The configuration of electrons

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• The number of electrons in an atom of an element corresponds to the element's Atomic Number (Z) and is equal to the number of protons in the nucleus of the atom.
• The number of electrons in the positive ion (cation) of an element corresponds to the element's Atomic Number (Z) minus the charge on the ion (atom has lost electrons).
• The number of electrons in the negative ion (anion) of an element corresponds to the element's Atomic Number (Z) plus the charge on the ion (atom has gained electrons).
• Simple Electron Configuration shows the number of electrons in each energy level
• Lowest Energy levels are filled first
      (ie, first energy level is filled first followed by the second then the third etc)
  

  Energy Level	Maximum Number of Electrons
  First	2
  Second	8
  Third	18
  Fourth	32

• Filled energy levels correspond to the electron configuration of Group VIII elements
      (the Noble or Inert Gases)
• Subshell Electron Configuration shows the number of electrons in each subshell
      (or sub-level) within each energy level
  

  Group of the Periodic Table	subshell being filled	Maximum Number of Electrons
  I and II	s	2
  III, IV, V, VI, VII, VIII	p	6
  Transition Metals	d	10
  Lanthanides and Actinides	f	14

• Orbital Notation shows the number of electrons in each orbital within an energy level

Simple Electron Configuration

Maximum number of electrons in each energy level (shell):
1^st(K)=2, 2^nd(L)=8, 3^rd(M)=18, 4^th(N)=32

The helium atom has 2 electrons in the first energy level.
Its simple electron configuration is 2

The neon atom has 10 electrons,
2 electrons in the first energy level and 8 electrons in the second energy level.
Its simple electron configuration is 2,8

The argon atom has 18 electrons,
2 electrons in the first energy level, 8 electrons in the second energy level, 8 electrons in the thrid energy level.
Its simple electron configuration is 2,8,8

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