A form of notation which shows how the electrons in an atom are distributed among the various atomic orbitals and energy levels. The format consists of a series of numbers, letters and superscripts.

For example, the electron configuration for helium is 1s2. The large number "1" refers to the principle quantum number "n" which stands for the energy level. It tells us that the electrons of helium occupy the first energy level of the atom. The letter "s" stands for the angular momentum quantum number "l". It tells us that the two electrons of the helium atom occupy a spherical orbital. The exponent "2" refers to the total number of electrons in that orbital or subshell. In this case, we know that there are two electrons in the spherical orbital at the first energy level.

The electron configuration of an element can be abbreviated by using the preceding noble gas. For example, the electron configuration of magnesium is 1s22s22p63s2, but it can be abbreviated as [Ne]3s2, as the electron configuration of neon is 1s22s22p6.

To find an atom's electron configuration you must use the diagonal rule.

Atoms have different general electron configurations based on their placement in the periodic table.

                    1 1 1 1 1 1 1 1 1
  1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
 +-+                               +-+
1| |                               | |
 +-+-+                   +-+-+-+-+-+-+
2| | |                   | | | | | | |
 +-+-+                   +-+-+-+-+-+-+
3| | |                   | | | | | | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4| | | | | | | | | | | | | | | | | | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5| | | | | | | | | | | | | | | | | | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6| | @ | | | | | | | | | | | | | | | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7| | @ | | | | | | | | | | | | | | | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       1 1 1 1 1
     1 2 3 4 5 6 7 8 9 0 1 2 3 4
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  @:| | | | | | | | | | | | | | |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  @:| | | | | | | | | | | | | | |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

n will refer to the period of the element.

The following list will be ordered by group. It refers to the last sublevel that electrons enter:

  1. ns1
  2. ns2
  3. n-1d1
  4. n-1d2
  5. n-1d3
  6. n-1d4
  7. n-1d5
  8. n-1d6
  9. n-1d7
  10. n-1d8
  11. n-1d9
  12. n-1d10
  13. np1
  14. np2
  15. np3
  16. np4
  17. np5
  18. np6

The next list is for the Lanthanide and Actinide series:

  1. n-2f1
  2. n-2f2
  3. n-2f3
  4. n-2f4
  5. n-2f5
  6. n-2f6
  7. n-2f7
  8. n-2f8
  9. n-2f9
  10. n-2f10
  11. n-2f11
  12. n-2f12
  13. n-2f13
  14. n-2f14

The outer electron configuration is the s'es and p's of the highest principal quantum number. What the **** does that mean? Well, it means the s'es and p's of the highest number in front of the s and p.

Thus, the electron configuration for Mg is 1s2 2s2 2p6 3s2. The outer electron configuration is therefore 3s2.

For sulfur, S, atomic number 32, the electron configuration is 1s2 2s2 2p6 3s2 3p4. The outer electron configuration is 3s2 3p4.

Exception: elements in groups 6 and 11 rearrange themselves to become more chemically stable because of the octet rule.

When creating chemical bonds, electrons can move around, and electron configurations can change.

Indicates the number of electrons in each subshell. It contains a number, which stands for the principal energy level, followed by a letter, which is the subshell, followed by a superscript numeral which is the number of electrons found in that subshell.

Example: 1s2 2s2 2p6 3s2 3p6 is the electronic configuration of Argon. The sum of all the superscripts is 18, which equals the number of electrons which Argon has, which is equal to it's atomic number.

The principal energy levels are numbered starting at 1, and increases by one for each level. The sublevels are named in this order s, p, d, f, g, h, i, etc. (all in alphabetical order after f, but it's a moot point considering h, i, etc. don't even exist, neither naturally nor artificially (yet)).

The number of subshells each principal energy level has (also known as 'shell') is equal to the number which names it. The 1st shell has one subshell (s). 2nd shell has two subshells (s,p). 3rd shell has 3 subshells (s, p, d), etc.

Each subshell can hold a limited number of electrons. The nth subshell can hold 4n - 2 number of electrons. Therefore the 's' subshell (1st) holds two (4*1 - 2), the 'p' subshell (2nd) holds 6 (4*2 - 2), d = 10 electrons, f = 14 electrons, etc.

When one reaches the higher levels the order of the shells and subshells can get confusing. A shell can start filling before the last one finished.

The order of the subshells can be found by creating the following chart.

1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 6h
7s 7p 7d 7f 7g 7h 7i

In order to find the order you must imagine, or draw, arrows coming from the top right to the bottom left going through the last sublevel on each row, and the sublevel below it (on the chart). The order which is found when consulting the chart is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, etc.

Atoms are divided into levels of energy. Energy levels are the regions around the nucleus where electrons are likely to be moving. Electrons can move between one level to another, but cannot be in between levels. To move from one level to another, the electron must gain or lose the right amount of energy.

A Quantum is the amount of energy required to move an electron to the next highest energy level. Energy levels grow closer as they move further away from the nucleus. The number of electrons vary from element to element, obviously. For example, Oxygen (O) has 8 electrons in it. This is determined by subtracting the number of protons(8) from the Atomic mass(16.00).

This is where electron configuration comes in. The levels contain orbitals, which can hold a certain number of electrons. There are four different orbitals:

S Orbitals
P Orbitals
D Orbitals
F Orbitals

The lowest energy level has one sublevel - the S, and the second has two, and so on. S orbitals can only hold 2 electrons, P can hold 6, D can hold 10, and F can hold 14.

Aufbau Diagram:
5s 5p 5d 5f
4s 4p 4d 4f
3s 3p 3d
2s 2p
1s

So, if Chlorine (Cl) has 17 electrons, it's electron configuration is
1s12s22p63s23p5
As you see, the last orbital isn't filled all the way, and that's ok. The Aufbau principle states that elctrons enter the orbitals of lowest energy first. Also, the Pauli exlusion principle states that electrons in the same orbital must have opposite spins.

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