Question

Question #617e4

Answer 1

Normally, atoms need 8 electrons to fill their outer shell. This is called the octet rule. As your question notes, there are some exceptions to the octet rule. Hydrogen only needs 2 electrons to fill its outer shell, since is that has an electron configuration of `1s^1`, and an s orbital can hold 2 electrons.

2 other exceptions are Be and B, which need 4 and 6 electrons respectively.

Now, to find out how many electrons an atom has already in its outer shell, count the column it is in. For example, Cl is an element in column 7, so it has 7 electrons in its outer shell.