The part of an atom that gives an element its identity is the nucleus. It is made up of two kinds of particles, the proton and the neutron. These are extremely dense. A teaspoonful of either of these particles, packed tightly together, would weigh tons. Protons and neutrons have just about the same mass, but the proton has an electric charge while the neutron does not.
The simplest element, hydrogen, has a nucleus made up of only one proton; there are usually no neutrons. This is the most common element in the universe. Sometimes a nucleus of hydrogen has a neutron or two along with the proton, but this does not occur very often. These “mutant” forms of hydrogen do, nonetheless, play significant roles in atomic physics.
The second most abundant element is helium. Usually, this atom has a nucleus with two protons and two 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 also responsible for the terrific explosive force of a hydrogen bomb.
Every proton in the universe is just like every other. Neutrons are all alike, too. The number of protons in an element’s nucleus, the atomic number, gives that element its identity. The element with three protons is lithium, a light metal that reacts easily with gases such as oxygen or chlorine. The element with four protons is beryllium, also a metal. In general, as the number of protons in an element’s nucleus increases, the number of neutrons also increases. Elements with high atomic numbers, like lead, are therefore much denser than elements with low atomic numbers, like carbon. Perhaps you’ve compared a lead sinker with a piece of coal of similar size, and noticed this difference.