Atoms become larger as we go down a column of the
periodic table. We can examine trends in ionic radii across a row of the
periodic table by comparing data for atoms and ions that are isoelectronic
atoms or ions that have the
same number of electrons. The table below summarizes data on the radii of a
series of isoelectronic ions and atoms of second- and third-row elements.
Radii for Isoelectronic Second-Row and Third-Row Atoms or Ions
| Atom or Ion | Radius (nm) | Electron Configuration | ||
| C4- | 0.260 | 1s2 2s2 2p6 | ||
| N3- | 0.171 | 1s2 2s2 2p6 | ||
| O2- | 0.140 | 1s2 2s2 2p6 | ||
| F- | 0.136 | 1s2 2s2 2p6 | ||
| Ne | 0.112 | 1s2 2s2 2p6 | ||
| Na+ | 0.095 | 1s2 2s2 2p6 | ||
| Mg2+ | 0.065 | 1s2 2s2 2p6 | ||
| Al3+ | 0.050 | 1s2 2s2 2p6 |
The data in this table are easy to explain if we note that these atoms or ions all have 10 electrons but the number of protons in the nucleus increases from 6 in the C4- ion to 13 in the Al3+ ion. As the charge on the nucleus becomes larger, the nucleus can hold a constant number of electrons more tightly. As a result, the atoms or ions become significantly smaller.
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