In science, an atom is a constituent of matter consisting of z negatively charged electrons bound predominantly by the Coulomb force to a tiny, positively charged nucleus consisting of Z protons and (A - Z) neutrons, where Z is the atomic number and A is the mass or nucleon number. [1] The negatively-charged electrons move about the positively-charged nucleus in regions called "orbitals", generally defined by the 90% probability region of movement, whose shape is determined by a combination of an attraction for the positive charge as this conflicts with like-like negative charge repulsion effects of the crowding of negatively-charged electrons around the nucleus.

The human molecule, i.e. one average 70-kg (154-lb) person, is composed of 6.9 x 10E27 atoms, of 26 varieties, from hydrogen H, the smallest containing one proton and one electron, to iodine I, the largest containing 53 protons, 74 neutrons, and 53 electrons. [2]

References
1. Licker, Mark, D. (2004). McGraw-Hill Concise Encyclopedia of Chemistry. New York: McGraw-Hill.
2. (a) Emsley, John. (2001). Nature's Building Blocks - an A-Z Guide to the Elements. New York: Oxford University Press.
(b) Thims, Libb. (2008). The Human Molecule, (preview). Morrisville, NC: LuLu.