Chapter 2 The Chemical Context of Life Basic Terms Element = cannot be broken down to other substances Examples: Na, O, C, Cl Compound = combination of 2 or more different elements Examples: NaCl, CO2, CH4 Requirements for Life 92 natural elements? 25 are essential to life Carbon, oxygen, hydrogen, and nitrogen make up 96% of living matter Trace elements are needed, too, though Iodine?lack of = goiter!! Atomic Structure Atom = smallest possible amount of an electron that retains the element?s properties Subatomic particles: Protons = + charge Electrons = - charge Neutrons = neutral charge Atomic Structure Atomic Number: # of protons (also equals # of electrons?) Mass Number: # of protons + # of neutrons Examples Isotopes