The Chemical Context of Life Chapter 2 Chemical Elements • Matter - any substance that has mass and takes up space. • Element - cannot be broken down to other substances by chemical reactions. • 92 naturally occurring • life requires about 25 elements – Most common elements (CHNOPS) = 98% • Compound - substance consisting of 2 or more elements combined in a ratio. Atoms and Molecules • The simplest unit of matter that retains its chemical properties is the atom. • Atoms are composed of subatomic particles. – Protons – Neutrons – Electrons • Atoms that are chemically combined are called compounds or molecules. Chemical Bonds • Bond - formed by electrical interactions atoms; behavior of electrons • Two types of strong interactions – ionic – covalent • Weak interaction – hydrogen bonds Exit Questions • 1. Draw and label an atom. Include the following labels: nucleus, energy levels, neutrons, protons, and electrons. • 2. What is the difference between an ionic bond and a covalent bond?