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The electron configuration of an atom is the order and arrangement of the electrons in
the electron cloud. Where an electron is placed in the electron cloud is determined by the quantity of energy
in that electron. This quanta of energy is contained in little packets called photons. Thus, the
gaining and losing of photons determines the order, arrangement and movement of electrons in the electron
Learning how electrons are arranged in an atom helps us to understand chemical bonding, formation of
compounds and how chemical reactions take place.
Electron Arrangement Rules
The rules for electron configurations are for stable atoms; which means there is no
charge. Also, remember these are general rules and there are exceptions.
1. The number of electrons equals the number of protons.
2. Electrons are distributed one at a time, starting with energy level one, then energy level two,
etc. until all electrons are distributed. (Aufbau Principle)
3. The number of subshells in an energy level is equal to the number of the energy level.
4. The s subshell has only one possible position, the p subshell has three, the d subshell has five
and the f subshell has seven. Each possible position is an orbital.
5. Only two (2) electrons can occupy each orbital.
6. As a sublevel is being filled, each orbital must get an electron before there is any pairing.
7. The maximum number of electrons is two (2) in any s subshell, six (6) in any p subshell, ten (10)
in any d subshell, and fourteen (14) in any f subshell.
8. The orbital number is in the form: nxy
n = the energy level
x = the sublevel
y = the number of electrons in that subshell
Now let's look at how electron configurations and orbital notations of the elements are represented using our
knowledge of electron arrangements.
Electron Configuration and Orbital Notation
Using the above rules, you can easily diagram the distribution of the electrons in an atom. Just determine how
- electrons the atom has
- energy levels to be used
- sub levels to be used
- the orbitals there will be in each sub energy level
Let's look at carbon as an example.
According to the first rule, carbon has six electrons and according to the second rule there are two energy
Using rule three, you can determine that:
- the first energy level will have only one sub level and only one orbital (s).
- The second energy level will have two sub levels and four orbitals ( one s and three p).
Now, using this information distribute carbon's six electrons:
- two electrons go into the first energy level's only orbital (s orbital)
- then two go into the second energy level's first orbital (s orbital)
- then one in the second energy level's first p orbital
- finally one in the second energy level's second p orbital
carbon = 1s2 2s2 2p1 2p
If you truly want to master electron configurations, you must practice, practice and then practice some
Electron Arrangement (PDF)