Atomic Size: Bigger Down a Group, Smaller Across a Period

0:00Atoms of different elements have different sizes. Is there a pattern in the periodic table?

0:07An atom's size is its atomic radius: the distance from the nucleus to the outer edge of the atom.

0:14Here, each element is drawn as a circle sized to its atomic radius.

0:19A column of the table is a group, and a row is a period.

0:25Going down group 1, from hydrogen to potassium, the atoms get bigger.

0:31Why? Look at lithium. Its electrons are arranged in shells, layers around the nucleus.

0:38Each step down adds a new shell: lithium has two, sodium has three, and potassium has four.

0:47So the outer electrons are farther from the nucleus, and the atom is bigger.

0:53Now go across period 3, from left to right. The atoms get smaller.

1:00Every atom in period 3 has three shells. Compare sodium with chlorine.

1:07But each step right adds a proton, a positive particle in the nucleus. Sodium has 11 protons, chlorine has 17.

1:17More protons pull harder on the electrons, so the shells are drawn in and the atom is smaller.

1:24So more electrons do not always mean a bigger atom. Chlorine has 17 electrons, sodium has only 11, yet chlorine is smaller.

1:36The extra electrons join the same outer shell, so no new layer is added, and the stronger pull wins.

1:44In general, atoms get bigger down a group as shells are added, and smaller across a period as protons are added.