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Isotopes (Edexcel IGCSE Physics: Double Science)
Revision Note
Isotopes
- For a particular element, the number of protons is always the same, but the number of neutrons can be different
- This is because the number of protons determines the element e.g. carbon atoms have 6 protons and iron atoms have 26 protons
- An isotope is defined as:
An atom, or atoms, of the same element that have an equal number of protons but a different number of neutrons
- Each element can have more than one isotope
Isotopes of hydrogen
- Some isotopes are more unstable than others due to the imbalance of protons and neutrons, which means
- They may be more likely to decay
- They may be less likely to occur naturally
- For example, about 2 in every 10 000 atoms of hydrogen are the isotope deuterium
- The isotope tritium is even rarer (about 1 in every billion billion atoms of hydrogen)
Worked example
State the number of protons, neutrons and electrons in these two isotopes of chlorine:
, 
Answer:
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
- Number of neutrons in chlorine-35 = 35 − 17 = 18
- Number of neutrons in chlorine-37 = 37 − 17 = 20
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
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