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Radii of Atoms Compared to Radii of Ions

Atoms consist of a positively charged nucleus, made up of protons and neutrons, orbited by a cloud of negatively charged electrons. Normally, the total charge of an atom is zero because the number of electrons is equal to the number of protons. In certain cases, such as when the atoms in a molecule separate while in solution, an atom can gain or lose electrons relative to this neutral configuration. When this happens, the resulting charged atom is called an ion.

Electrons, Anions and Cations
- When an atom loses one or more electrons, the total charge becomes positive because there are now more protons than electrons. A positively charged ion is called a cation. When an atom gains electrons, the total charge becomes negative because there are more electrons than protons. A negatively charged ion is called an anion.
Atomic Radius Versus Ionic Radius
- An atom's radius, meaning the distance from the nucleus to the furthest orbiting electrons, changes as electrons are gained or lost. Cations, which are positively charged due to the loss of electrons, have a smaller radius than the neutral atom. Anions, which are negatively charged due to the gain of electrons, have a larger radius than the neutral atom.
Charge
- The overall charge of an ion depends on how many electrons are gained or lost relative to the neutral configuration. If an atom loses two electrons, then it has a total charge of +2. If it gains three electrons, then it has a total charge of -3, and so on. The greater the magnitude of the charge, the greater the difference between the radius of the neutral atom and the radius of the ion.
Why the Radius Changes
- Electrons orbit the positively charged nucleus because opposite charges attract, so the negative electrons are attracted to the positive nucleus. Electrons also interact with electrons, effectively shielding each other from the influence of the positive nucleus. If electrons are taken away to form a cation, this shielding effect is reduced and the remaining electrons are more strongly attracted to the nucleus, so they move closer and reduce the radius. As electrons are added, the shielding effect is magnified, the electrons are less attracted to the nucleus, and the radius increases.

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