The number of protons in an atom is called its atomic number . Within limits, the number of neutrons and electrons in an atom can vary. Thus, when we want to know how much an atom of something weighs, we can just add up the number of protons and neutrons. The half life of a radioactive chemical is the amount of time it takes for half of the starting quantity to undergo radioactive decay. Thus, if you started with 1 gm of a substance with a half life of 1 year, after one year, you would have 0.5 gm left, after two years you would have only 0.25 gm, after three years, 0.125, etc.
Molecular weight equals the sum of the atomic weights of the atoms in the molecule.
Normally, the number of protons and electrons match so the charge is balanced out. Sometimes, however, the number of electrons can vary. Ions are atoms with electrons added or removed resulting in an overall positive or negative charge. These quantities are referred to as energy shells or orbitals. These are not orbits like the planets around the sun, but an attempt to show the energy quantities as pictures.