Which of the following atoms is reactive?

Question

2, 8, 8

2, 8

2

or 2, 8, 1

Answer 1

An atom is only considered 'reactive' when it needs to lose it valence electrons (for metals) or gain electrons (for non-metals) so as to achieve stability with a full outermost electron shell.

The first atom you have, [2,8,8], it has a full outermost shell (last number is 8, meaning it has an octet). Thus it is not likely to react and gain or give away any valence electrons. Similarly for your 2nd atom, [2,8].
The 3rd atom has only 2 electrons in its 1 and only shell. It has a full outermost shell too (duplet, since 1st shell can only take in max 2 electrons), hence it is stable.
The last atom belongs to Group I, with 1 valence electron. Hence it will be very willing to give away its one valence electron to obtain a stable octet. Thus the last atom is reactive.
By the way, the first 3 atoms belong to the noble gases (Group 0).

Answer 2

2 8 1

Answer 3

2, 8, 1
this is highly reactive because of the extra valence electron