Essential Chemistry for Biology 09 27 2013 Matter Matter anything that occupies space and has mass Elements Matter is composed of chemical elements Elements substances that cannot be broken down into other substances There are 92 naturally occurring elements on Earth 25 elements are essential for life 4 most abundant Carbon 6 Hydrogen 1 Oxygen 8 Nitrogen 7 Atoms Each element consists of one kind of atom Atom Smallest unit of matter that still retains the properties of an element Atoms are composed of subatomic particles Protons positively charged and HAS MASS Electrons negatively charged and have NO MASS Neutrons electrically neutral and HAS MAS Protons Neutrons Electron Cloud Elements differ in the number of subatomic particles in their atoms Atomic Number Number of protons determining which element it is top Mass Number Sum of both protons and neutrons bottom Electron Arrangement Electrons orbit atoms at specific distances called electron shells The first INNER shell can hold up to 2 electrons All subsequent OUTER shells can hold up to 8 electrons Atoms with incomplete outer electron shells are reactive in an attempt to fill or satisfy their outer electron shell Chemical Bonds and Molecules Chemical reactions enable atoms to give up or acquire electrons to complete their outer shells These reactions result in atoms joining together with other atoms forming molecules Atoms are held together in molecules by chemical bonds Covalent Bonds Covalent Bond Forms when 2 atoms share outer shell electrons The number of covalent bonds an atom can form is equal to the number of electrons needed to fill its outer shell ex 2 electrons needed 2 possible bonds Single Bond designed for a pair of shared electrons Double Bond designed for two pairs of shared electrons Chemical Reactions Cells constantly change molecules by breaking existing chemical bonds and forming new ones Chemical reactions do not create or destroy matter they only rearrange it Water Covers 75 of Earth s surface Makes up between 70 and 95 of a cell s volume is a polar molecule that forms hydrogen bonds Polar Molecules Polar molecules such as water Those in which electrons are not shared equally between atoms Hydrogen Bonds Hydrogen Bonds The polarity of water results in electrical attractions between neighboring water molecules