The Chemical Context of Life CHAPTER 2

Matter anything that takes up space and has mass pure forms are called elements combined forms are called compounds 92 elements occur in nature…25 elements are essential for life 96% of living matter is composed of 4 elements: C,H,O,N most of the remaining 4% is composed of P, S, Ca, and P

Atomic Structure atom – smallest unit having the properties of its element subatomic particles: e, p+, n0 nucleus (p+ & n0) electron shells 1st: max 2e 2nd: max 8e

Periodic Table of Elements provides information about atoms atomic number atomic mass

isotopes = atoms with extra n0 radioactive isotopes atomic number = # of p+ atomic mass = p+ + n0 isotopes = atoms with extra n0 examples: 12C, 13C, and 14C radioactive isotopes unstable atoms that give off particles & energy as the nucleus decays

Electron Configuration chemical behavior of an atom depends mostly on the # of e in the outermost shell = valence electrons

Energy Level of Electrons electrons can gain and lose potential energy gain energy  move to electron shells farther away from nucleus lose energy  move to electron shells closer to nucleus  the amount of potential energy held by an electron (energy level) is a function of the energy shell it occupies

Chemical Bonds electrons are directly involved in the chemical reactions between atoms atoms either share or transfer valence electrons to form chemical bonds: Covalent Ionic Hydrogen Van der Waals

Covalent Bonds: results from the sharing of pairs of valence electrons by 2 atoms classified by # of pairs shared (single, double, or triple) degree of sharing determines polarity (polar, nonpolar) – depends on the electronegativity of the atoms involved

Polar Covalent Bonds in Water

Ionic Bonds: result from a transfer of valence electrons ions produced when electrons are transferred anion = negatively charged ion cation = positively charged ion due to extreme differences in electronegativity (metals & nonmetals) attraction between oppositely charged ions = the ionic bond forms ionic compounds (salts)

Hydrogen Bonds weak electrical attractions between the slightly negative end of one polar molecule and the slightly positive end of another polar molecule

van der Waals interactions weak electrical attractions between two nonpolar molecules due to changing “hotspots” of positive & negative charge as electrons move around in the electron cloud

Molecular Shape & Function molecules have characteristic sizes & shapes molecular shape is crucial to biological function because it determines how molecules recognize & respond to each other with specificity

Chemical Reactions require physical contact between reactants result from the making (synthesis rxns) & breaking (decomposition rxns) of chemical bonds some are reversible; others are irreversible are affected by: concentration of reactants, presence of enzymes, environmental conditions (pH, temperature)