Why is the periodic table that shape?

Because the shape is the underlying physics — the blocks, rows and columns are a direct picture of how electrons fill available energy states around a nucleus.

The organising principle. Elements are ordered by atomic number — the number of protons. Mendeleev arranged by atomic mass and left gaps for undiscovered elements, correctly predicting their properties; the switch to atomic number resolved the anomalies his ordering produced.

Why columns behave alike. Elements in the same group (column) have the same number of outer-shell electrons, and chemical behaviour is almost entirely about outer electrons. That is why the alkali metals are all violently reactive, and why the noble gases are all inert — a full outer shell has no incentive to react.

Why the rows have different lengths. Each period (row) corresponds to filling a new energy level, and the levels hold different numbers of electrons because they contain different types of orbitals:

s-orbitals hold 2 electrons — the two columns on the left.

p-orbitals hold 6 — the six columns on the right.

d-orbitals hold 10 — the ten columns of transition metals in the middle. They appear only from period 4 onward, which is why the first rows are short and the table suddenly widens.

f-orbitals hold 14 — the lanthanides and actinides.

Why those two rows are printed separately at the bottom. They are not a footnote — they belong in the middle of periods 6 and 7. They are extracted purely so the table fits on a page. The long-form table, which includes them in place, is the physically honest version and is extremely wide.

The trends follow from the same structure: atomic radius decreases across a period as nuclear charge pulls electrons in tighter; ionisation energy rises across and falls down; electronegativity rises toward fluorine.

Alternative arrangements exist — spiral and three-dimensional versions — and the conventional layout persists because it is legible, not because it is uniquely correct.