Atmosphere: the air surrounding the earth, which contains 78% nitrogen, 21% oxygen, and 1% of other trace gases (including carbon dioxide and water vapor)

Atom: smallest portion into which an element can be divided and still retain its properties; made up of a dense positively charged nucleus surrounded by negatively charged electrons

Atomic Number: number of protons in the nucleus of an atom of an element

Biogeochemical Cycles: the biological, geological, and chemical processes that move an element through the earth's air, water, rocks, and organisms

Biosphere: the part of the planet that supports life

Carbohydrates: macromolecule that has carbon, hydrogen, and oxygen; examples are sugars and starches

Covalent Bond: bond in which electrons are shared between atoms

Electron: subatomic particle that orbits around the nucleus of an atom; has a negative charge

Fossil Fuels: energy products that come from the compressed, liquefied, or vaporized remains of once living organisms; includes coal, oil, and natural gas

Greenhouse Effect: atmospheric process in which certain gases trap heat at earth's surface; allows shortwave radiation in, but blocks infrared (heat, longwave radiation) from leaving the atmosphere

Lipids: macromolecule that makes up fats, oils, and waxes

Neutron: subatomic particle found in the nucleus of an atom; has no charge

Nucleic Acid: macromolecule made of a sugar, a nitrogen base, and a phosphate group; usually forms chains

Organic carbon: carbon found in living or once living organisms

Protein: macromolecule made up of chains of amino acids

Proton: subatomic particle found in the nucleus of an atom; has a positive charge