A number bond is a mental picture of the relationship between a number and the parts that combine to make it. The concept of number bonds is very basic, an important foundation for understanding how numbers work. A whole thing is made up of parts. If you know the parts, you can put them together (add) to find the whole. If you know the whole and one of the parts, you take away the part you know (subtract) to find the other part.
Number bonds let children see the inverse relationship between addition and subtraction. Subtraction is not a totally different thing from addition; they are mirror images. To subtract means to figure out how much more you would have to add to get the whole thing.

Here is a number bond powerpoint that you can use with your students. The pictures on the slides can be used to complete the number bond and write the number sentences.

Here are number bond cards that you can use with your students

Here are worksheets that students can do to practice number bonds

Here are cards with missing numbers in the number bond and in the proportionate part-part-whole mat