Nonmetals form negatively charged ions called anions. They do this by gaining electrons rather than losing them, which gives them more electrons than protons and produces a net negative charge. The number of electrons a nonmetal gains follows a predictable pattern based on its position on the periodic table.
Why Nonmetals Gain Electrons
Atoms are most stable when they have eight electrons in their outermost energy level, a principle known as the octet rule. Nonmetals sit on the right side of the periodic table, meaning they already have five, six, or seven valence electrons. Rather than losing that many electrons to reach an empty shell, it’s far easier for them to gain just one, two, or three electrons to complete a set of eight. This full outer shell matches the electron arrangement of the noble gases (helium, neon, argon), which are the most chemically stable elements.
When a nonmetal atom picks up extra electrons, it doesn’t gain any new protons to balance them out. The result is a particle with a negative charge. For example, a chlorine atom has 17 protons and 17 electrons, making it electrically neutral. When it gains one electron, it becomes a chloride ion with 17 protons and 18 electrons, giving it a 1− charge.
Common Charges by Group
The charge a nonmetal ion carries depends on how many electrons it needs to complete its octet. This lines up neatly with the element’s group (column) on the periodic table:
- Group 17 (halogens): fluorine, chlorine, bromine, and iodine each gain 1 electron, forming ions with a 1− charge (F⁻, Cl⁻, Br⁻, I⁻).
- Group 16: oxygen and sulfur each gain 2 electrons, forming ions with a 2− charge (O²⁻, S²⁻).
- Group 15: nitrogen and phosphorus each gain 3 electrons, forming ions with a 3− charge (N³⁻, P³⁻).
The pattern is straightforward: the closer a nonmetal is to the noble gases on the right edge of the table, the fewer electrons it needs to gain and the smaller its negative charge.
How Anions Are Named
When a nonmetal becomes a single-atom (monoatomic) anion, its name changes. You drop the ending of the element’s name and replace it with the suffix “-ide.” Chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide, nitrogen becomes nitride. This naming convention tells you immediately that you’re dealing with a negatively charged ion rather than a neutral atom.
Anions Are Larger Than Their Parent Atoms
Gaining electrons changes the physical size of the atom. The added electrons increase repulsion among all the electrons in the cloud, while the number of protons pulling them inward stays the same. This causes the electron cloud to expand. Anions are always larger than the neutral atoms they came from. Fluorine, for instance, has an atomic radius of about 42 picometers, but the fluoride ion balloons to roughly 133 picometers, more than three times larger.
Polyatomic Anions
Not all anions are single atoms. Groups of nonmetal atoms can bond together covalently and then gain or lose electrons as a unit, creating what are called polyatomic ions. Most polyatomic ions are anions, and they show up constantly in chemistry. Some common examples:
- Nitrate (NO₃⁻): one nitrogen and three oxygens with a 1− charge.
- Sulfate (SO₄²⁻): one sulfur and four oxygens with a 2− charge.
- Phosphate (PO₄³⁻): one phosphorus and four oxygens with a 3− charge.
- Carbonate (CO₃²⁻): one carbon and three oxygens with a 2− charge.
- Hydroxide (OH⁻): one oxygen and one hydrogen with a 1− charge.
These polyatomic anions are the building blocks of many salts, minerals, and biological molecules. Phosphate, for example, works with calcium to build bones and teeth. Bicarbonate (HCO₃⁻) helps regulate your blood’s pH. Chloride ions help control fluid balance, blood volume, and blood pressure.
Nonmetal Ions vs. Metal Ions
The key distinction is direction. Metals, on the left side of the periodic table, have only one, two, or three valence electrons. It’s easier for them to lose those electrons, forming positively charged ions called cations. Nonmetals do the opposite, gaining electrons to form anions. When metals and nonmetals react together, the metal donates its valence electrons to the nonmetal, so both end up with full outer shells. This electron transfer is what creates ionic compounds like sodium chloride (table salt), where sodium gives one electron to chlorine, producing Na⁺ and Cl⁻.