Isotopes (AQA GCSE Physics: Combined Science)

• 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

Step 1: Determine the number of protons

• • The atomic number is the number of protons
  • Both Chlorine-35 and Chlorine-37 have 17 protons

Step 2: Determine the number of neutrons

• • The mass number is the number of protons and neutrons
  • Chlorine-35 neutrons: 35 - 17 = 18 neutrons
  • Chlorine-37 neutrons: 37 - 17 = 20 neutrons

Step 3: Determine the number of electrons

• • The number of electrons is equal to the number of protons
  • Both chlorine-35 and chlorine-37 have 17 electrons