Students should be able to: determine the number of fundamental particles…

The idea

Counting particles is a three-step routine. Protons equal the atomic number. Neutrons equal mass number minus atomic number. Electrons equal protons minus the charge, so a 2⁺ ion has lost two electrons and a 1⁻ ion has gained one; subtracting the signed charge handles both cases in one step. Isotopes exist because the number of neutrons in a nucleus can vary while the proton count stays fixed, so the element is unchanged but the mass number differs. Because isotopes have the same number of electrons and therefore the same electron configuration, they react in exactly the same way; the mark scheme credits same electron configuration and ignores answers based only on the proton count.

What earns the marks

• Number of protons taken directly from the atomic number.
• Number of neutrons calculated as mass number minus atomic number.
• The electron count adjusted correctly for the charge on the ion.
• Isotopes defined as atoms with the same number of protons but different numbers of neutrons.
• The same electron configuration explaining identical chemical properties.

Common mistakes and how to put them right

• Adding electrons for a positive ion instead of removing them.

• Explaining identical chemistry by saying isotopes have the same number of protons, which examiners ignore.

• Saying isotopes have different chemical properties because their masses differ.

• Forgetting the charge entirely and giving an ion the same electron count as the atom.

Exam tips

• Write protons, neutrons and electrons as three labelled lines; a table earns the process marks even if one value slips.
• Answer the isotope chemistry question with the words same electron configuration, then add same number of electrons as support.
• Never write same number of neutrons in an isotope answer; that statement is explicitly refused.