Often, when we are teaching young learners about addition sentences we refer to part/part/whole. So, in the above example, 2 and 3 are "parts" and they add together for the whole of 5. A fancier way of saying part/part/whole would be addend/addend/sum.
All of this is important because we are trying to get young learners to think algebraically. When I show a student the problem 2+___=5 I'd like them to automatically start solving for the blank. "OK, one addend is 2 and the other addend is blank. The sum is 5. What addend do we have to add to 2 to get 5? Ahhhh! I know! The missing addend is 3!"
Consider this bit of algebra for elementary aged students:
As long as the kids are thinking in terms of addends things like this can be a breeze! On the left side we see that two addends which are the same add up to the sum of square. It reads "triangle plus triangle equals square." The problem goes on to say that square=12. If a young learner is thinking clearly about addends he will say, "Now what two addends can I use that are the same add up to 12? Hmmmm.... let me think about what double I know that will give me 12. I have it: double six adds up to 12. If each of the addends shown as triangles are 6 then the problem works out! Triangle plus triangle equals square. Triangle plus triangle equals 12. 6+6=12!"
