Recall that the nucleus of each element has a characteristic positive…

The idea

Protons carry positive charge and neutrons carry none, so a nucleus is always positively charged. Because the proton number is fixed for a given element, that positive charge is characteristic of the element: every carbon nucleus has six protons and the same nuclear charge, whatever its mass. Isotopes of that element keep the same proton number and charge but contain different numbers of neutrons, so their masses differ. For example, carbon-12 has six protons and six neutrons while carbon-13 has six protons and seven neutrons; both have the same positive charge but carbon-13 is heavier. In a multiple-choice question, separate the two ideas: charge depends on protons, mass depends on protons plus neutrons.

What earns the marks

• States that the nucleus carries a positive charge because of its protons, while neutrons contribute no charge.
• Explains that the proton number is fixed for each element, so its nuclear charge is characteristic of that element.
• States that isotopes of an element have the same number of protons and so the same nuclear charge.
• Explains that isotopes differ in mass because they have different numbers of neutrons.
• Applies the idea to a named isotope pair, keeping proton number constant while neutron number changes.

Common mistakes and how to put them right

• Claims the nucleus is neutral or negative.

  Put it right: State that protons make the nucleus positively charged.

• Says isotopes differ in charge.

  Put it right: Keep the charge the same because the proton number is unchanged.

• Attributes the mass difference to electrons.

  Put it right: Explain that the different neutron numbers change the mass.

• Assumes all atoms of an element have equal mass.

  Put it right: Allow for isotopes with different neutron numbers.

Exam tips

• Link charge to protons only and mass to protons plus neutrons before reading the options.
• Reject any option that gives isotopes of one element different charges.
• Use a familiar pair such as carbon-12 and carbon-13 to test each option quickly.