Elements that do not form positive ions are non-metals.

The idea

This statement gives a practical way to classify an element as a non-metal: if it does not form positive ions, it is a non-metal. Metals, on the left and centre of the periodic table, lose outer-shell electrons to form positive ions such as Na⁺, Mg²⁺ and Al³⁺. Non-metals, mostly on the right and top, gain or share electrons; they form negative ions such as O²⁻ and Cl⁻, or covalent bonds, and do not form positive ions. For example, oxygen gains two electrons to become O²⁻, and chlorine gains one electron to become Cl⁻. Hydrogen is a non-metal that forms H⁺ in acids, so the statement is a general rule with that familiar exception. Use electron configuration and position in the periodic table to decide whether an element tends to lose or gain electrons.

What earns the marks

• Metals lose electrons from their outer shell to form positive ions, for example Na → Na⁺ + e⁻ and Mg → Mg²⁺ + 2e⁻.
• Non-metals gain electrons to form negative ions, for example O + 2e⁻ → O²⁻ and Cl + e⁻ → Cl⁻.
• Elements that do not form positive ions are classified as non-metals, so the absence of positive-ion formation is the key test in this statement.
• Non-metals can also bond covalently by sharing electrons, so they do not need to form ions at all in many compounds.
• Position in the periodic table supports the classification: metals are mainly on the left and centre, non-metals mainly on the right and top.
• Hydrogen is a non-metal that can form H⁺ in acidic solution, so it is a recognised exception to the simple rule.

Common mistakes and how to put them right

• Assuming every non-metal forms negative ions in all compounds.

  Put it right: Many non-metals, such as carbon and hydrogen in covalent compounds, share electrons instead of forming ions.

• Treating hydrogen as a metal because it can form a positive ion.

  Put it right: Hydrogen is a non-metal, and its formation of H⁺ in acids is a specific exception to the general rule.

• Confusing the charge number with the number of electrons gained or lost.

  Put it right: O²⁻ shows a 2⁻ charge because oxygen gains two electrons, not because it gains one.

• Thinking that all elements on the right of the periodic table are non-metals without checking.

  Put it right: Use electron loss or gain and ion formation as the deciding evidence.

Exam tips

• Link each classification to electron transfer: metals lose electrons to form positive ions, non-metals gain or share electrons.
• When explaining an example, write the ion with a superscript charge, such as Mg²⁺ or O²⁻, and state the number of electrons gained or lost.
• If a question asks you to identify a non-metal, look for an element that forms negative ions or covalent bonds rather than positive ions.
• Use the periodic table position as supporting evidence, but base your final answer on ion formation or electron transfer.