Isotopes (OCR GCSE Physics A (Gateway)) : Revision Note

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 an equal number of protons but a different number of neutrons

• In the diagram below are three isotopes of Hydrogen:

Hydrogen has three isotopes, each with a different number of neutrons

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

• For example, about 2 in every 10,000 Hydrogen atoms is 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

This section of a periodic table shows Chlorine as having a mass number of 35.5, but other elements have an integer mass number

• 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

• 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