Ion

An ion is a charged particle formed when an atom gains or loses one or more electrons; in an ion the number of electrons is not equal to the number of protons. Malay: ion · Chinese: 离子.

Because protons carry the positive charge and electrons the negative charge, an atom is neutral only while the two are balanced. Losing electrons leaves more protons than electrons, giving a positive ion (cation); gaining electrons gives more electrons than protons, giving a negative ion (anion). Metals tend to lose electrons and form positive ions, while non-metals tend to gain electrons and form negative ions.

Example. A sodium atom has the electron arrangement 2.8.1. It loses its single valence electron to become a sodium ion, Na⁺, with the arrangement 2.8. It still has 11 protons but now only 10 electrons, so it carries a charge of +1. A chlorine atom (2.8.7) instead gains one electron to become a chloride ion, Cl⁻ (2.8.8).

Confusion to avoid. Do not confuse an ion with an atom. A neutral atom has equal numbers of protons and electrons; an ion does not. The number of protons never changes when an ion forms, only the number of electrons changes, so the ion is still the same element, just charged. Note too that losing electrons makes a positive ion, which students often get the wrong way round.

In the exam you may need to work out the charge of an ion from its electron arrangement, or to write an ion symbol with the correct charge, such as Mg²⁺ or O²⁻. Write the charge at the top right of the symbol, and remember that the size of the charge equals the number of electrons transferred.

The idea of the ion introduced here becomes the heart of the Chemical Bonding chapter, where ionic bonds form as atoms transfer electrons. Our teachers link the two ideas from the start so the later chapter feels like a continuation rather than something new.