Question

both helium and beryllium have two electrons in the valence shell. helium is a noble gas whereas beryllium is a metal. justify

Answer 1

Because Helium have only S orbit in its configuration but beryllium have S and P orbits in its configuration and you know S orbit have a tendency to take only two electrons in one shell, but in the case of beryllium its atomic number is 4 so it has 2 electrons in S orbit and 2 in P orbit. P orbit has 3 shells in which it can fill 6 electrons, beryllium has only 2 electrons in P orbit and remaining 1 orbit will be empty that's why it is not a noble gas.