Three things set the size of an atom: how strongly the nucleus pulls, how many shells the electrons occupy, and how much the inner electrons shield the outer ones. Change one at a time and every trend on the Periodic Table follows.
Across a period the radius decreases: each step adds one proton and one electron to the same shell, shielding barely changes, so the outer electrons are pulled in harder. Down a group the radius increases: each period adds a whole new shell, further out and more shielded. A cation is smaller than its atom (the outer shell is lost), an anion is larger (extra repulsion), and in an isoelectronic series the ion with the most protons is the smallest.
Fig. 1: atomic radius across Period 3 (typical values)
Section 01
Across a period.
Step through Period 3 in the simulator and watch the atom shrink from sodium to chlorine. Then step down Group 2 and watch it grow. Before each step, predict whether the radius rises or falls and say which of the three factors is doing the work.
Nuclear charge rises by one for each element across the period.
Same shell: the new electron goes into the same outer shell (n = 3 for Period 3), so it is no further from the nucleus.
Shielding stays roughly constant: the inner shells (n = 1 and 2) are unchanged, and electrons in the same shell shield each other poorly.
Result: the outer electrons feel a stronger attraction and are pulled closer, so the radius decreases.
Section 02
Down a group.
Each period down a group starts a new principal quantum shell. That single change outweighs the extra protons.
More shells: the outer electron is in a shell further from the nucleus.
More shielding: every extra complete inner shell sits between the nucleus and the outer electron and cancels part of the nuclear pull.
Nuclear charge rises too, but the extra distance and shielding win, so the radius increases down the group.
Stage 1 · Learn
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Why does atomic radius decrease from sodium to chlorine across Period 3?
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Which atom has the largest atomic radius?
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Compared with a sodium atom, a Na+ ion is:
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Compared with a chlorine atom, a Cl− ion is:
Section 03
Ions and the isoelectronic series.
Forming an ion changes the number of electrons but never the number of protons, so the same nucleus is pulling on a different-sized cloud.
Cation smaller than its atom. Na (1s22s22p63s1) loses its whole outer shell to become Na+ (1s22s22p6); the radius falls from about 0.186 nm to 0.095 nm. The 11 protons now pull on only 10 electrons, so those are held more tightly too.
Anion larger than its atom. Cl gains an electron into the same outer shell. The nuclear charge is unchanged but there is more electron-electron repulsion, so the cloud expands: about 0.099 nm to 0.181 nm.
Isoelectronic ions have the same number of electrons, so only the nuclear charge differs. N3− > O2− > F− > Na+ > Mg2+ > Al3+: all have 10 electrons, and more protons means a smaller ion.
Examiner trap
Ionic radius across Period 3 is not a smooth decrease. It falls from Na+ to Mg2+ to Al3+ (all 10 electrons, n = 2 outer shell), then jumps up at P3−, S2− and Cl−, which have 18 electrons and an n = 3 outer shell, and then falls again along that second isoelectronic set.
Stage 2 · Exam
Exam-style questions
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The ions N3−, O2−, F−, Na+, Mg2+ and Al3+ all have 10 electrons. Which is the smallest?
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Which statement best explains why a sulfur atom is smaller than a magnesium atom?
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Which ion has a larger radius than the atom from which it is formed?
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Across Period 3, between which pair of consecutive ions is the change in ionic radius largest?
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