A.P. BIOLOGY
CHAPTER 2: THE CHEMICAL CONTEXT OF LIFE
I. MATTER CONSISTS OF CHEMICAL ELEMENTS IN PURE FORM AND IN
COMBINATIONS CALLED COMPOUNDS.
Matter - anything that takes up space and has mass. Early Greeks believed in four forms of
matter: air, water, fire, and earth. Examples of matter include rocks, metals, oils, gases,
and students
Mass - a measure of the amount of matter an object contains (this is not to be confused with
weight)
All matter is made up of Elements, which are substances that cannot be broken down to
other substances by chemical reactions.
Compound - a substance consisting of two or more elements combined in a fixed ratio.
NaCl is a common example.
A. Essential Elements of Life
-All matter is made up of elements
-There are 92 naturally occurring elements
-Elements are designated by a symbol of one or two letters, ex. Na
-About 25 elements are essential to life
-4 elements make up 96% of living matter
C = Carbon
O = Oxygen
H = Hydrogen
N = Nitrogen
-Calcium (Ca), Phosphorus (P), Potassium (K), Sulfur (S), Sodium (Na),
Chlorine (Cl), Magnesium (Mg), and trace elements make up the remaining 4% of an
organism’s mass.
-Trace Element- an element required by an organism in extremely minute quantities.
Examples include Iodine and Iron. These elements are still essential for life.
II. AN ELEMENT’S PROPERTIES DEPEND ON THE STRUCTURE OF ITS ATOMS
Atom- The smallest possible unit of matter that retains the physical and chemical
properties of its element. They are incredibly small!
A. Subatomic Particles
-Atoms are made up of subatomic particles
-The three most stable subatomic particles are:
a. Neutrons (no charge=neutral)
b. Protons (+1 electrostatic charge)
c. Electrons (-1 electrostatic charge)
-Neutrons and protons form a dense core called the atomic nucleus, while
electrons form a cloud around the nucleus.
-Atoms and subatomic particles have such a small mass that they are measured in
units called daltons (amu). Neutrons and Protons have a mass close to one dalton.
B. Atomic Number and Atomic Mass
a. Atomic number=the number of protons in an atom of a particular element.
-Ex. 11Na has 11 protons
-In a neutral atom, # of protons = # of electrons
b. Mass number = the sum of the protons and neutrons in the nucleus of an atom
-Can find the # of neutrons by subtracting the atomic number from the mass
number
-Ex 23Na still has 11 protons, 11 electrons and has 12 neutrons
c. Atomic mass-the weighted mean of the masses of an element’s isotopes. Based
upon the number of neutrons and protons.
C. Isotopes
Isotopes are atoms of an element that have the same atomic # but a different mass
number
-They have the same # of protons, but a different # of neutrons
-Some isotopes are radioactive
a. Radioactive isotope = unstable isotope in which the nucleus spontaneously decays,
emitting subatomic particles and/or energy as radioactivity.
-Has a fixed ‘half-life’
b. Half-life=Time for 50% of radioactive atoms in a sample to decay.
-Radioactive isotopes can be used for
1. Dating geological strata and fossils a.k.a. “Carbon dating”
2. Radioactive tracers
3. Treatment of cancer
D. The Energy Levels of Electrons
-An atom’s electrons vary in the amount of energy they possess.
-Energy is defined as the capacity to cause change (like by doing work)
-Potential energy is the energy that matter possesses because of its location or structure.
(Think of the gates on a dam)
-The further electrons are from the nucleus, the more potential energy they possess.
-The different states of potential energy that electrons have in an atom are called energy
levels. The average distance of an electron from the nucleus is represented symbolically
by electron shells.
E. Electron Orbitals
-An Orbital is the three-dimensional space where an electron is found 90% of the time.
III. THE FORMATION AND FUNCTION OF MOLECULES DEPEND ON CHEMICAL
BONDING BETWEEN ATOMS.
Atoms combine by chemical bonding to form molecules
Chemical Bond = Attractions that hold molecules together
Molecules = Two or more atoms held together by covalent bonds
A. Covalent bonds
1. Covalent Bonds=Chemical bond between atoms formed by sharing a pair of
valence electrons.
-Structural formula for a Hydrogen Molecule is H-H
-Molecular formula for a Hydrogen Molecule is H2
-Single covalent bond-atoms that share a single pair of valence electrons
-Double covalent bond-atoms that share two pairs of valence electrons
-Triple covalent bond-atoms that share three pairs of valence electrons
2. Nonpolar and polar covalent bonds
-Electronegativity-The ability of an atom to attract electrons
-The more electronegative an atom, the more strongly it attracts shared
Electrons
-Nonpolar covalent bond=Covalent bond formed by an equal sharing of
electrons between atoms.
a. Occurs when Electronegativity of both atoms is about the same
b. Molecules made of one element usually have Nonpolar covalent bonds
-Polar covalent bond=Covalent bond formed by an unequal sharing of
electrons between atoms. ****Example: Water****
a. Occurs when the atoms involved have different electronegativities
b. Shared electrons spend more time around the more electronegative atom
B. Ionic Bonds
-Ionic Bonds=Bonds formed by the electrostatic attraction after the complete
transfer of an electron from a donor atom to an acceptor
a. Ion = charged atom or molecule
b. Anion = an atom that has gained one or more electrons and has become negatively
charged
c. Cation= an atom that has lost one or more electrons and has become positively
charged.
-Ionic Compounds (salts) are compounds formed by ionic bonds. (See Figure 2.13)
C. Weak chemical bonds
-Weak chemical bonds are critical to Biology.
1. Hydrogen bond=Bond formed by the charge attraction when a hydrogen atom
covalently bonded to one electronegative atom is attracted to another electronegative
atom.
-Weak attractive force
-Opposite portions of polar molecules attract each other
-WATER
2. Van der Waals interactions = weak interactions that occur between atoms and molecules
that are very close together and result from charge asymmetry in electron clouds.
D. Molecular Shape and Functions
-molecular shape is crucial because it determines how most molecules of
life recognize and respond to one another.
-molecules with only two atoms are linear
-molecules with more than two atoms have more complex shapes
IV. CHEMICAL REACTIONS MAKE AND BREAK CHEMICAL BONDS
Chemical reactions = process of making and breaking chemical bonds
leading to changes in the composition of matter.
-Process where reactants undergo changes into products
-Matter is constantly being rearranged! Think Photosynthesis!
Chemical equilibrium= equilibrium established when the rate of forward
reaction equals the rate of the reverse reaction