An ion is an atom, or a group of atoms, that has lost or gained electrons and so carries a charge. Metal atoms lose outer electrons to form positive ions, and non-metal atoms gain electrons to form negative ions. Examiners ask you to explain this using electron arrangements, not just to name the charge.
This skill opens bonding and structure and sits underneath ionic formulas, melting points and conductivity later in the topic. It builds on atoms, elements and isotopes, where you learn that the number of protons equals the number of electrons in a neutral atom.
What is really happening when an ion forms?
A neutral atom has the same number of protons and electrons, so the charges cancel. If the atom loses electrons, protons outnumber electrons and the particle is positive. If it gains electrons, electrons outnumber protons and the particle is negative.
The electrons that move are in the outer shell. Atoms of the elements in the main groups reach a full outer shell when they lose or gain the right number. That full shell is why ions such as Na⁺ and Cl⁻ are stable compared with the atoms they came from.
How to explain ion formation, step by step
- Write the electron arrangement of the atom from its proton number.
- Decide the move. Metals lose the outer electrons. Non-metals gain enough electrons to reach a full outer shell.
- Write the new electron arrangement after the move.
- Count the charge: charge = number of protons − number of electrons.
- Write the ion with its charge, such as Mg²⁺, and say which electrons were transferred.
Worked example
Explain how magnesium and oxygen form ions, and give the formula of magnesium oxide.
Magnesium: proton number 12, so the electron arrangement is 2,8,2. It loses its 2 outer electrons and becomes 2,8. It now has 12 protons and 10 electrons, so the charge is 12 − 10 = 2+. The ion is Mg²⁺.
Oxygen: proton number 8, so the electron arrangement is 2,6. It gains 2 electrons and becomes 2,8. It now has 8 protons and 10 electrons, so the charge is 8 − 10 = 2−. The ion is O²⁻.
Formula: the 2 electrons lost by one Mg atom are gained by one O atom, so the charges balance in a 1:1 ratio. The formula is MgO.
A full answer in words: “Each magnesium atom transfers two outer electrons to an oxygen atom. Mg²⁺ and O²⁻ are formed, and both have a full outer shell.”
The mistake to watch for
A slip that appears in practice answers is to say that protons move.
Mistaken answer: “Sodium loses a proton to become Na⁺.”
This reverses the real idea. Losing a proton would change the element itself.
The correction is that sodium loses one electron. The 11 protons stay in the nucleus, and with 10 electrons left the charge is 11 − 10 = 1+. Whenever you write an explanation, name the particle that moves, and check that you wrote electron.
Check yourself
Try these without looking back, then open each answer.
1. Aluminium has proton number 13. Give its electron arrangement and explain how Al³⁺ forms.
Show answer
Aluminium atom: 2,8,3. It loses its 3 outer electrons and becomes 2,8. It has 13 protons and 10 electrons, so the charge is 13 − 10 = 3+.
Al³⁺ (2,8)
2. Fluorine has proton number 9. What ion does it form, and what is its electron arrangement?
Show answer
Fluorine atom: 2,7. It gains 1 electron and becomes 2,8. It has 9 protons and 10 electrons, so the charge is 9 − 10 = 1−.
F⁻ (2,8)
3. Calcium (2,8,8,2) reacts with chlorine (2,8,7). Use the electron transfers to give the formula of calcium chloride.
Show answer
Calcium loses 2 electrons to form Ca²⁺. Each chlorine atom gains 1 electron to form Cl⁻. One Ca²⁺ needs two Cl⁻ to balance the charge, since 2+ and 2 × (1−) cancel.
CaCl₂
Where this leads next
Once ions feel secure, move to representing a simple covalent bond, where atoms share electrons instead of transferring them. Then test the whole topic in the bonding and structure practice set. When you later balance equations that contain ionic formulas, the equation balance reasoning trainer gives quick atom-count feedback.
Some students can recite the rules but lose marks because the electron movement is not stated clearly. That is a pattern our teachers look for in online one-to-one Chemistry tuition.