
Worked out. Worked problem: GeCl4 is more stable than GeCl2, while PbCl2 is more stable than PbCl4 — explain. Solution: germanium and lead are the 4th- and 5th-period elements of group 14. The group oxidation state for group 14 is +4, but the oxidation state two units lower (+2) becomes increasingly stable down the group because of the inert-pair effect. Since lead lies below germanium, this effect — and hence the stability of the +2 state — is stronger for lead than for germanium. This is why GeCl4 (germanium in its +4 group oxidation state) is more stable than GeCl2, while for lead the situation reverses and PbCl2 (lead in its lower, ine …
Worked out. Worked problem: nitrogen does not form NCl5 or NF5, but phosphorus can — explain. Solution: phosphorus and the other, heavier members of group 15 have vacant d-orbitals available in their valence shell, which lets them form both MX3-type and MX5-type halide compounds. Nitrogen, however, is a second-period element whose valence shell has no d-orbitals at all, so it cannot expand its octet the way phosphorus can — this is why nitrogen forms only trihalides (such as NF3) and can never form a pentahalide like NCl5 or NF5, while phosphorus readily forms PCl5 and s …