The central atom's number of valence electrons in makes it an exception to the octet rule. nh3 sef2 bf3 asf3 cf4

Explanation:

You must draw the Lewis structure for each molecule, trying to give every atom an octet.

In the structures below, every molecule except one has a central atom with octet. That molecule is  

The B atom has only six electrons in its valence shell, so it violates the octet rule.

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.