The s, p, d and f blocks

The block is named after the kind of orbital receiving the last electron. Two exceptions: helium (1s²) belongs to the s-block but sits with group 18 because its shell is full; hydrogen (1s¹) can lose an electron like group 1 or gain one like group 17, so it is placed on its own at the top.

s-block: groups 1 (alkali metals, ns¹) and 2 (alkaline earth metals, ns²). Reactive metals with low ionization enthalpies, forming 1+ or 2+ ions; metallic character and reactivity rise down the group, so they never occur free. Their compounds are mainly ionic, except those of Li and Be.

p-block: groups 13 to 18, outer configuration ns²np¹ to ns²np⁶. The s- and p-blocks together are the representative or main group elements.

Each period ends in a noble gas, ns²np⁶; its full valence shell is hard to disturb, so reactivity is very low. Before them come the halogens (group 17) and chalcogens (group 16), with strongly negative electron gain enthalpies; they readily take one or two electrons to reach a noble gas configuration.

d-block (transition elements): groups 3 to 12, filling inner d orbitals; general configuration (n−1)d¹⁻¹⁰ns⁰⁻², with Pd (4d¹⁰5s⁰) the exception. All are metals, mostly with coloured ions, variable oxidation states and paramagnetism, and are often catalysts.

Zn, Cd and Hg, with (n−1)d¹⁰ns², miss most typical transition-metal features. Sitting between the very active s-block metals and the less active groups 13 and 14, the d-block acts as a bridge, which explains the name transition elements.

f-block (inner transition elements): lanthanoids Ce (58) to Lu (71) and actinoids Th (90) to Lr (103), outer configuration (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns². All are metals, and members of one series are very alike.

Early actinoids have more complex chemistry than the matching lanthanoids because they show many oxidation states. Actinoids are radioactive, many made only in nanogram amounts or less; elements after uranium are the transuranium elements.

Predicting from the table: Z = 117 falls in group 17, [Rn]5f¹⁴6d¹⁰7s²7p⁵; Z = 120 would be in group 2, [Uuo]8s².