The Chemical Context of
Life
Chapter 2
Biology – Campbell • Reece
Chemical Elements and
Compounds

Matter consists of chemical elements in pure
form and in combinations called compounds


Element – a substance that cannot be broken
down to other substances by chemical reactions


What is matter?
Ex. Oxygen (O), Nitrogen (N), Sodium (Na)
Compound – a substance consisting of two or
more different elements combined in a fixed
ratio

Ex. NaCl, H2O
Figure 2.2 The emergent properties of a compound
+
→
Table 2.1 Naturally Occurring Elements in the Human Body

25 of the 92 natural
elements are known
to be essential for life
 C, H, O, N make up
96% of living matter
 P, S, Ca, K and a few
others make up the
other 4%
 Trace elements –
those required in
minute quantities
Figure 2.4 Goiter
Atoms

Atom – smallest unit of matter that retains the
properties of an element


Neutrons – neutral charge (n0)
Protons – positive charge (p+)
• Neutrons and protons are found in the nucleus

Electrons – negative charge (e-)
• Move in a cloud around the nucleus
Atomic Number & Atomic Mass
 Neutrons
and protons have a mass of 1
dalton (same as atomic mass unit)
 Atomic number – number of protons

2He
number – sum of the protons and
neutrons
 Mass

4He
mass – mass of an atom (same
as the mass number)
 Atomic
Isotopes
 All
atoms of an element have the same
number of protons
 Isotopes – have a different number of
neutrons (and therefore weigh more)

Carbon-12, Carbon-13, Carbon-14
isotopes – one in which the
nucleus decays spontaneously, giving off
particles and energy
 Radioactive

Used in dating fossils, as tracers in medicine
Figure 2.7 A PET scan, a medical use for radioactive isotopes
Energy Levels of Electrons
An atom’s electrons
vary in the amount of
energy they possess
 The different stages
of energy that
electrons have are
called energy levels,
or electron shells


Electrons in the shell
closest to the nucleus
have the lowest
energy
Electron Configuration
 The
chemical behavior of an atom is
determined by its electron configuration
 Valence electrons – the electrons in the
outermost shell (valence shell)
 An element is stable when its valence
shell is full
Electron Orbitals
– the three-dimensional space where
an electron is found 90% of the time
 Orbital

Each orbital can hold 2 e-
 1s

orbital is spherical in shape (1)
1st shell holds 2 e-
 2s
orbital is also spherical (1)
 2p orbitals are dumbbell-shaped (3)

2nd shell holds 8 e-
Figure 2.11 Electron orbitals
Covalent Bonds
 The
sharing of a pair of valence electrons
by two atoms


Single covalent bond – one pair is shared
Double covalent bond – two pairs are shared
– two or more atoms held
together by covalent bonds
 Molecule



Structural formula – H-H
Lewis dot structure – H:H
Molecular formula – H2
– a combination of two or more
different elements
 Compound
Nonpolar & Polar Covalent Bonds
 The
attraction of an atom for the electrons
of a covalent bond is called its
electronegativity
 Nonpolar covalent bond – the electrons
are shared equally

Ex. O2
covalent bond – the electrons are
not shared equally
 Polar

Ex. H2O
Figure 2.13 Polar covalent bonds in a water molecule
Ionic Bonds
 Sometimes
two atoms are so unequal in
their electronegativity that the more
electronegative atoms takes the electron
forming an ion
 Cation – a positive ion (Na+)
 Anion – a negative ion (Cl-)
 The attraction between the opposite
charges forms an ionic bond
 Compounds formed by ionic bonds are
NOT molecules
Other Types of Bonds


Hydrogen bonds – form
when a hydrogen atom
covalently bonded to one
electronegative atom is
also attracted to another
electronegative atom
(weak)
Van der Waals
Interactions – attractions
formed by the everchanging “hot spots” of
positive and negative
charges (as the electrons
move around)
Figure 2.17 Molecular shapes due to hybrid orbitals
Molecular shape
is crucial in
Biology because it
determines how
most biological
molecules
recognize and
respond to one
another.
Chemical Reactions
 The
making and breaking of chemical
bonds, leading to changes in the
composition of matter
 Some reactions go to completion, but most
are reversible


3H2 + N2 ↔ 2NH3
Chemical equilibrium – the relative
concentrations of products and reactants stop
changing
Unnumbered Figure (Page 38) Chemical reaction between hydrogen and oxygen