Question

both helium and beryllium have two electrons in valence shells. helium is noble gas where as beryllium is a metal. justify

Answer 1

It can be understood by the concept of electronic configuration
First take He
Z= 2
Maximum number of electrons in a shell is given by 2n²
Helium's electrons enters in the k shell and completes it and become inert

Now let's take Be
Z= 4
In first (k) shell
No of maximum electrons which enters = 2
Electrons left = 2
They will enter in l shell
Since maximum no of electrons which can enter in l shell is 8
therefore Be has a valency of 2 and is a metal