Please help me with this question:

Dear Student,
1) ​The atomic number of elements increases by a factor of 1 as we move from left to right across a period. We know that elements in a period involve the filling of same valence shell. So in a period, the electronic configuration mainly involve changes in the valence shell. For example, consider carbon, nitrogen and oxygen. All these elements belong to the 2nd period. The atomic number of carbon is 6, of nitrogen is 7 while that of oxygen is 8. So, the electronic configuration of carbon is 2, 4; of nitrogen is 2, 5 while that of oxygen is 2, 6. Though the number of shells remain the same, number of valence electrons increases by one, as we move across any given period  from left to right.
2) While going from top to bottom in a group, the number of shells increases successively by one but the number of valence electrons remains the same.
for example,boron has a configuration [He]2s22p1 and down the group aluminium has a electronic configuration of [Ne]3s23p1.​​​​

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