Electrons are tiny, negatively charged particles that spin around the nucleus of atoms.
The mass of an electron is reckoned to be 1/2000th of the mass of a hydrogen atom (the lightest atom).
They move in pathways called orbitals or shells. The number of electrons in an atom depends on the atom: For example, hydrogen has one electron, helium has 2 and copper has 29.
The more electrons there are, the more orbitals or shells are required to hold them.
The first orbital can hold 2 electrons, then it is full.
The second orbital can hold 8 electrons, then it is full.

In order to calcualte how many electrons an orbital or shell can hold, use the formula: 2n2, where n = the number of the orbital. For example:-

  • For shell 1, substitute the 1 for n, giving us 2x(1)2 = 2 x 1 = 2.
  • For shell 2, substitute the 2 for n, giving us 2x(2)2 =2 x 4 = 8.
  • For shell 3, substitute the 3 for n, giving us 2x(3)2 =2 x 9 = 18.

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