The opening in a leaf that lets things in
and out is the what?

The stoma  (plural would be stomata) is where air can get in. They are found on the underside of a leaf.

It is called the stomata

The atomic nucleus made up from protons (yellow) and neutrons (blue) and held together by nuclear forces. nuclear forces (also known as nuclear interactions or strong forces) are the forces that act between two or more nucleons. they bind protons and neutrons (“nucleons”) into atomic

Answer(28.02g/mol);