An Ion With a Negative Charge: Formed by Gaining Electrons
An ion with a negative charge is called an anion. It forms when an atom or molecule gains one or more electrons. Since electrons carry negative charge, the particle then has more electrons than protons and a net negative charge.
How an anion forms
In a neutral atom, the number of protons equals the number of electrons. Protons are positively charged and electrons are negatively charged, so their charges balance. When an atom gains electrons, its proton count stays the same while its electron count increases. The atom becomes a negatively charged ion.
The Encyclopaedia Britannica defines an ion as an atom or group of atoms with an electric charge. The IUPAC Gold Book defines an anion as a monatomic or polyatomic ion with a net negative charge. In plain terms, “negative ion” and “anion” mean the same thing.
Example: chloride ion
A neutral chlorine atom has 17 protons and 17 electrons. After it gains one electron, it has 17 protons and 18 electrons. This extra negative charge gives it a net charge of −1, written as Cl⁻. It remains chlorine because its number of protons has not changed.
Why atoms gain electrons
Atoms can gain or lose electrons as they form ions. For many elements, this change produces a more stable electron arrangement. Nonmetals often gain electrons, while metals often lose them. This is a general pattern, not a rule for every atom in every situation. The outcome depends on the element and its chemical environment.
An electron transfer also has a source: when one atom gains an electron, another particle commonly loses it. In sodium chloride, for example, sodium forms Na⁺ as chlorine forms Cl⁻. Their charges balance in the compound, while each ion retains its own charge.
Charge and formulas
An anion can have a charge of −1 or a greater negative charge, depending on how many electrons it gains. The ion’s charge matters when writing a compound’s formula because the positive and negative charges must balance overall.
For example, chloride has a −1 charge, while magnesium commonly forms Mg²⁺. A neutral magnesium chloride formula therefore contains two chloride ions for each magnesium ion: MgCl₂. The subscript gives the number of chloride ions needed to balance the magnesium ion’s charge.
Anions can also consist of groups of atoms. The sulfate ion, SO₄²⁻, is a polyatomic anion with an overall charge of −2. In formulas and reactions, it is treated as a charged unit.
Anions and cations
An anion has a negative charge; a cation has a positive charge. The difference comes from the balance between protons and electrons:
| Change in electron count | Result |
|---|---|
| Gains one or more electrons | Anion, net negative charge |
| Loses one or more electrons | Cation, net positive charge |
| No change from a neutral atom | Neutral atom |
To check an ion’s charge, compare its number of protons with its number of electrons. More electrons than protons means the ion is negative. More protons than electrons means it is positive.
Key Takeaways
- An anion is an ion with a net negative charge.
- It forms when an atom or molecule gains one or more electrons.
- The number of protons stays the same during ordinary chemical reactions; the electron count changes.
- Chlorine gains one electron to form Cl⁻.
- An ion’s charge helps determine the proportions of ions in a neutral compound.
