The basic elemental building block of condensed matter and gases. 

The atomic mass (amu) is associated with the nucleus of the atom, and is defined as one-twelfth of the mass of an atom of C12. The atomic weight of an element is the weighted average of the atomic masses of the constituent isotopes of the element.

The chemistry of the atom is associated with its electron distribution. The number of electrons on a neutral atom is equal to the number of protons in the nucleus of the atom. This is known as the atomic number, Z.

The removal or addition of electrons to a neutral atom creates ions that have a net negative or positive charge.