The Chemical Basis of Life

Structure of an Atom Atom has a nucleus that consists of protons (+) and neutrons (0) Electrons(-) circle the nucleus

Structure of the Atom Atomic number = total number of protons –Neutral atoms have the same number of electrons as protons Mass number = total number of protons and neutrons –Atomic mass

Electrons and Reactivity The number of electrons present in atoms outside energy level determines how it reacts with other atoms –1 st energy level holds 2e –Next energy levels hold 8e If an atom is missing valence electrons, then it will react with other atoms Octet Rule all atoms (except H and He) want 8 valence electrons

Periodic table Arranged by atomic number Periods are horizontal, Families (groups) are vertical Three main categories –Metals – to the left; low electronegativity –Non-metals – to the right; high electronegativity –Metalloids – zig-zag line

Combining Elements Compound = substance containing 2+ elements –Example: H 2 O; NaCl Chemical bonds join atoms to one another Bonds form between atoms that do not have complete outermost energy levels

Ionic Bonds Ion = an atom that has gained or lost an electron; an atom with a charge Ionic bonds occur when atoms transfer electrons. Metal to Non-metal

Covalent Bonds Covalent bonds are formed between atoms that share electrons; forms molecule Non-metal to non-metal

Covalent Bonds Nonpolar covalent bond –Between atoms with similar electronegatvities –EX: CH 4 Polar covalent bond –Between atoms with different electronegativities –EX: H 2 O

Hydrogen Bonds A weak attraction between oppositely charged atoms of a polar molecule Energy is required to break a bond; releases energy when a bond is formed

Properties of Water Water is a polar molecule Water has special properties because it is polar

Cohesion and Adhesion Cohesion = polar molecules of the same kind to stick to one another Adhesion = attraction between unlike molecules Surface tension = adhesion + cohesion

Temperature moderation Water resist temperature change Cools you down when you sweat (evaporative cooling)

Low density of Ice Ice is less dense than liquid form

Water is a solvent A solution is a uniform mixture of 2 or more substances (solvent dissolves the solute) Water is the solvent in an aqueous solution

Acids and Bases Some substances add ions to an aqueous solution –hydrogen = H + –hydroxide = OH - Acids = add H + to aqueous solutions Bases = removes H + from aqueous solutions by adding OH- Hydrogen ions and hydroxide ions need to be in balance in living things

pH scale Describes how acidic or basic Ranges from 0 (most acidic) to 14 (most basic) –Each unit is a tenfold increase in H+ concentration Pure water has a pH of 7 (neutral); so do most cells

Buffer A buffer is a substance that resists change in pH It removes H+ when there are too many, and donates when there are not enough.