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– 1st 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: H2O; 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: CH4

• Polar covalent bond– Between atoms with different

electronegativities– EX: H2O

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.