Isotopes (Edexcel IGCSE Physics)

• 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)

Differences Between Isotopes

• 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

• The charge of the nucleus of a particular element is always the same
• 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 roughly equal numbers of isotopes with a mass number of 35, and of 36
• The number of electrons and protons in different isotopes remains the same
• Isotopes tend to be more unstable due to the 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-36 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-36 neutrons: 36 - 17 = 19 neutrons

Step 3: Determine the number of electrons

• • The number of electrons is equal to the number of protons
  • Both Chlorine-35 and Chlorine-36 have 17 electrons