Explain how you can use an atom’s mass number and atomic number to determine the number of protons, electrons, and neutrons in the atom.

You can determine it of the mass atom
mass atom is mostly protons and neutron because they are heavier than electron that have mass of 1/1856 of protons, so ignore the electron. 

atomic number is the number of protons, so the electron have the same number of protons. 

To find neutron, you need to do math. 
You can use atomic number of protons, but ignore electron, their mass is too small to change. so put the atomic mass subtracted by the number of protons and find the total, there you can find neutron. also the mass is very specific, so you will need to estimate the closet number of mass.

False, as a substance changes from a solid to a liquid and from a liquid to a gas the molecules get farther apart from each other