The Chemical Context of Life
Chapter 2
MATTER CONSISTS OF CHEMICAL ELEMENTS
AND COMBINATIONS CALLED COMPOUNDS
Elements and Compounds
• Matter is made up of elements
• Elements cannot be broken down any further by
normal chemical reactions
– 92 natural elements recognized by chemists
– Gold (Au), Copper (Cu), Carbon (C), Oxygen (O)
• Compounds are two or more elements combined
in a fixed ratio
– Table salt (NaCl), Water (H2O)
– Properties of compounds are different than its
individual elements
The Elements of Life
• C, H, O, and N makes up
96% of all living matter
• Essential Elements are
required by all organisms
• Trace Elements
– Only required in minute
quantities
– Often used as Cofactors for
Enzymes
Check Your Understanding
• Differentiate between an element and a compound.
Provide examples of each.
• Which elements make up 96% of all living matter?
• Differentiate between an essential element and a
trace element. Provide example of each. Why are
trace elements still important within an organisms
diet?
AN ELEMENT’S PROPERTIES DEPEND
ON THE STRUCTURE OF ITS ATOMS
Subatomic Particles
• Atomic Nucleus
– Protons (positive charge;
mass of 1 Dalton)
– Neutrons (no charge;
mass of 1 Dalton)
• Orbitals are where an
atom’s electrons are
found 90% of the time
– Electrons (negative
charge; no mass)
Helium
Atomic Mass and Atomic Number
• Atomic Number denotes
the number of protons
– Neutral (uncharged) atoms
have equal number of
protons and electrons
• Mass Number denotes the
number of protons and
neutrons in the nucleus
– Atomic Mass ≈ Mass Number
Isotopes
• Possess more neutrons than
normally present
– Alters the atomic mass
– Many isotopes are stable
• Radioactive Isotopes are
inherently unstable
– Gives off particles and energy
– Often used to tag proteins, to
track the movement of fluids
in the body, to identify tissues
(PET scan)
Energy Levels
• Energy is the capacity to
cause change
– Potential Energy is a type of
energy due to position or
structure
• Electrons have potential
energy due to distance from
atomic nucleus
– Electron Shells
Electron Distribution
• Valence Electrons are the
outermost electrons in an
atom’s orbital
– Valence Number is 2 for the
first orbital and 8 for the
second and third orbital
• Valence Shell is the outermost
orbital of a particular atom
• An atom’s chemical behavior is
determined by the
interactions of its valence
electrons
Check Your Understanding
• Differentiate between a proton, a neutron, and an electron.
• Differentiate between atomic number and atomic mass. How
do you determine an atoms number of protons, electrons,
and neutrons if given only those two pieces of information?
• What is an isotope? What are they commonly used for in
medicine?
• What is meant by the valence number of an atom? What is
the valence number of electrons in the first, second, and third
electron orbitals? Why is the valence number important to an
atom?
THE FORMATION AND FUNCTION OF
MOLECULES DEPENDS ON CHEMICAL
BONDS BETWEEN ATOMS
Chemical Bonds
• Attractions that keep atoms close together
– Due to interactions between electrons of neighboring
atoms
• Strongest chemical bonds in Aqueous solutions
are Covalent Bonds
– Typical of biological systems
• Ionic Bonds are generally strong when NOT
present in aqueous solutions
Electronegativity
• Strength of attraction for a
given atom to the electrons
in a chemical bond
• Determines if electrons are
shared, how they are
shared, or if they are
transferred completely
Covalent Bonds
• Sharing of a pair of valence
electrons between 2 atoms
– Single Bonds
– Double Bonds
• Molecules are created by a
covalent bond between 2 or
more atoms
Types of Covalent Bonds
Nonpolar Covalent
Polar Covalent
Equal Sharing
Unequal Sharing
Ionic Bonds
• One atom is so electronegative, it
takes an electron from its partner
• Result is one negatively-charged
and one positively-charged atom
(Ion)
– Cations are positively charged
– Anions are negatively charged
• Resulting ions are held together
by electrostatic interactions
• Often results in salts such as
NaCl, KCl, MgCl2
Weak Chemical Bonds
• Hydrogen Bonds occur
between polar molecules
– Hydrogen of one is attracted
to the electronegative atom
of another
• van der Waals Interactions
are weak bonds based on
the close proximity of
molecules
– Can be significant as the
number of interacting
molecules increases
Molecular Shape and Function
• Shape is determined by
the way atoms bond to
each other
• Shape determines how
molecules interact with
other molecules
– Determines a molecule’s
function
Check Your Understanding
• Define electronegativity and describe how it determines the
type of bonds an atom can form.
• Differentiate between a covalent and an ionic bond.
• Differentiate between a polar covalent and a nonpolar
covalent bond.
• Differentiate between a cation and an anion.
• What is a hydrogen bond and how does it differ from both
covalent and ionic bonds?
• How do the chemicals endorphin and morphine illustrate the
principle of “form fits function” even at the molecular level?
CHEMICAL REACTIONS MAKE AND
BREAK CHEMICAL BONDS
Chemical Reactions
• Making and breaking of
chemical bonds
– Reactants are the starting
materials
– Products are the end
materials
• Many biological reactions
are Reversible
• Most reactions occur until
an Equilibrium is reached
Reactants
Products