It is a good idea to try to put the most common symbols to memory. The structure of the nucleus and the electron layers surrounding it combine to determine the atom's atomic number and atomic weight. The nucleus is made up of protons and neutrons. Within the atom, the electrons are very light compared to the protons and neutrons. The nucleus makes up most of the atomic weight, however, the total number of protons, neutrons, and electrons in orbit determine what the complete atomic weight of the atom will be.
Lets look at a few examples.
The atom which has an atomic weight of 1 is hydrogen. Here we find one proton and no neutrons. Now, compare the simple atom of hydrogen with something which is a little more complicated. Oxygen, with eight protons and eight neutrons in orbit, has an atomic weight of 16. The number of electron layers in a particular atom helps to determine the precise relationship between it and other atoms in a chemical compound.