Atom  

Atoms are the most basic units of all matter and cannot be broken down further by chemical means. Everything in the universe, including every solid, liquid and gas, and except for energy, is matter, is composed of atoms.

Individual atoms are so small that they cannot be seen even with the most powerful microscopes, and so small that their size cannot be measured, or even imagined. For example, an average sized human hair is roughly one million carbon atoms in width. There are probably many more atoms in a grain of sand than there are grains of sand on the earth. Likewise, a 160-pound human body may contain very roughly seven octillion (7,000,000,000,000,000,000,000,000,000) atoms.

Every atom consists of three types of basic, sub-atomic particles: protons, neutrons and electrons. The numbers of these particles determines the physical and chemical properties of an atom.

Atoms are classified onto elements according to the number of protons in their nucleus. This number can range from one, in the case of hydrogen, to about 118, for oganesson. Of these, about 90 occur in nature in appreciable amounts. Atoms with 99 to 118 protons only result from particle accelerator experiments, and all of them are radioactive and very short-lived.