Atoms are particles of matter. They are the smallest unit of matter that defines the chemical elements and their isotopes. Every substance, be it solid, liquid, or gas is made up of atoms. Atoms are tiny. The size of atoms is measured in picometers (trillionths of a meter). A single strand of human hair is about one million carbon atoms wide.
Every atom is composed of a nucleus made of protons and neutrons (hydrogen-1 has no neutrons). The nucleus is surrounded by a cloud of electrons. The electrons in an atom are bound to the atom by the electromagnetic force, and the protons and neutrons in the nucleus are bound to each other by the nuclear force. Over 99.94% of the atom's mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. Normally, an atom's electrons balance out the positive charge of its protons to make it electrically neutral. If an atom has a surplus or deficit of electrons, then it will have an overall charge, and is called an ion.
The number of protons in the nucleus determines what chemical element the atom belongs to (e.g. all copper atoms contain 29 protons). The number of neutrons determines what isotope of the element it is. The electron cloud of the atom determines the atom's chemical properties and strongly influences its magnetic properties.
Atoms can attach themselves to each other by chemical bonds to form molecules, network solids, metal alloys, crystals, and other solid solutions. The tendency for atoms to bond and break apart is responsible for most of the physical changes we observe in nature, and this is studied by the science of chemistry.
Atoms and sub-atomic particles behave in peculiar ways that cannot be explained through the classical laws of physics. The field of quantum mechanics was developed to explain the structure and behavior of atoms.
Not all matter is made up of atoms, but atoms do comprise all the types of matter than can be seen and touched. Astronomical observations indicate that most of the Universe's matter is "dark matter", composed of particles of a currently unknown type.