Which of these would you expect to have the least momentum?

Momentum is simply defined as "mass in motion." All objects have mass; so moving objects will have momentum. The amount of momentum of an object depends on object’s mass and its moving velocity.

Therefore, Momentum = mass × velocity

From above equation it is clear that momentum is directly proportional to an object's mass and directly proportional to the object's velocity. It means object of high mass and high velocity will have higher momentum than the object having low mass and low velocity.

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.