Recall that in an atom the number of protons equals the number of…

The idea

A neutral atom contains equal numbers of protons and electrons. Each proton carries relative charge +1 and each electron carries relative charge −1, so equal counts give a total charge of zero and the atom is electrically neutral. For example, a carbon atom has 6 protons and therefore 6 electrons, giving 6 × (+1) + 6 × (−1) = 0. The neutron contributes no charge and does not affect neutrality. If the counts differ, the particle is an ion: more electrons gives a negative ion, fewer electrons gives a positive ion. Use the proton count to find the electron count in a neutral atom, then check the charges cancel.

What earns the marks

• States that a neutral atom has equal numbers of protons and electrons.
• Explains that each proton's +1 charge cancels one electron's −1 charge, giving zero total charge.
• Applies the rule to an example, such as 6 protons and 6 electrons giving a net charge of zero.
• Recognises that an unequal count produces an ion rather than a neutral atom.

Common mistakes and how to put them right

• Assumes the neutron count must equal the proton count for neutrality.

  Put it right: Correct it because neutrons carry no charge and do not affect it.

• Claims an atom is neutral because it has no charged particles.

  Put it right: Correct it because protons and electrons are charged but their charges cancel.

• Treats an atom with more electrons than protons as neutral.

  Put it right: Correct it to a negative ion.

Exam tips

• Count protons first, then set electrons equal to that number for a neutral atom.
• Add the charges explicitly to show they sum to zero rather than asserting neutrality.
• Remember neutrons are neutral and play no part in the charge balance.