Recall that atoms of the same elements can differ in nuclear mass by…

The idea

Atoms of one element always share the same proton number, but they need not share the same neutron number. Variants of an element with different neutron counts are isotopes, and because neutrons carry mass, isotopes differ in nuclear mass while keeping the same nuclear charge and chemical identity. For example, chlorine-35 has 17 protons and 18 neutrons, while chlorine-37 has 17 protons and 20 neutrons; both are chlorine, but the second is heavier. A reliable method is to subtract the proton number from the mass number to find the neutron count, then compare two atoms of the same element. In a multiple-choice question, accept an option that keeps the proton number fixed and varies the neutron number, and reject options that change the element or move electrons.

What earns the marks

• States that atoms of the same element can have different numbers of neutrons.
• Links a difference in neutron number to a difference in nuclear mass.
• Keeps the proton number the same when comparing two atoms of one element.
• Rejects options that change the element's identity or alter electron numbers to explain the mass difference.

Common mistakes and how to put them right

• Assumes atoms of one element must have identical mass.

  Put it right: Allow different neutron counts and therefore different nuclear masses.

• Changes the proton number to explain a mass difference.

  Put it right: Keep protons fixed and vary neutrons instead.

• Confuses neutron number with electron number.

  Put it right: Derive neutrons by subtracting the proton number from the mass number.

Exam tips

• Use mass number minus proton number to find neutrons before judging each option.
• Check that the proton number stays the same in any pair described as the same element.
• Rule out options that mention electrons, since the statement concerns nuclear mass.