Atomic Number:

The simplest element, hydrogen, has a nucleus made up of one proton; there are no neutrons. This is the most common element in universe. Sometimes a nucleus of hydrogen has a neutron or 2 along with the proton, but this does not occur often. These "mutant" forms of hydrogen do, play important roles in atomic physics.

The 2nd  most abundant element is helium. Usually, this atom has a nucleus having 2 protons and 2 neutrons. Hydrogen is changed into helium inside the sun, and in the process, energy is given off. This makes the sun shine. The process, called fusion, is responsible for the explosive force also of a hydrogen bomb.

Neutrons are all identical, too. The number of protons in an element's nucleus, atomic number, gives which element the identity of it. The element with 3 protons is lithium, a light metal which reacts easily with gases like oxygen or chlorine. The element with four protons is beryllium, also a metal. Generally as the number of protons in an element's nucleus increases, the number of neutrons increases. Elements with high atomic numbers, like lead, are there- fore denser than elements having low atomic numbers, like carbon. Perhaps you have compared a lead sinker with the piece of coal of similar size, and noticed this difference.