Question

how many electrons can be accomodated in first shall of any atom?​

Answer 1

Answer:

two electrons

Explanation:

Each shell can contain only a fixed number of electrons: The first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on.

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Answer 2

Answer : 2 Electrons.

Explanation : As per the Madelung Ordering Rule, the Electronic Configuration of Atoms approximately follows the series $2n^{2}$, that is, the principle quantum number of the shell ( $n= 1, 2, 3 ...$ ) is inserted into the expression and the value produced is the maximum number of atoms that can be accommodated in the said shell.

However, there are certain things to be noted about this rule. First and foremost, the expression only applies to atoms which are neutral and in their ground state ; that is to say, they are not ionised. Furthermore, the expression works faultlessly until Proton Number 20, i.e, Calcium, but beyond that the Aufbau Principle takes over and the shells start filling up with electrons based on the fact that the lowest available energy levels are to be filled up before higher energy levels.

So, beyond Calcium, the shells do not follow the predictable pattern of $2, 8, 8, 3$ ... $2, 8, 8, 4$ ... etc. but instead the shell with the principle quantum number of 3 ( the M shell ) starts filling up, hence the Electronic Configurations are in the following pattern ; $2,8,9,2$ ... $2,8,10,2$ ... which obeys the Madelung Ordering Rule, but not in a straightforward way.

For this same reason, Ununoctium, the last element in the Periodic Table, has the Electronic Configuration of  $2, 8, 18, 32, 32, 18, 8$. One would expect the shell with the principle quantum number of 5 ( the O shell ) to have a maximum of $2(5)^{2} =50$ electrons, but this is not the case due to the Aufbau Principle. Similarly, one would expect the shell with the principle quantum number of 6 ( the P shell ) to have a maximum of $2(6)^{2} =72$ electrons, but again, this is not the case.

It is also important to note that the outermost shell of any atom, will have 8 electrons, or an octet, as this is the most stable "Noble Gas" valency electron configuration. It is for these reasons that we say that the Madelung Ordering Rule is followed approximately.