Moving left to right along a period, atoms shrink even though electrons are being added — lithium is 167 picometres across and fluorine, six electrons later, is 42.
Each step to the right adds a proton to the nucleus but puts the new electron in the shell already in use. The extra positive charge pulls the whole shell in harder, and that outweighs the added electron.
It only holds across a period. Go down a group and a new shell is added, so atoms grow. It also understates what happens after the f-block: the fifteen lanthanides shield the nuclear pull so poorly that the contraction across them leaves hafnium the same size as zirconium one period above, despite a whole extra shell — which is why the two were not told apart until 1923.
Estimate with the rule, then check it against the calculator that models it properly.
Open Periodic Table →Moving left to right along a period, atoms shrink even though electrons are being added — lithium is 167 picometres across and fluorine, six electrons later, is 42. Each step to the right adds a proton to the nucleus but puts the new electron in the shell already in use.