s-, p-, d-, and f-blocks

The periodic table looks like a strangely shaped grid with a tall block on the left, a wide block on the right, a long slab in the middle, and two rows pulled out at the bottom. Those four regions are the blocks — s, p, d, and f — and each one is simply the set of elements whose outermost electron is going into an s, p, d, or f orbital. The shape of the table is not arbitrary; it is a direct print-out of how orbitals fill.

Read it left to right. The s-block is the two columns on the far left (groups 1 and 2, plus helium): their last electron enters an s orbital, and since an s subshell holds 2 electrons, the block is 2 wide. The p-block is the six columns on the right (groups 13-18): a p subshell holds 6, so the block is 6 wide. The d-block is the 10-column slab in the middle — the transition metals — because the five d orbitals hold 10 electrons; it sits one row 'late' because (n-1)d fills after ns. The f-block is the 14-column pair of rows floated below (the lanthanides and actinides), since the seven f orbitals hold 14. Periods are the horizontal rows (one principal shell each); groups are the vertical columns of similar valence configuration.

Blocks are the master map of inorganic chemistry. They predict behaviour at a glance: s-block elements are reactive metals that lose their few outer electrons easily; p-block runs from metals through metalloids to the reactive nonmetals and noble gases; the d-block transition metals show variable oxidation states, colour, and magnetism from their partly filled d orbitals; the f-block elements are the lanthanides and actinides, dominated by buried f electrons. One honest wrinkle: helium sits atop the noble gases (group 18) for its chemistry even though it is technically an s-block element — a reminder that the table organises behaviour, and a few placements are conventions.