Periodic Trends Lab

The periodic table is not a poster to memorise — it is a map. Every trend in it follows from two opposing effects, and once you hold both in your head, the whole table becomes predictable.

The two competing effects

Going across a period (left → right): you add a proton to the nucleus and an electron to the same shell. Shell number stays put, nuclear charge rises, so the pull on electrons tightens. Atoms get smaller and ionisation energy climbs.

Going down a group (top → bottom): you add a whole new electron shell each time. The outer electrons sit further out and are screened by the inner ones. Atoms get bigger and ionisation energy falls.

The three properties

• Atomic radius — halves the table: largest at bottom-left (caesium, francium), smallest at top-right (helium, fluorine).
• Ionisation energy — the energy to remove the outermost electron. Peaks at the noble gases (a full shell is very stable), troughs at the alkali metals (one loose electron that is eager to go).
• Electronegativity — how strongly an atom pulls shared electrons. Fluorine is the champion at 3.98. Noble gases have no value at all: they do not share.

Try this

• Map radius, then click Group 1 (alkali metals). The values climb steadily down the group.
• Map ionisation energy and click Period 3. It rises left to right — but not smoothly: there is a dip at aluminium and another at sulphur, caused by subshell details.
• Map electronegativity and click the halogens. Fluorine at the top is the most electronegative element there is.

Why it matters

These trends predict which elements form ions, whether a bond is ionic or covalent, how reactive a metal is, and which combinations make useful semiconductors, catalysts and battery materials. The table is arranged the way it is precisely because these properties repeat — that periodicity is the whole point.