Ionic Radius
Atoms rarely exist as isolated, neutral spheres. In chemistry and biology, you encounter ions constantly - sodium and potassium flowing through nerve channels, calcium triggering muscle contraction, chloride balancing fluid osmolarity. Understanding how gaining or losing electrons changes an atom’s size is essential for predicting ionic compound properties, lattice energies, and solubility patterns on the MCAT.
Cations Are Smaller Than Their Parent Atoms
When a neutral atom loses one or more electrons to form a cation, it gets smaller. Two things happen simultaneously. First, losing electrons reduces electron-electron repulsion in the outer shell, allowing the remaining electrons to be pulled closer to the nucleus. Second, the nuclear charge stays the same - you still have the same number of protons pulling on fewer electrons. The effective nuclear charge per electron increases, and the electron cloud contracts.
In many cases, the atom loses its entire outermost shell. Sodium, for example, goes from [Ne]3s¹ to [Ne] when it becomes Na⁺. The valence shell is gone entirely, and the ion now has the compact electron configuration of neon. Na has an atomic radius of 186 pm, but Na⁺ shrinks to just 95 pm - nearly half the size.
Anions Are Larger Than Their Parent Atoms
When a neutral atom gains one or more electrons to form an anion, it gets larger. The added electrons increase electron-electron repulsion in the outer shell, but the nuclear charge has not changed. The same number of protons now must hold onto more electrons, and they cannot do it as tightly. The electron cloud puffs outward.
Periodic trends, and the single cause behind them
Scroll sideways to see the whole map.
The one cause underneathAcross a period, protons are added but the new electrons go into the same shell, so they shield each other poorly and effective nuclear charge climbs. Down a group, a whole new shell is added and the outer electrons are both further out and screened by everything beneath. Every trend on this page is one of those two sentences.
The two ionisation-energy dipsBe to B falls because boron's outermost electron is in a higher-energy 2p rather than 2s. N to O falls because oxygen is the first period-2 element forced to put two electrons in one 2p orbital, and their mutual repulsion makes one easier to remove. Both dips are on the plot, not smoothed away.
Ionic radiusCations are always smaller than their parent atom, often dramatically so when a whole shell is lost: Na is 186 pm, Na⁺ is 102 pm. Anions are always larger, because added electrons increase repulsion without adding any protons.
Chlorine, for example, goes from [Ne]3s²3p⁵ to [Ne]3s²3p⁶ (the neon core plus a full third shell) when it becomes Cl⁻. The atomic radius of Cl is 99 pm, but Cl⁻ swells to 181 pm.
Summary of Size Changes
| Species | Electrons | Radius (pm) | Change |
|---------|-----------|-------------|--------|
| Na | 11 | 186 | - |
| Na⁺ | 10 | 95 | Shrinks (lost 1 e⁻) |
| Cl | 17 | 99 | - |
| Cl⁻ | 18 | 181 | Expands (gained 1 e⁻) |
| Mg | 12 | 160 | - |
| Mg²⁺ | 10 | 65 | Shrinks dramatically (lost 2 e⁻) |
| O | 8 | 60 | - |
| O²⁻ | 10 | 140 | Expands dramatically (gained 2 e⁻) |
Notice the pattern: the more electrons removed, the greater the shrinkage. The more electrons added, the greater the expansion.
Isoelectronic Series
An isoelectronic series is a set of atoms and ions that all have the same number of electrons. Because they share the same electron count, the only variable that differs is the number of protons in the nucleus. This makes isoelectronic series the cleanest possible test of how nuclear charge affects size.
Consider the following species, all of which have 10 electrons:
| Species | Protons | Electrons | Ionic Radius (pm) |
|---------|---------|-----------|-------------------|
| O²⁻ | 8 | 10 | 140 |
| F⁻ | 9 | 10 | 136 |
| Ne | 10 | 10 | ~38 (van der Waals) |
| Na⁺ | 11 | 10 | 95 |
| Mg²⁺ | 12 | 10 | 65 |
The Isoelectronic Rule
Within an isoelectronic series, the species with more protons is smaller. This is because more protons means a greater nuclear charge pulling on the same number of electrons, compressing the electron cloud more tightly.
To rank an isoelectronic series from largest to smallest, simply rank by increasing number of protons:
O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺
The species with the fewest protons (O²⁻, with 8) is the largest. The species with the most protons (Mg²⁺, with 12) is the smallest. Every additional proton tightens the grip on those 10 shared electrons.
How to Approach MCAT Ionic Radius Questions
The MCAT tests ionic radius in two main ways:
Type 1: Compare an atom to its own ion. Apply the simple rule - cations shrink, anions expand. Example: “Is Fe²⁺ larger or smaller than Fe?” Smaller, because it lost electrons.
Type 2: Rank ions in an isoelectronic series. First, confirm all species have the same electron count. Then rank by number of protons - more protons means smaller radius. Example: “Rank N³⁻, O²⁻, F⁻, Ne, Na⁺ from largest to smallest.” All have 10 electrons. Order by increasing protons (7, 8, 9, 10, 11): N³⁻ > O²⁻ > F⁻ > Ne > Na⁺.
Ionic Radius and the Periodic Table
Ionic radii follow the same general periodic trends as atomic radii within the same type of ion. Among cations, radius increases going down a group (more shells) and decreases going across a period (higher charge, often fewer shells). The same applies to anions.
However, be careful when comparing cations to anions. A cation from the left side of a period is often much smaller than an anion from the right side of the same period, even if the cation has a higher atomic number. For example, Na⁺ (95 pm) is far smaller than Cl⁻ (181 pm), even though sodium has a higher atomic number than chlorine. The loss versus gain of electrons creates a stark size difference.
A cation is smaller than its parent atom (fewer electrons, same nuclear charge, reduced repulsion). An anion is larger than its parent atom (more electrons, same nuclear charge, increased repulsion). This is one of the most commonly tested ionic radius concepts on the MCAT.
O²⁻ > F⁻ > Ne > Na⁺ > Mg²⁺. All five species have 10 electrons. In an isoelectronic series, the ion with the fewest protons is the largest (weakest nuclear pull on the shared electrons). O²⁻ has 8 protons (largest), while Mg²⁺ has 12 protons (smallest).
Mg²⁺ has 12 protons pulling on 10 electrons, while O²⁻ has only 8 protons pulling on 10 electrons. The higher nuclear charge in Mg²⁺ compresses the electron cloud much more tightly. In any isoelectronic series, more protons always means a smaller radius.