In an atom the number of electrons is equal to the number of protons in…

The idea

Every proton carries a relative charge of +1 and every electron a relative charge of -1. In an atom the two counts are identical, so the positive and negative charges cancel exactly and the atom has no overall charge. Neutrons play no part in this balance, because they are uncharged. A carbon atom has six protons and therefore six electrons, total charge zero. Take one electron away and the balance is broken: there are now more protons than electrons and the particle is a positive ion. Add an electron and it becomes a negative ion. So an object only becomes charged when electrons move, never because protons have moved or been created.

What earns the marks

• Stating that the number of electrons equals the number of protons.
• Stating that the charges on a proton and an electron are equal and opposite.
• Concluding that the charges cancel so the atom has no overall charge.
• Stating that neutrons are uncharged and so do not affect the balance.

Common mistakes and how to put them right

• Matching the number of electrons to the mass number instead of the atomic number.

• Saying neutrons balance the charge of the protons.

• Stating that an atom is neutral because it contains neutrons.

• Offering an ion as an example of a neutral atom.

Exam tips

• Say equal and opposite charges, then say they cancel; both steps are usually needed for two marks.
• Check whether the question says atom or ion before you equate electrons and protons.
• Neutrons deserve a mention only to say they carry no charge.