Explain what happens to the mass that is lost when a nucleus forms?

Answer 1

The mass isn't really lost, it's converted to energy via E=mc². The same thing happens in chemical reactions, it's just not noticeable.

For example, burning one mole of carbon produces about 393500 J.

Using E=mc²

393500 J=m(3.00x10^8 m/s)²

m=393500 J/(3.00x10^8 m/s)²

=4.37x10^-12 Kg

Note that a Joule(J) is a (Kg∙m²)/s² so that's why the units cancel out to give Kg.

Anyways, you see that the mass "lost" is very small, if you don't understand how small 4.37x10^-12 Kg is, write it out, it's 0.00000000000437 Kg. A good analytical balance wouldn't even be able to detect such a small "loss" in mass. However when we're dealing with nuclear reactions we're usually talking about converting mass to energy and because E=mc² and the speed of light(c) is such a large number we get a lot of energy from a small "loss" in mass! That's the only reason we care about the small "loss" in mass when we're talking about nuclear reactions.

Notice that I put the word "loss" in quotations because like I said, the mass is not really lost, it's just converted to energy.

Answer 2

The mass that is lost when a nucleus forms is converted into nuclear binding energy that holds the nucleus together.