Size and mass of atoms: Students should be able to calculate the numbers of protons, neutrons and electrons in an atom or ion, given its atomic number and mass number.

The idea

The atomic number equals the number of protons and identifies the element. The mass number equals protons plus neutrons, so neutrons are found by subtracting the atomic number from the mass number. In a neutral atom, electrons equal protons. In an ion, the charge tells you the electron change: a positive charge means electrons have been lost, so electrons equal protons minus the charge; a negative charge means electrons have been gained, so electrons equal protons plus the charge.

For example: ²⁴₁₂Mg²⁺ has 12 protons, 12 neutrons and 10 electrons, while ³²₁₆S²⁻ has 16 protons, 16 neutrons and 18 electrons.

What earns the marks

• States that the atomic number equals the number of protons.
• Calculates neutrons by subtracting the atomic number from the mass number.
• States that electrons equal protons in a neutral atom.
• Adjusts the electron count for an ion by subtracting the positive charge or adding the magnitude of the negative charge.
• Applies the method correctly to a supplied atom or ion, for example ²⁴₁₂Mg²⁺ giving 12 protons, 12 neutrons and 10 electrons.

Common mistakes and how to put them right

• Adding the charge to the electron count for a positive ion.

  Put it right by subtracting the charge because cations have lost electrons.

• Using the mass number as the proton count.

  Put it right by using the atomic number for protons.

• Forgetting to adjust electrons for an ion and giving the neutral-atom value.

  Put it right by checking the sign and magnitude of the charge before finalising the electron number.

Exam tips

• Write the three quantities in a consistent order, for example protons, neutrons, electrons, to avoid mixing them up.
• Show the neutron calculation explicitly as mass number minus atomic number.
• For ions, state whether electrons are gained or lost before giving the final number.