Protons, neutrons and electrons: relative charge and relative mass.

The idea

Relative charge and relative mass are comparisons, not measurements in coulombs or kilograms. On the relative scale the proton has charge +1 and mass 1, the neutron has charge 0 and mass 1, and the electron has charge −1 and mass 1/1840, usually quoted as 0.0005. That tiny electron mass is why almost all the mass of an atom sits in the nucleus while almost all its volume is electron cloud. The equal and opposite charges on the proton and electron are why a neutral atom must contain equal numbers of each, and why removing one electron leaves a 1⁺ ion. Marks are given for the relative values in a table, so learn them as a set rather than trying to recall absolute masses such as 1.67 × 10⁻²⁷ kg.

What earns the marks

• The relative charges +1 for the proton, 0 for the neutron and −1 for the electron.
• The relative masses 1 for both the proton and the neutron.
• The electron mass given as 1/1840 or 0.0005 on the same scale.
• Stating that the electron mass is negligible so the nuclear particles account for the mass of the atom.

Common mistakes and how to put them right

• Quoting absolute masses in kilograms when the question asks for relative mass.

• Writing the electron's relative mass as zero rather than 1/1840 or 0.0005.

• Giving the neutron a small charge because the name sounds like it should be slightly negative.

• Leaving the sign off the electron charge so it reads 1 rather than −1.

Exam tips

• Draw the three-by-three table (particle, relative charge, relative mass) from memory before you start the question.
• If the question gives you masses in kilograms, divide by the proton mass to get back to the relative scale.
• Use the electron's negligible mass explicitly when explaining why mass number counts only protons and neutrons.