Number bonds

A whole in the top circle and two parts beneath it. Move one counter across, see every pair that makes the number, and hide one value to ask the class for it.

How to use it

Choose the whole with the − and + buttons (2 to 20). The two parts appear with counters in rows of five, red up to five and yellow after.
Press the arrow buttons, or the left and right arrow keys, to move one counter from one part to the other. The number sentences underneath change at once.
Press Hide one number (or Q) and ask what is missing. Press Space to show the answer and Space again for a new question about the same whole.

Ideas for class

Set the whole to 10 and move one counter at a time from one part to the other, saying each pair aloud: nine and one, eight and two, seven and three.
Show a whole of 7 with parts 3 and 4, then ask how you could use it to find 7 − 3 without counting back. The bond holds both facts.
Hide the whole and show two parts, such as 6 and 5, then ask for the total. Students who split 5 into 4 and 1 to make ten first are using the bond.
Click through all the pairs for 8 in the list and ask which pairs are really the same fact written the other way round.
Ask students to predict which part will be larger after two moves before you press the button, then check.
Set the whole to 12 or 14 and ask students to find a pair that includes ten, so that the part beyond ten becomes a clear step.
Use the fact family line to write all four sentences for one bond on the board, and ask where each number sits in the diagram.

Questions

What is a number bond?

A picture of how a number is made from two smaller numbers. The whole sits in one circle and its two parts in two more, joined by lines, so that students see addition and subtraction as the same relationship.

Why do the counters change colour at five?

Showing the first five in red and the rest in yellow lets students see a part as five and some more. That is the structure behind making ten and bridging past ten, and it makes larger parts quicker to read than a pile of identical dots.

Can I use numbers larger than 20?

Not in this tool, which stops at 20 so the counters stay clear. For larger numbers use base-ten blocks or the place value chart, which show a number in tens and ones.

What does the fact family line show?

All the number sentences that one bond gives: two additions and two subtractions, with repeats removed when both parts are equal. It lets students see that one fact tells them three others.

Is the practice score kept anywhere?

No. The questions are made fresh from a counter that restarts when you leave the page, and nothing about what students answered is stored or sent. The whole and the split you last used are remembered in this browser only.

Nothing you do here is sent to us. Names, lists and settings stay in this browser. No sign-in, no ads.

Using number bonds in class

Number bonds turn a single fact into a pattern. A student who sees that 10 is 6 and 4, and also 7 and 3, begins to see that moving one unit from one part to the other keeps the whole the same. That idea is the basis for adding and subtracting mentally, so the move buttons matter more than the answers.

Start with wholes to 5, where every pair can be shown with fingers.
Move to 10, the most important whole, and spend several days on it.
Then use 10 as a stepping stone to the teens: 9 + 4 is 9 + 1 + 3.
Finish by hiding one number and asking for it, which is the missing addend in a different form.

Common mistakes

Seeing the diagram as three unrelated numbers. Point to the lines and say that the two parts together make the whole.
Reading 3 + 7 and 7 + 3 as two different facts to memorize. Show them as one pair.
Answering the whole when a part is hidden, writing the sum instead of the difference. Ask which circle holds the biggest number.