Nonmetals form ions that are larger than their neutral atoms. Any element that gains electrons to become a negative ion (an anion) will expand in size. This includes the halogens (fluorine, chlorine, bromine, iodine), the chalcogens (oxygen, sulfur, selenium), and the nitrogen group elements (nitrogen, phosphorus). The size increase can be dramatic: fluorine’s neutral atom has a radius of about 42 pm, but its ion (F⁻) balloons to 133 pm, more than three times larger.
Why Gaining Electrons Makes an Ion Bigger
When a neutral atom picks up one or more extra electrons, two things happen at once. First, there’s more electron-electron repulsion. Electrons are all negatively charged, so they push each other apart, and adding more of them into the same set of orbitals forces the electron cloud to spread out. Second, the effective nuclear charge drops. The number of protons in the nucleus hasn’t changed, but now each proton has to “share” its pull across more electrons. The result is a weaker grip on the outer electrons, allowing them to drift farther from the nucleus.
For neutral halogens and chalcogens, the anion is typically about 0.8 Å (80 pm) larger than the parent atom. The more electrons an atom gains, the bigger the jump. Nitrogen gains three electrons to form N³⁻, which has a radius of 146 pm compared to nitrogen’s atomic radius of about 65 pm.
Why Metals Do the Opposite
Metals lose electrons to form positive ions (cations), and this has the reverse effect. Removing electrons reduces electron-electron repulsion and increases the effective nuclear charge felt by the remaining electrons, pulling them in tighter. Sodium, for instance, shrinks from a relatively large neutral atom down to Na⁺ with a radius of just 98 pm. Magnesium loses two electrons and compresses even further to 79 pm as Mg²⁺. The more electrons removed, the smaller the cation gets.
Specific Examples With Numbers
The contrast between neutral atoms and their ions is easiest to see with concrete values:
- Fluorine: neutral atom ~42 pm, F⁻ ion 133 pm
- Oxygen: neutral atom 0.48 Å (48 pm), O²⁻ ion 1.40 Å (140 pm)
- Sulfur: neutral atom 0.88 Å (88 pm), S²⁻ ion 1.84 Å (184 pm)
- Nitrogen: neutral atom ~65 pm, N³⁻ ion 146 pm
In every case, the ion is roughly two to three times the size of the neutral atom. The pattern holds across all nonmetals that form anions. Elements that gain more electrons (like oxygen gaining two, or nitrogen gaining three) tend to show an even larger size increase than elements that gain just one.
The Isoelectronic Series Explains It Clearly
One of the clearest ways to see this principle is to compare ions that all have the same number of electrons but different numbers of protons. Nitrogen (N³⁻), oxygen (O²⁻), fluorine (F⁻), sodium (Na⁺), magnesium (Mg²⁺), and aluminum (Al³⁺) all contain exactly 10 electrons. Their sizes line up in a smooth trend based purely on how many protons are in the nucleus:
- N³⁻: 146 pm (7 protons pulling on 10 electrons)
- O²⁻: 140 pm (8 protons)
- F⁻: 133 pm (9 protons)
- Na⁺: 98 pm (11 protons)
- Mg²⁺: 79 pm (12 protons)
- Al³⁺: 57 pm (13 protons)
With the same electron count, every additional proton pulls the electron cloud inward. N³⁻ has the fewest protons and is the largest. Al³⁺ has the most and is the smallest. This is why anions are always bigger than their parent atoms (fewer protons per electron) and cations are always smaller (more protons per electron).
Periodic Table Trends for Ionic Size
Among anions specifically, size increases as you move down a group on the periodic table. Chloride (Cl⁻) is larger than fluoride (F⁻) because chlorine’s electrons occupy a higher energy level, placing them farther from the nucleus. The same pattern holds for sulfide being larger than oxide, and phosphide being larger than nitride.
Moving across a period from left to right, the trend for nonmetal anions is a decrease in ionic radius. This happens because the nuclear charge increases with each step across the row, pulling the electron cloud tighter even though each atom is gaining electrons to fill its outer shell. So within the same row, an ion like O²⁻ (140 pm) is slightly larger than F⁻ (133 pm), partly because oxygen gains two electrons while fluorine gains only one.
The short answer for any chemistry assignment: every nonmetal that gains electrons to form an anion produces an ion larger than its neutral atom. The classic textbook example is fluorine, but oxygen, sulfur, nitrogen, chlorine, and the rest of the nonmetals all follow the same rule.