Chapter 2 Notes
The Chemical
Context of Life
Concept 2.1
Organisms are composed of matter:
anything that takes up space or has
mass
Element: a substance that cannot be
broken down by chemical reactions
Compound: substance consisting of two
or more elements in a fixed ratio
Concept 2.1
Life requires about 25 elements
4 of those make up 96% of living matter
Carbon, Oxygen, Hydrogen, and
Nitrogen
Trace Elements are those that are
required in only minute quantities
-ex. Iron, iodine
Concept 2.1
Concept 2.2
Atom: smallest unit of matter that still
retains the properties of an element
Subatomic particles: Protons (+),
Neutrons (0), Electrons (-)
Protons and Neutrons have a mass of 1
dalton. Electrons have no mass
Concept 2.2
Atomic number: # of protons
Mass number: sum of protons +
neutrons
Isotopes: different atomic forms of an
element.
-ex. Carbon-12 (99%), Carbon-13
(1%), Carbon-14 (<1%)
Concept 2.2
C-12 and C-13 are stable. C-14 is
unstable, and radioactive. It will decay
giving off particles and energy.
Carbon-14 will decay into Nitrogen
Concept 2.2
An atom’s electrons vary in the amount of
energy they possess.
Energy: the ability to do work.
Potential Energy: energy that matter
stores because of its position or location
Electrons have potential energy because
of their position in relation to the
nucleus.
Concept 2.2
(a) A ball bouncing down a flight
of stairs provides an analogy
for energy levels of electrons
Third shell (highest energy
level)
Energy
absorbed
Second shell (higher
energy level)
First shell (lowest energy
level)
Atomic
nucleus
Energy
lost
Concept 2.2
The different states of potential energy
that electrons have in an atom are
called energy levels or electron shells.
- the first shell has the lowest energy.
The second shell has more than the
first, etc.
Valence electrons: those in the
outermost shell
Concept 2.2
Hydrogen
1H
Atomic mass
First
shell
2
He
4.00
Atomic number
Helium
2He
Element symbol
Electrondistribution
diagram
Lithium
3Li
Beryllium
4Be
Boron
5B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Sodium Magnesium Aluminum
12Mg
11Na
13Al
Third
shell
Concept 2.3
Atoms will bond with others to gain
stability
Covalent bonds: sharing of a pair of
valence e- by two atoms
-ex. Hydrogen atoms will share their
electrons. They become H-H
Concept 2.3
Concept 2.3
Electronegativity: the attraction of an
atom for the electrons of a covalent
bond
Nonpolar: when the electrons are
shared equally
Polar: when one atom is bonded to a
more electronegative atom
Concept 2.3
–
O
H
H
+
H2O
+
Concept 2.3
Ionic bonds: when two atoms are so
unequal in their attraction for electrons
that one atom will strip the electrons
from its partner
Ion: a charged atom; cation has a
positive charge, anion has a negative
charge (ca+ion; a negative ion)
Compounds formed by ionic bonds are
salts
Concept 2.3
Na
Cl
Na
Sodium atom
Cl
Chlorine atom
Concept 2.3
Na
Cl
Na
Cl
Na
Sodium atom
Cl
Chlorine atom
Na+
Sodium ion
(a cation)
Cl–
Chloride ion
(an anion)
Sodium chloride (NaCl)
Concept 2.3
Na+
Cl–
Concept 2.3
The advantage of weak bonding is that
the contact can be brief
Hydrogen bonds: H is covalently
bonded to an electronegative atom and
attracted to another electronegative
atom.
-ex. Water and ammonia
Concept 2.3

+
Water
(H2O)
+
Hydrogen
bond

Ammonia
(NH3)
+
+
+
Concept 2.4
Chemical reactions: making and
breaking of chemical bonds.
- starting material is reactants
- ending material is products
Chemical equilibrium: point at which
reactions offset one another.
Concept 2.4