Question

In 20 words or more, how does the number of electrons on the outer most energy level of an atom correlate with the position of that element in the periodic table.

Answer 1

According to the modern periodic law, the properties of elements are repeated after a certain interval when they are arranged in increasing order of their atomic number( no. of electrons).

In a periodic table, each period(horizontal rows) indicates the value of n for the outermost shells. That is, n= 1,2,3,4.... indicates successive filling of the 1st, 2nd , 3rd and higher principle energy levels. For example- Li ( 1s² 2s¹) means its outermost electron is filled in the second shell and hence it belongs to the 2nd group.

Each groups (vertical column) consists of number of elements having the same electronic configuration of the outermost shell (valence shell) and these groups are numbered from 1 to 18.

For example group 1 alkali metals will have 1 electron in their valence shell (ns¹). Similarly group 15 elements will have 5 electrons in their outmost shell (ns ² np³).

Combining the above two factors if we have an element whose atomic no. is 15 it will have a configuration of  

1s²2s²2p⁶3s²3p³ . The valence shell configuration is 3s²3p³.Since the principle energy shell is 3( n=3) it is a 3rd period member and the total valence electron is 5 therefore it belongs to group 15. Hence the element is phosphorous (P).