Trend: atomic radius

A caesium atom has about two and a half times the radius of a fluorine atom: 343 pm against 135 pm in PubChem.

An atom has no hard edge, because its electrons thin out gradually like fog. Chemists still need a size, so they define one from how close atoms come to each other. The model uses the van der Waals radius from PubChem: half the distance between two atoms that touch without bonding. Turn on "Height: Atomic radius" and the table turns into a landscape. The tallest ridge is on the far left at the bottom, and the lowest ground is in the top right.

Down a group: bigger

Going down a group, each element has one more shell of electrons than the one above. A new shell sits further from the nucleus, so the atom gets larger. In group 1 the PubChem values climb from lithium at 182 pm to sodium at 227 pm, potassium at 275 pm and caesium at 343 pm. One picometre is a trillionth of a metre, so even caesium, the largest stable atom, is only about a third of a nanometre across in radius.

Across a period: smaller

This one surprises most people. Moving left to right along a period, every atom has more protons and more electrons than the last, yet the atoms get smaller. The reason is that the new electrons all go into the same outer shell, while the nucleus gains positive charge. Electrons in the same shell are poor at shielding each other from the nucleus, so the outer electrons feel a stronger and stronger pull and are drawn closer in. In period 2 the radius falls from lithium at 182 pm to fluorine at 135 pm.

The pull that an outer electron actually feels is called the effective nuclear charge. It is the nuclear charge minus the part cancelled out by inner electrons. Across a period it rises steadily. Down a group it changes much less, because the extra protons are matched by an extra layer of shielding electrons, and the distance effect wins.

Reading the model honestly

Real data is bumpier than the textbook rule. The noble gases have van der Waals radii that are larger than their neighbours, because they are measured as touching atoms that never bond, while some other radii in the same table come from bonded crystals. The transition metals change only a little across the d block. The trend is strongest in the s and p blocks. Tiles for the heaviest synthetic elements stay grey and flat, because nobody has measured their size.

Size matters for chemistry. A big atom holds its outer electrons loosely, so large metal atoms such as caesium give them up easily. A small atom such as fluorine holds its electrons tightly and pulls on other atoms' electrons too. Radius is the first of the trends, and the next unit, on electronegativity, follows from it.

Atoms change size when they become ions. A sodium atom that loses its single outer electron also loses its whole third shell, so the Na⁺ ion is much smaller than the atom: its ionic radius is about 102 pm. A chlorine atom that gains an electron adds repulsion between electrons in the same shell, and the Cl⁻ ion swells to about 181 pm. These two numbers set the spacing in a salt crystal, which you can see in the molecule model in the bonds course.