Atoms, compounds and isotopes

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Atoms, compounds


  • Atomic number and mass number describe an atom
  • The atomic number is the number of protons
  • The mass number is the total number of protons and neutrons
  • Atoms of the same element all have the same number of protons
  • So atoms of different elements will have different numbers of protons
  • To get the neutron number, just subtract the atomic number from the mass number
  • Electrons aren't counted in the mass number because their relative mass is very small


  • Compounds are formed when atoms of two or more elements are chemically combined
  • For example, CO2 is a compund formed from a chemical reaction between carbon and oxygen
  • Carbon + oxygen = carbon dioxide
  • It's difficult to separate the two original elements out again
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