The periodic table

Mendeleev built his table by atomic mass, and it worked well enough that he could leave gaps and predict the missing elements. But mass ordering has inversions — argon is heavier than potassium yet clearly belongs before it — and the modern table is ordered by atomic number, the proton count. That is not a technicality: the proton count fixes the electron count, and the electrons are what chemistry is made of.

The table's shape follows from how those electrons fill shells. Elements in the same group (column) have the same number of outer electrons, and since bonding is an outer-electron affair, they behave alike — the alkali metals all react violently with water, the noble gases all sit inert with a full outer shell. Elements in the same period (row) share only a shell number and are chemically unrelated: sodium and chlorine are neighbours in period 3 and could hardly differ more.

The trends are what make the table predictive rather than merely tidy. Across a period the nuclear charge rises while electrons enter the same shell, so the pull inward strengthens and atoms get smaller, not larger — an unintuitive result that catches many people. Ionisation energy rises with it, and metallic character falls. Down a group each element adds a whole new shell, so atoms grow, outer electrons are held more loosely, and ionisation energy falls. Those two directions together explain most of descriptive inorganic chemistry without memorising it element by element.