Question

What is the total number of pairs of electrons having same quantum
number(l and ml) for Be ?

Answer 1

Explanation:

The only quantum number which is to be determined here is the magnetic quantum number ml

ml takes the values from -l to +l

So as per your query, ml = -1, 0, 1

Now the thing here to note is that the quantum numbers are the state of the electron in the atom. Think of it in terms of wavefunction. It has nothing to do with the number of electrons. I can take a carbon atom, extract all the electrons from it but one and make it achieve a state n=2, l=0, ml=0, s=-1/2 (or place it in 2s orbital). The set of numbers signify an ‘Address ‘ of an electron (for us an address mean spatial parameters but that's not same for an electron, it has other parameters as well, like angular momentum, spin and magnetic Moment). However as someone has already done the math in this thread of answers, what we can find is the maximum number of electrons an atom can accommodate. i.e. 3.