What exactly is an atom? What is it made of? What does it look like? The pursuit of the structure of the atom has married many areas of chemistry and physics in perhaps one of the greatest contributions of modern science! See more »

Gas, in physics and chemistry, any substance that expands and spreads indefinitely when not confined in a container. See more »

Your eyes can only perceive things in the presence of light. Learn how the photoelectric effect supports the particle theory of light in this article. See more »

Matter, in the physical sciences, anything that occupies space and has mass. Every physical object--the sea, a star, a book, or a person--is made up of matter. See more »

The basic structure of an atom is made up of neutrons, protons and electrons, and its atomic number is calculated by adding up the number of protons and neutrons in the atom's nucleus. Learn about the basic structure and the atomic number of an atom in this article. See more »

Wavelengths are used to measure the size of a wave. Learn about wavelengths in this article. See more »

Liquid, a state of matter with properties midway between those of solids and gases. See more »

Plasma, in physics, a highly ionized gas, usually at high temperatures, that conducts electricity and is affected by a magnetic field. See more »

Solid, in physics and chemistry, one of the three states of matter. The other two states of matter are gas and liquid. See more »

Vapor, the gaseous form of substances that are usually liquids or solids. The process by which a substance becomes a vapor is called vaporization. See more »