The electron arrangement (order in which orbitals fill) of atoms is illustrated in how the elements are laded out in the periodic table.
The rows in the periodic table represent the successive energy levels that are filled by the electrons. The periodic table can be broken into blocks or areas, based on the last orbital to be filled by the atoms. If we count over from the right of each block we know how many electrons are in this last orbital. The order in which electrons fill is also illustrated as we move from left to right in any row of the table. The table below illustrates these blocks.
Periodic Table
Row
s orbitals
      
d orbitals
      
p orbital
      
1
1s
                              
1s
2
2s
                    
2p
3
3s
                    
3p
4
4s
3d
4p
5
5s
4d
5p
6
6s
4f
5d
6p
7
7s
5f
6d
7p

Row
          
f orbitals
          
6
Lanthanide series (4f)
7
Actinide series (5f)
For example, electron configuration of Magnesium
If we examine a periodic table and locate Magnesium (atomic number 12), we find it in the third row or energy level. Therefore the 1s orbital requiring 2 electrons,2s orbital requiring 2 electrons and the 2p orbital requiring 6 electrons are all filled. These require a total of 10 electrons (these are termed the core electrons). The last two electrons are being placed in the 3s orbital. Moving from left to right in the rows the order was illustrated by the table above 1s ---> 2s---> 2p----> 3s.
periodic table
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Assignment # 7
Periodic Table and Electron Configuration