When an Atom Gains or Loses Electrons It Becomes an Ion: A Guide to Ions and Charge

When an atom gains or loses electrons, it becomes an ion, a particle with an electric charge. A neutral atom has the same number of protons and electrons, so its positive and negative charges balance.

If an atom loses electrons, it has more protons than electrons and becomes a positively charged ion, called a cation. If it gains electrons, it has more electrons than protons and becomes a negatively charged ion, called an anion. The number of protons does not change in an ordinary chemical reaction, so the atom remains the same element.

Ions can be single atoms or groups of bonded atoms. In either case, the defining feature is a net electric charge Encyclopaedia Britannica.

One example: sodium and chlorine

A neutral sodium atom has 11 protons and 11 electrons. If it loses one electron, it has 11 protons and 10 electrons. Its net charge is +1, so it is written as Na⁺.

A neutral chlorine atom has 17 protons and 17 electrons. If it gains one electron, it has 17 protons and 18 electrons. Its net charge is −1, so it is written as Cl⁻.

The electron lost by sodium can be gained by chlorine. The resulting sodium and chloride ions have opposite charges, so they attract. In sodium chloride, their charges balance in a one-to-one ratio. The nuclei remain unchanged throughout this electron transfer.

To work out an ion’s charge, subtract the number of electrons from the number of protons:

Net charge = number of protons − number of electrons

A positive result means the ion is a cation; a negative result means it is an anion. This proton-and-electron comparison is the basis of ion notation Khan Academy.

Why atoms form ions

Atoms can gain or lose electrons during chemical reactions. Which outcome is more likely depends on the element and its chemical surroundings. Metals often lose electrons and form cations, while nonmetals often gain electrons and form anions. These are common tendencies, not rules that predict every compound.

For many main-group elements, an element’s position in the periodic table can help predict the charge of a common ion. Group 1 metals often form +1 ions, and Group 2 metals often form +2 ions. Halogens in Group 17 often form −1 ions. Some elements can form ions with different charges. Iron, for example, can form iron(II) and iron(III) ions, with charges of +2 and +3.

The tendency to form a particular ion is related to an atom’s electron arrangement and the conditions of the reaction. It is more accurate to treat simple group patterns as useful guides than to assume that atoms always gain or lose electrons to reach a particular arrangement. Electron transfer and electron sharing are both ways atoms can form chemical bonds Chemistry LibreTexts.

What an ion’s charge tells you

An ion’s charge describes the difference between its number of protons and electrons. It also helps explain how ions interact. Ions with opposite charges attract; ions with the same charge repel. In an ionic compound, positive and negative charges balance overall. For example, one Na⁺ ion and one Cl⁻ ion have a combined charge of zero.

When an ionic compound dissolves in water, its ions may separate and move through the solution. Those mobile charged particles can carry electric current. Whether a compound dissolves, and how well its solution conducts electricity, depends on the substance and conditions.

Ions also occur in living systems. Sodium, potassium, calcium, and chloride ions take part in processes that involve movement of charged particles across cell membranes. Their roles vary by ion and by biological context; the general point is that their charges affect how they interact with other particles.

Key Takeaways

• An atom becomes an ion when it gains or loses electrons.
• Losing electrons produces a positive ion, or cation. Gaining electrons produces a negative ion, or anion.
• In ordinary chemical reactions, the number of protons stays the same, so the element’s identity does not change.
• Calculate an ion’s charge by subtracting its number of electrons from its number of protons.
• Oppositely charged ions attract, and ionic compounds have charges that balance overall.