An element is defined as a substance made up of atoms with a specific number of protons. That means that every atom with 1 proton is always a hydrogen atom. Elements cannot be broken down further by chemical means and always retain specific properties, even when bonding.
The three subatomic particles that make up atoms are electrons, protons and neutrons. Protons never vary within an element, or the atom would become an entirely new element. The atomic number of each element denotes how many protons are in any given atom of that element. The outermost electrons are responsible for chemical bonding and are known as valence electrons. Depending on the element in question, valence electrons can be gained or lost freely. The availability of valence electrons in the atom of a particular element can be determined through the use of quantum numbers in advanced calculations.
Atoms of a specific element can have variances in neutrons. These are known as isotopes and are named by combining the total number of protons and neutrons in the atom. For example, isotopes of carbon — atomic number 6 — include carbon-12 and carbon-14, both of which have 6 protons but have 6 and 8 neutrons respectively.