Atomic structure


Calculate the atomic number and number of p-electrons of an atom whose valence shell electronic configuration is 4s^2


The atom would be in Period 4 Group 2 which is Ca.

The variance shell electronic configuration is 4s^2.

The principal quantum number (4) gives you the Period number. So, starting from element ‘H’ in period 1 and counting down to the period 4 (or fourth row). That gets us to the element ‘K’. Now, start counting to the right through the s-block. 2 elements including the K, which will land us to ‘Ca’. This is the element we’re looking for with 4s^2.

Here’s the snap from atomic table for a little bit more clarity:


Atomic num=20??
Num of p-electrons=??
Didn’t get the idea…:confused::confused:


You never mentioned the number of p-electrons. A common equation, we often encounter is 4s^2 4p^5 for which the explanation is same for upto 4s^2 but will continue further to get the full result.

As it needs us to get an element from the p-block, we need to get through the d-block upto ‘Zn’. Once there, count five more atoms for 4p^5 configuration which will land you at ‘Br’. This is the desired element. It being, atom in period 4, group 17 - ‘Br’.

Don’t go on atomic number here, the simple way to calculate would be to look at the atomic table and just count the way around. As it’s 4 s, you just count upto 4th row down and then move to the right for s-block and then past the d-block as it needs you to find the element in p-block. Within p-block as it’s powered by 5, just count upto the 5th element there in that row which is ‘Br’ in our case here.

Hope that clarifies :slight_smile:


Thank you…