Right. I'm going to go from absolute basics, and work up, to explain to Wybit what he's flailing about trying to understand... then if I get time I'll answer the OP...

Firstly... each atom of a particular substance (hydrogen, carbon, uranium, lead, gold) has the same number of protons.

Atoms also (mostly, except some hydrogens) contain neutrons. The exact number of neutrons may differ slightly, although not generally by very much.

Neutrons and protons weigh, to a reasonable approximation, the same amount. Which makes the maths easier. The weight of an individual proton or neutron is (roughly) 1.66 x 10 ^{-27} kg. This unit of mass is known as a Dalton. So one proton weighs (roughly) 1 Da, one neutron also weighs (roughly) 1 Da. That's all a bit approximate, because, actually, the exact weights of protons and neutrons vary a tiny bit depending on what atom they're in.

An atom of hydrogen my be hydrogen-1, hydrogen-2 or hydrogen-3, the atomic number (number of protons) is 1 in all cases, and there may be 0, 1 or 2 neutrons giving an atomic weight of (more or less) 1, 2 or 3 Da.

An atom of carbon contains 6 protons, and 6, 7 or 8 neutrons. These are described respectively as carbon-12, carbon-13, and carbon-14 . The definition of the Dalton is 1/12 of the weight of one atom of carbon-12 (which is why I said all the other masses were approximate).

Incidentally, weights are often expressed in grams/mole, where one mole of any substance is 6.022 x 10 ^{-22} atoms (or molecules). The mole is a number chosen such that that number of atoms of carbon-12, 1 mole of carbon-12, weighs 12 g. (And thus 1 mole of hydrogen gas, H_{2} weighs about 2 g).

The molecular weight given on the periodic table for a particular element is the mass per mole.. it's an average number that tries to take into account the masses and abundances of the different naturally occurring isotopes of the element, thus chlorine is given as having a molecular weight of 35.5 because about 3/4 of naturally occuring chlorine atoms are chlorine-35, and the other quarter are chlorine-37.