Everything around you is made of atoms. The air, the chair, the screen you are reading. An atom is the smallest unit of an element that retains the properties of that element. Split it and you get subatomic particles, not a smaller piece of the same substance. Atoms are mostly empty space. The nucleus at the center holds nearly all the mass, but it is tiny compared to the electron cloud around it.

An atom has three main parts. Protons carry positive charge and sit in the nucleus. Neutrons carry no charge and also sit in the nucleus. Electrons carry negative charge and orbit the nucleus in cloud-like shells. The number of protons determines the element. Hydrogen has one. Carbon has six. Gold has seventy-nine. The number of neutrons can vary, producing isotopes of the same element. The number of electrons can also vary, producing ions. Atoms combine to form molecules, and molecules form the materials we see and touch.

Atomic structure

• Protons. Positive charge, define the element.
• Neutrons. No charge, add mass, create isotopes.
• Electrons. Negative charge, determine chemistry.
• Nucleus. Central core holding protons and neutrons.

Atoms are about one ten-billionth of a meter across. A single drop of water contains more atoms than there are stars in the observable universe.