A compound or a mixture.
If it is two or more elements are mixed, it is a mixture. If two or more elements are chemically reacted, it is a compound ( example FeS)
Hope this helps.
Atomic radius increases down a group
Nitrogen has 5 valence electrons.
The definition from what I know because I remember it from biology I think is "a theory explaining the structure of the earth's crust and some other associated phenomena as resulting from the interaction of a rigid lithospheric plates that move slowly over the underlying mantle." I'll put the quotations for you to copy what I said so it doesn't sound like you got it off me and I hopefully spelt lithospheric right