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Atoms Molecules Ions Chemical Equations On-line practice


  1. Atoms always have as many electrons as protons.
  2. Atoms usually have as many neutrons as protons.
  3. The number of protons in an atom is fixed.  Adding a proton makes a new kind of atom.  Adding a neutron makes an isotope of that atom ( a heavier version of that atom ).
  4. Therefore, isotopes are atoms of the same element with different number of neutrons.  They have the same number of protons (ie. proton number) but different atomic mass

Example :

Isotopes of Hydrogen

Hydrogen has three isotopes, with atomic mass 1, 2 and 3.  They all contain the same number of protons and electrons but differ in the number of neutrons.


Isotopes of Carbon

  1. Isotopes of the same element have the same chemical properties and form compounds with the same formula.
  2. There are some differences in physical properties eg. boiling point.
  3. There are two types of isotopes : radioactive and non-radioactive.