Atoms bond by either transferring electrons from one atom to another or by…

The idea

Atoms bond to gain a full outer shell and become more stable. In ionic bonding, a metal atom transfers one or more electrons to a non-metal atom, forming positive and negative ions that attract electrostatically. For example, sodium transfers its single outer electron to chlorine, giving Na⁺ and Cl⁻, which pack into a giant ionic lattice. In covalent bonding, two non-metal atoms share one or more pairs of electrons, so both count the shared pair towards a full outer shell. For example, each hydrogen in H₂ shares one electron pair, and oxygen in O₂ shares two pairs. The type of bonding depends on the atoms involved: metal plus non-metal usually transfers electrons, while non-metal plus non-metal usually shares them.

What earns the marks

• Ionic bonding involves transfer of one or more electrons from a metal atom to a non-metal atom, producing oppositely charged ions.
• The ions formed are held together by strong electrostatic attraction in a giant ionic lattice, not by shared electrons.
• Covalent bonding involves sharing one or more pairs of electrons between non-metal atoms, so each atom counts the shared pair towards a full outer shell.
• The number of electrons transferred or shared relates to the group of the atom and the need to achieve a full outer shell.
• Dot-and-cross diagrams can represent both processes: transferred electrons shown on separate ions, shared pairs shown in the overlap region.
• The physical properties of the resulting substance, such as melting point or conductivity, can be linked back to whether ions or molecules are formed.

Common mistakes and how to put them right

• Saying that ionic bonding is a sharing of electrons.

  Put it right by stating that ionic bonding is transfer of electrons, while sharing is covalent bonding.

• Writing ion charges without superscript signs, for example writing Na⁺ as Na with a postfix plus.

  Put it right by using Unicode superscript signs such as Na⁺ and O²⁻.

• Thinking that atoms share electrons to fill only one atom's outer shell.

  Put it right by explaining that both atoms count the shared pair towards a full outer shell.

• Describing covalent bonds as a transfer of electrons between non-metals.

  Put it right by stating that non-metal atoms share electron pairs.

Exam tips

• When asked to explain bonding, name the process first (transfer or sharing) and then link it to the atoms involved.
• Use dot-and-cross diagrams to show transferred electrons on separate ions and shared pairs in the overlap, and label the charges clearly.
• Check that any ion you write uses superscript charge notation, for example Mg²⁺ and O²⁻, before moving on.
• If a question asks why a substance conducts when molten, link your answer to charged particles being free to move, not to shared electrons.