Isotopes (Oxford AQA IGCSE Combined Science Double Award)

Isotopes

• Although the number of protons in a particular element is always the same, the number of neutrons can be different

• Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons

• Isotopes of the same element therefore have the same atomic number but different mass numbers

• In the diagram below are three isotopes of hydrogen:

Isotopes of hydrogen

• Isotopes occur naturally, but some are more rare than others

• For example, about 2 in every 10,000 hydrogen atoms are deuterium

  • Tritium is even more rare (about 1 in every billion billion hydrogen atoms)

• The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass

  • This is because neutrons have no charge but do have mass

• In the periodic table, the mass number of chlorine is often given as 35.5

A section of the periodic table

• The mass number of chlorine is given as 35.5 because it has 2 isotopes, one with a mass number of 35 and the other with a mass number of 37

• Chlorine-35 is about three times more abundant than chlorine-37, so the given mass number of chlorine is closer to 35 than 37 because the mass number is a weighted average

• The number of electrons and protons in different isotopes remains the same

• Some isotopes are unstable as they have an imbalance of protons and neutrons