The Periodic Table

The periodic table is organized by atomic number, but the layout itself carries information. Elements in the same column, called a group, all have the same number of valence (outer-shell) electrons, and since chemical behavior is driven by those outer electrons, elements in a group react in similar ways. Lithium, sodium and potassium all sit in the first column and all react with water, because each has a single, loosely held outer electron ready to be lost.

Move across a row (a period) instead of down a column, and the number of electron shells stays fixed while the number of protons in the nucleus keeps climbing. More protons means a stronger pull on the same shell of electrons, so the atom gets pulled in tighter and the atomic radius shrinks step by step. That shrinking resets the moment a new row starts: the first element of a new period has to open a brand-new outer shell, farther from the nucleus and partly shielded by the filled shells underneath it, so the radius jumps back up before the shrinking trend starts over.

Down a column, the story is different: every element adds a full new shell, so the outer electron sits farther from the nucleus and is shielded by more layers of inner electrons. A farther, more shielded electron is held more loosely, so it is easier to pull away. That is why reactivity among metals like lithium, sodium and potassium increases going down the column, even though all three share the same outer-electron count and the same basic reaction with water.