An atom is surrounded by a cloud of electrons. This cloud is organised into separate probability distributions each of which can contain up to two electrons. These probability distributions are referred to as electron orbitals (harping back to the time before particle/wave duality and quantum physics when the electrons were thought to orbit the nucleus like the moon around the earth).

The orbitals are grouped into sets by their symmetry there are 4 known sets of orbitals which are called s, p, d, and f. The number of orbitals increases for each type so that while there is only 1 s orbital per shell there are 7 f orbitals. Each orbital holds 2 electrons.

The letters come from name given to the appearance of the lines from the orbitals in spectra.

There are shells of orbitals that increase in energy at the higher shells. Shell 1 only contains an s orbital and each shell adds the next orbital type so shell 2 has s and p orbitals etc. The energies of the shells overlap so that for instance the 4s shell has a lower energy than the 3d shell. The actual filling is shown below.

  1s				 2 electrons	  2 total 
  2s			2p	 8 electrons	 10 total 
  3s			3p	 8 electrons	 18 total 
  4s		3d	4p	18 electrons	 36 total 
  5s		4d	5p	18 electrons	 54 total 
  6s	4f	5d	6p	32 electrons	 86 total 
  7s	5f	6d	7p	32 electrons	118 total
This filling order while generally correct does have slight irregularities due to the closeness (energetically) of certain orbital groups during partial filling.

It is the reactivity and properties of the outer electrons that determine the general chemistry of an element. Thus the ordering of the shells and orbitals explains the order of the elements and the size of the various rows in the periodic table.

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