Knowledge of isotopes made it possible to explain why the order based on…

The idea

Early periodic tables arranged elements by relative atomic weight, but a few pairs appeared to be in the wrong order for their chemistry. Isotopes explain this: atoms of the same element can have different mass numbers because they have different numbers of neutrons, yet they have identical proton number and identical chemical properties. The weighted mean relative atomic mass of an element depends on the abundances of its isotopes, so it need not follow a simple whole-number sequence. For example, argon has a higher relative atomic mass than potassium, yet argon is a noble gas placed before potassium because proton number, not mass, fixes position. Knowing isotopes exist shows that ordering by atomic weight can misplace an element, whereas ordering by proton number removes the anomaly.

What earns the marks

• State that isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• Explain that isotopes of an element have the same proton number and therefore the same chemical properties.
• Explain that relative atomic mass is a weighted mean that depends on the abundance of each isotope.
• Use a named example, such as argon before potassium, to show that atomic weight order can conflict with chemical behaviour.
• Conclude that ordering by proton number, rather than atomic weight, gives the correct sequence and removes anomalies.

Common mistakes and how to put them right

• Saying isotopes have different numbers of protons.

  Put it right by stating that proton number is the same and only neutron number differs.

• Claiming that relative atomic mass is always a whole number.

  Put it right by explaining it is a weighted mean of isotope masses.

• Thinking the isotope explanation changes chemical properties.

  Put it right by stating that chemical behaviour depends on electron arrangement, which is unchanged.

Exam tips

• Link the word isotope directly to proton number and neutron number before discussing the periodic table.
• Use one named pair, such as argon and potassium, to show the anomaly rather than describing it in the abstract.
• Finish by stating that proton number gives the modern order, so the explanation answers the whole statement.