Electron Configuration

Electron configuration is the arrangement of electrons in successive sublevels and orbitals.   To determine the electron configuration of the elements electron distribution mnemonics is used.

In the electron distribution mnemonics the sublevels are already arranged according to increasing energy as shown below: 

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f  6d 7p

Now in writing electronic configuration we have to follow the Pauli's Exclusion Principles which states that, no two electrons can have four quantum numbers, and that orbitals can only accommodate 2 electrons with opposite spins.    Since, we only have 1 s orbital in s sublevel, only two electrons can occupy in s sublevel, and we have 3 p orbitals in p sublevel, and therefore total of 6 electrons can be accommodated in p sublevel.  In d sublevel there are 5 d orbitals, accommodating 2 electrons in each orbital there are 10 total electrons that can be accommodated in d sublevel, and in f sublevel there are 7 f orbitals and having  2 electrons each orbital, total of 14 electrons are in sublevel f.  An so we have to take note in writing electrons configuration the following:  s can have only 1 or two electrons, p can have only 1 to 6 electrons, d can have 1 to 10 electrons, and f sublevels can have 1 to 14 electrons.  

We use atomic number in writing the electron configuration, since atomic number is the number of protons at the same time the number of electrons.  Let us have an example:

What is the electron configuration of first 10 elements? 

₁H has only 1 electron and so the the electron is only located at the lowest sublevel 1s.  The electron configuration is :  1s¹

₂He has 2 electrons. The electron configuration is 1s²

₃Li :  1s² 2s¹

₄Be:  1s² 2s²

₅B : 1s² 2s² 2p¹

₆C : 1s² 2s² 2p²

₇N : 1s² 2s² 2p³

₈O : 1s² 2s² 2p⁴

₉F :  1s² 2s² 2p⁵

₁₀Ne : 1s² 2s² 2p⁶

We have to take note that in writing electron configuration, electron distribution mnemonics must be followed.