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Biol 1441 - Dr Westmoreland SI leader: Viet Ho Chapter 2: Matter is anything that takes up space and has mass. Element is a substance that cannot be broken down chemically to other substances by chemical reactions. A compound is a substance consisting of two or more different elements combined in a fixed ratio. Essential Elements of Life: Carbon, oxygen, hydrogen, and nitrogen make up approximately 96% of living matter. 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. Atomic Number and Atomic Mass Isotopes Electron Configuration and Chemical Properties Valence electrons are outer electrons Valence shells are the outermost electron shell Chemical behavior depends on the number of valence electrons. An atom with an incomplete valence shell is reactive. Chemical bonds Covalent Bonds: is the sharing of a pair of valence electrons by 2 atoms. Electronegativity: is the attraction of a particular kind of atom for the electrons of a covalent bond Nonpolar covalent bond: the electrons of the bond are shared equally. Ex: N2 Polar covalent bond: the electrons of the bond are not shared equally. Ex: HCl Ionic Bonds: Two atoms are so unequal in their attraction for valence electrons that the more electronegative atom strips an electron completely away from its partner. Ion: a charged atom (or molecule) Cation: a positive ion Anion: a negative ion The transfer of an electron is not the formation of a bond; rather, it allows a bond to form because it results in two ions. Any two ions of opposite charge can form an ionic bond. Weak Chemical Bonds A hydrogen bond forms when a hydrogen atom covalently bonded to one electronegative atom is also attracted to another electronegative atom. Chemical reactions is making and breaking of chemical bonds, leading to changes in the composition of matter.