mjhjmtu

My first, yet simple, puzzle on this site.

By myself, I’m as light as can be

But throw me a ball, and suddenly I’ll be heavy

Give me another, and I’ll get quite excited

Because shortly thereafter, I will become a he instead.

What am I?

Are you

a hydrogen atom/nucleus?

By myself, I’m as light as can be

Hydrogen is the lightest element on the periodic table. Specifically, this would refer to the standard isotope of protium, with just a single proton in the nucleus.

But throw me a ball, and suddenly I’ll be heavy

Deuterium, a hydrogen isotope with one proton and one neutron, is called heavy water.

Give me another, and I’ll get quite excited

Tritium, a hydrogen isotope with one proton and two neutrons, is in an excited state ...

Because shortly thereafter, I will become a he instead.

... and soon decays into helium-3, denoted by "He" on the periodic table.

I think the word "excited" was the first key for me in solving this. That word seems likely to have a non-obvious meaning that fits the context of whatever the solution is, and the only second meaning I could think of was in physics. From the first line I guessed the answer might be a photon, so I spent a while barking up that tree and thinking about particle interactions involving photons on Feynman diagrams. Then I realised what the "he" must mean, and from there it was easy to construct the right answer after a quick look at a list of isotopes on Wikipedia.

Are you:

Hydrogen (nucleus)

By myself, I’m as light as can be

Lowest atomic weight from all the elements

But throw me a ball, and suddenly I’ll be heavy

Add an electron, atomic weight increases

Give me another, and I’ll get quite excited

Add more electrons or even a proton+neutron, it will go through chemical change

Because shortly thereafter, I will become a he instead.

From that, it becomes He, helium

You are an

electron

By myself, I’m as light as can be

Electrons have essentially as little mass as possible without being massless

But throw me a ball, and suddenly I’ll be heavy

the ball is a proton (or a proton-neutron pair)

Give me another, and I’ll get quite excited

Then you will be a helium ion

Because shortly thereafter, I will become a he instead.

helium ions are so good at stabilizing that it will pull an electron from somewhere and become stable. He is the atomic symbol for helium

Are you

Hydrogen?

By myself, I’m as light as can be

Hydrogen is the lightest element.

But throw me a ball, and suddenly I’ll be heavy

I believe this is referring to adding a neutron to the hydrogen atom. It'll make tritium, which makes it nearly twice as heavy.

Give me another, and I’ll get quite excited

Radioactive isotope.

Because shortly thereafter, I will become a he instead.

Turns into helium.