Unit 2:
The Chemical
Context of Life
I. Elements and Compounds
A. Matter – anything
that takes up space
and has mass
B. Element – can’t be
broken down to other
substances by
chemical reactions
1) Essential elements – 20-25% of the 92 elements
needed by organisms to stay healthy and
reproduce
2) Of these essential elements, 96% of living
matter is carbon, hydrogen, oxygen, nitrogen.
3) Ca, P, K, S, and few other elements account for
most of the remaining 4%
4) Trace elements are only required in minute
amounts (Iodine)
C. Compound – 2 or more elements combined
in a fixed ratio
II. Atoms and Molecules
A. Atom – smallest unit of matter that still
retains the properties of an element
1) Nucleus contains positive protons and neutral
neutrons.
2) Negative electrons form a cloud around the
nucleus.
3) The neutron and proton have almost identical
masses
• Atomic # = # of protons (2He); also # of
electrons
• Mass # = protons + neutrons (4He)
• Atomic Mass = mass of atom (in Daltons) =
mass #
4) Isotopes – atom with different # of neutrons
than protons, & therefore different mass
5) Radioactive Isotopes – nucleus decays
spontaneously, giving off particles & energy.
B. Chemical Bonds – sharing or transferring
valence electrons, creating attractions that
hold atoms together
1) Covalent – sharing pairs of valence electrons;
results in molecule
• Single, double, and triple bonds are covalent
• Valence = bonding capacity = # unpaired
electrons
• Electronegativity = attraction for the electrons
in a covalent bond
• Nonpolar C. B. = electrons of covalent bonds
are equally shared by all atoms
• Polar C. B. = an atom has higher
electronegativity, so does not share electrons
equally in covalent bonds.
2) Ionic Bonds – two atoms are so unequal in
electronegativity, the more electronegative
atom strips an electron completely from the
other; transfer of electrons
• Produces ions = charged atom or molecule
• Cations are positive ions and anions are
negative ions.
• Ionic compounds are also called salts.
3) Hydrogen Bonds – a hydrogen atom that is
covalently bonded to an electromagnetic atom
is attracted to another electronegative atom; H
bonds are noncovalent.
4) Van der Waals Interactions – in nonpolar
covalent bonds, there can by instances when a
region of an atom is slightly charged. These
ever-changing “hot spots” allow atoms and
molecules to stick to each other.
III. Chemical Reactions
A. Energy and Reactions
1. Energy must be added to break bonds between
atoms
2. Forms of energy used
• Transferred as heat
• Electricity, Sound, and Light are other forms of
energy
B. Conservation of Energy – the total energy
before a reaction is equal to the total energy
of the products and their surroundings.
1. Chemical energy – energy stored within atoms
and molecules that can be released when a
substance reacts
C. Exothermic reactions – a chemical reaction
that releases energy, also called exergonic
D. Endothermic reactions – a chemical reaction
requiring energy in order to break bonds in
the reactants, also called endergonic
IV. Balancing Chemical Reactions
A. Reactants vs. Products
1. Reactants: substances before reaction
2. Products: substances formed after reaction
Ex: Na + Cl  NaCl
B. Conservation of Mass – matter can be
neither created nor destroyed
C. Balancing Chemical Equations
1. Identify the reactants and products
2. Begin balancing one element at a time
3. Add numbers (coefficients) in front of
chemical formulas (Don’t change chemical
formulas)
4. Continue going back and forth balancing each
individual element
5. Make a final count of each individual element
before going on