Periodic trends

A trend is a steady change in a property as you move across a row or down a column of the periodic table. Trends are the reason the table is so useful: they let you predict how an element behaves.

Why trends exist

Two things compete. Going across a period, each atom has one more proton, so the nucleus pulls harder on electrons that are all in the same shell. Going down a group, each atom has one more shell, so the outer electrons are further from the nucleus and are shielded by the inner ones.

Atomic size

Atoms get smaller across a period, because the stronger pull draws the electrons in. They get larger down a group, because shells are added. The largest atoms are at the bottom left of the table and the smallest at the top right.

Ionisation energy

This is the energy needed to remove one electron from an atom. It rises across a period and falls down a group. The lowest values in our data: caesium (3.89), francium (3.9), rubidium (4.18), potassium (4.34), sodium (5.14) electronvolts. The highest: helium (24.59), neon (21.57), fluorine (17.42), argon (15.76), nitrogen (14.53). That is why caesium gives up an electron so easily and helium does not react at all.

Electronegativity

This measures how strongly an atom pulls shared electrons in a bond. It rises across a period and falls down a group. The highest values: fluorine (3.98), oxygen (3.44), chlorine (3.16), nitrogen (3.04), krypton (3). The lowest: francium (0.7), caesium (0.79), potassium (0.82), rubidium (0.82), barium (0.89). Noble gases are usually given no value, because they rarely form bonds.

Metallic character

Metals lose electrons easily. So metallic character is strongest where ionisation energy is lowest: it falls across a period and rises down a group. Group 14 shows this well. Carbon at the top is a nonmetal, silicon and germanium are metalloids, and tin and lead at the bottom are metals.

Melting points

Melting points do not follow one simple rule, but there is a pattern: they are highest in the middle of the transition metals (tungsten melts at 3,422 °C) and for carbon, and lowest for the noble gases. See the records page.

A quick summary

There are small exceptions to every trend. Check them for yourself by sorting the list.