An atom is the smallest unit of that has the properties of an . It is composed of a dense core called the and a series of outer shells occupied by orbiting .

The nucleus, composed of and , is at the center of an atom. Protons have a electric charge while neutrons are . The of an atom is dictated by the number of protons, which in turn, determines the chemical of the atom. The of the element is determined by the number of neutrons.

The electron has an electric charge equal but the proton (that is, they cancel). A stable atom has the same number of electrons orbiting as it does protons in the nucleus; this makes the atom electrically . Atoms that are missing an electron (that is, have one more proton than electrons) will tend to attract additional electrons, while atoms that have an excess electron (one more electron than protons) will tend to eject the electron. The shell containing the outer-most electron is called the shell, and is the chemically active shell.

In size the entire atom has been thought to be approximately four-billionths of an inch, meaning that approximately 250,000,000 atoms of this size must be put into line to span 1 inch. Atoms are not usually alone, but instead come in groups called .