Recall the relative masses and relative electric charges of protons,…

The idea

Subatomic particles are compared using relative mass and relative charge, with the proton and neutron each given relative mass 1. The proton carries relative charge +1, the neutron 0, the electron −1 and the positron +1. The electron and positron are far lighter than a nucleon: their relative mass is about 1/1836, so it is often treated as negligible (approximately 0). A useful check is that the positron is the electron's antiparticle, so it matches the electron in mass but has the opposite charge sign. Learn the four particles as a table of two properties, then test yourself by covering one column and reciting the other.

What earns the marks

• States the proton has relative mass 1 and relative charge +1.
• States the neutron has relative mass 1 and relative charge 0.
• States the electron has relative charge −1 and a relative mass of about 1/1836, treated as negligible.
• States the positron has relative charge +1 and the same negligible relative mass as the electron.

Common mistakes and how to put them right

• Gives the neutron a charge of −1.

  Put it right: Correct it to 0 because the neutron is neutral.

• Gives the positron a negative charge.

  Put it right: Correct it to +1 because the positron is the positive antiparticle of the electron.

• Treats the electron as having relative mass 1.

  Put it right: Correct it to about 1/1836, negligible beside a nucleon.

Exam tips

• Learn the four particles as paired rows of mass and charge so neither property is forgotten.
• Use the antiparticle link: the positron mirrors the electron's mass but reverses its charge sign.
• Check the sign of every charge before selecting an option, since +1 and −1 are easily swapped.