GCSE Chemistry revision

Size and mass of atoms

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

AQA 4.1.1.5 Foundation & Higher

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Revision summary

Read on screen, then print for Cornell-style active revision.

An ion is a charged particle that can be a single atom, such as Na⁺, or a group of atoms, such as the hydroxide ion (OH ).
Ions are formed when atoms gain or lose electrons in order to achieve a full outer shell and become more stable.

The group number in the periodic table tells you how many electrons an atom has in its outermost shell — for example, Group 1 elements have one outer electron.
Atoms need to lose or gain electrons until their outermost shell is full to reach a stable electronic configuration.
Losing or gaining electrons requires energy, so atoms that only need to move one or two electrons are much more likely to form ions.

2. The Periodic Table and Outer Shell Electrons AQA

The group number in the periodic table tells you how many electrons an atom has in its outermost shell — for example, Group 1 elements have one outer electron.
Atoms need to lose or gain electrons until their outermost shell is full to reach a stable electronic configuration.
Losing or gaining electrons requires energy, so atoms that only need to move one or two electrons are much more likely to form ions.

Group 1 elements (alkali metals) lose one electron to form 1+ ions, as this requires very little energy.
Group 2 elements lose two electrons to form 2+ ions.
Group 6 elements gain two electrons to form 2− ions, and Group 7 elements gain one electron to form 1− ions.
Elements in Groups 3, 4, and 5 would need to lose or gain three or four electrons, which requires too much energy, so they rarely form simple ions.

3. Which Groups Commonly Form Ions? AQA

Group 1 elements (alkali metals) lose one electron to form 1+ ions, as this requires very little energy.
Group 2 elements lose two electrons to form 2+ ions.
Group 6 elements gain two electrons to form 2− ions, and Group 7 elements gain one electron to form 1− ions.
Elements in Groups 3, 4, and 5 would need to lose or gain three or four electrons, which requires too much energy, so they rarely form simple ions.

When an atom loses electrons to form a positive ion, the electron(s) are written on the right-hand side of the equation, e.g. Na → Na⁺ + e⁻.
When an atom gains electrons to form a negative ion, the electron(s) are written on the lefthand side, e.g. Cl + e⁻ → Cl⁻.
For atoms that lose or gain multiple electrons, the number of electrons is shown accordingly, e.g. Mg → Mg²⁺ + 2e⁻ for magnesium and O + 2e⁻ → O²⁻ for oxygen.

4. Writing Equations for Ion Formation AQA

When an atom loses electrons to form a positive ion, the electron(s) are written on the right-hand side of the equation, e.g. Na → Na⁺ + e⁻.
When an atom gains electrons to form a negative ion, the electron(s) are written on the lefthand side, e.g. Cl + e⁻ → Cl⁻.
For atoms that lose or gain multiple electrons, the number of electrons is shown accordingly, e.g. Mg → Mg²⁺ + 2e⁻ for magnesium and O + 2e⁻ → O²⁻ for oxygen.