BIOL 1201 : Cumulative Review
Document Summary
Elements cannot be broken down to other substances. A compound contains two or more elements in a fixed ratio. Life requires about 25 chemical elements carbon, oxygen, hydrogen, and nitrogen make up approximately 96% of living matter. Atomic structure determines the behavior of an element. An atom is the smallest unit of an element. An atom has a nucleus made up of positively charged protons and uncharged neutrons, as well as a surrounding cloud of negatively charged electrons. The number of electrons in an electrically neutral atom equals the number of protons. Most elements have two or more isotopes, different in neutron number and therefore mass. Some isotopes are unstable and give off particles and energy as radioactivity. Electron configuration determines the chemical behavior of an atom. Electrons occupy specific energy levels, or shells, of the atom. Chemical behavior depends on the number of valence electrons, those in the outermost shell. An atom with an incomplete valence shell is reactive.