additive numbers are numbers that are added together
opposite
As far as positive numbers are concerned, the additive inverse of a positive number is a negative number with the same magnitude (i.e. the additive inverse of 5 is -5). It would be wrong to simply say that "negative" and "additive inverse" are correct, because the additive inverse of a negative number is a positive number.
Opposite.
Another name for the opposite of a number is Additive Inverse.
The additive identity is 0.
additive numbers
Another name for a set of natural numbers is counting numbers.
Yes. Those would be numbers such as 5 and -5, which only have opposite signs - they are called additive inverses (of one another).
Zero.
The additive inverse is the sum of two numbers that will equal zero. The additive inverse of -18 is 18.
There is only one set and it does have an additive identity.