Explain how atoms may form positive ions by losing outer electrons

The idea

An atom is neutral because its nucleus holds positive protons matched by an equal number of negative electrons in shells around it. The outer electrons are the least tightly bound, so an atom can lose one or more of them and become a positive ion. Losing one electron leaves one more proton than electrons, giving a charge of 1⁺; losing two gives 2⁺. For example, a sodium atom with 11 protons and 11 electrons that loses its single outer electron becomes an Na⁺ ion with 11 protons and 10 electrons. The nucleus is unchanged, so the mass number stays the same while the proton number stays the same and only the electron count falls. Ionisation is therefore a change in electron number, not a change in the nucleus.

What earns the marks

• States that a neutral atom has equal numbers of protons and electrons.
• Identifies the outer electrons as the electrons most easily lost.
• Explains that losing electrons leaves more protons than electrons, giving a net positive charge.
• Applies the rule that losing one electron gives a 1⁺ charge and losing two gives a 2⁺ charge.
• Confirms that the nucleus and proton number are unchanged when an atom becomes a positive ion.

Common mistakes and how to put them right

• Claims that protons are lost to form a positive ion.

  Put it right by stating that electrons are lost and protons stay in the nucleus.

• Says the atom becomes positive because it gains protons.

  Put it right by explaining that the electron count falls while the proton count is fixed.

• Treats ionisation as a change in mass number.

  Put it right by noting that only electrons are removed so the nucleus is unchanged.

Exam tips

• Count protons and electrons separately before deciding the charge on the ion.
• Link the charge to the difference between proton number and electron number.
• Check that any explanation names electrons, not protons, as the particles lost.