Chapter 2:
The Chemical Context
of Life
Matter
 Anything
that has mass and
occupies space.
Atom
 Smallest
particle of an element
(still retains elemental properties)
Element
Matter made up of only one type of
atom.
 92 natural elements.
 Each element has a symbol.

Compound
 Compound
- Elements combined
in fixed ratios.
 A compound has characteristics
beyond those of its combined
elements.
Question?
What elements are necessary for life?
 Life requires about 25 chemical
elements.

Macroelements
Elements needed in large amounts or
quantities.
 Macro = Large
 Examples:
C HOPKNS CaFe Mg NaCl

Microelements
Elements needed in very small quantities.
 Also known as trace elements.
 Micro = small
 Examples:
Cu, Co, Zn, Mo, I, Mn

Atomic Subparticles
Protons
+ charge, 1 Dalton mass
 Neutrons
no charge, 1 Dalton mass
 Electrons
- charge, essentially no mass

Atomic Model
Atomic Number
The number of protons in the nucleus.
 Each element has its own atomic
number.

 If you change the atomic number, you no
longer have the same element.
Atomic Mass
The number of protons and neutrons in
the nucleus.
 The atomic mass can change.

Isotopes
Atoms of the same element with different
atomic mass
 Caused by changes in the number of neutrons
 Used as “tracers”, used to kill cancer/bacteria
cells, used to determine age of
fossils/geological formations

Types of Isotopes
Radioactive



Where the nucleus decays spontaneously, giving
off particles and energy.
Heavy
 Has a stable nucleus, but masses more
than the standard isotope for the element.
Energy
The ability to do work
 THINK = ATP
 ATP is how living organisms have the
ability to do work

 They USE ATP!!!
Potential Energy
 Is
the energy that matter stores
simply because of its position or
location
 Electrons have potential energy
because of their position relative
to the nucleus
Electron Energy Levels
 Energy
levels around the nucleus
of an atom
 1st level can have 2 electrons and
has the lowest potential energy
 Other levels can hold more than 2
electrons and have higher energy
levels
Electron Orbitals
 The
three dimensional space
where an electron is found 90%
of the time.
 Different orbitals have different
shapes.
 Each orbital can hold only 2
electrons.
Electron Orbitals
Chemical Behavior Of An Atom
Is determined by its electron
configuration in the energy levels and
orbitals
 This determines who is can bond with (if
anyone!)

Valence Electrons
The electrons in the outermost energy
level
 Electrons available chemical bonds
 Atoms/Elements with same # of valence
electrons will react similarly and will
have similar characteristics

Octet Rule
 The
most stable condition is to have
an outer level of 8 electrons
 Exception - 1st level is stable with
only 2 electrons
 When stable - no chemical reactions
will take place
 Ex: Ne, He, (Noble gases)
Electrons of the first elements
Chemical Bonds
Forces that join atoms together to form
molecules
 Usually caused by sharing or
transferring valence electrons

Bond Formation Depends On:
 The
number of valence electrons
that must be gained, lost, or
shared to reach the stable
condition.
Chemical Bond Types
Nonpolar Covalent
 Polar Covalent
 Ionic
 Hydrogen
 van der Waals forces

Electronegativity
 The
attractiveness of a specific kind of
atom towards e- in another atom
 Important in covalent bonds

Periodic Table trend:
 More electronegativity = stronger pull of e He has highest EN
 Increases as you go right and up the table
Nonpolar Covalent
When electrons are shared equally
between atoms
 Very strong bond
 Important in many molecules found in
living things
 Ex: carbon to hydrogen, hydrogen to
hydrogen, oxygen to oxygen

Nonpolar Covalent
Can be single, double, or triple between
two atoms
 Each nonpolar covalent bond involves a
pair of electrons

Polar Covalent
When electrons are shared unequally
between atoms
 Results in “polar” molecules that have
charged areas
 Use δ symbol
 Ex: Water, H to O bonds

Ionic Bonds
Formed when electrons are transferred
from one atom to another and ions are
formed
 Ex: NaCl


Why? Two
atoms electronegativity
are SO different that one atom
gains e- completely
Two Types of Ions

Cations - have
lost electrons (p+ > e-)
giving them a positive charge.
 Ex: Sodium (Na+)

Anions - have
gained electrons (p+ <
e-) giving them a negative charge.
 Ex: Chlorine (Cl-)
Ionic Bonds
Formed when cations (+) and anions (-)
attract each other
 Weak chemical bond

 Why? Environment easily affects strength of
this bond
 Ex: Salt  Solid in air/gas; Dissolves in
liquid
Ionic
Hydrogen Bonds
When a hydrogen atom bonded to one
molecule is attracted to the slightly
negative area (often N or O) of another
molecule
 Very weak individual bond

 Can be a “strong” force if there are many H
bonds.
Hydrogen bonds

Remember:
 H bonds occur BETWEEN MOLECULES
(not b/t atoms within ONE molecule)

Ex: H bonds hold water molecules
together
Hydrogen Bonds
Van der Waals
Result of e- ability to move at high speeds
 Creates “spots” where there are “pools” of
+ and – charges
 Weak chemical “bond”
 Ex: gecko’s feet

Bond
type
B/t atoms
w/in ONE
molecule?
B/t more
than one
different
molecules?
Weak or
Strong
Polar
covalent
Nonpolar
covalent
Yes
No
Strong
Yes
No
Strong
Ionic
Yes
No
Weak
Hydrogen
No
Yes (attraction
of H in one to atom in another)
Weak (unless
LARGE #)
Van der
Waals
No
Yes
Weak
Molecular Shape
 Each
molecule on Earth has a
characteristic shape
 Determined by the positions of the
atom’s orbitals

Shape related to function
Molecular Shape
 Molecular
shape is crucial because
it determines how most molecules
of life recognize and respond to
one another.
 Ex:
 Viruses (reproduction), bacteria
(reproduction), hormones/cell
recognition
Chemical Reactions
The making and breaking of chemical
bonds
 Reactions do not destroy matter, they
only rearrange it

Chemical Equations

A way to represent what is happening in
a chemical reaction
Ex: 2 H2 + O2
2 H2O
Parts of the Equation
Reactants:
- the starting materials
 Products:
- the ending materials
 Note - all atoms of the reactants must

be accounted for in the products
2 H2 + O2
2 H2O
Chemical Equilibrium
When the conversion of reactants to
products is balanced to the reverse
reaction
Ex:

3 H2 + N2
2 NH3
Chemical equilibrium
Reversible rxn
 When concentrations of react and prod
STOPS changing

 Doesn’t necessarily mean concentrations
are equal!!!!!

Rxn still continues
Summary
We will now put elements together to
form molecules and build the next level
in the hierarchy
 Ch 3, 4, 5 (Properties of Water and
Macromolecules)

Summary Continued
Recognize macro-elements and micro-elements
and their roles in biological organisms.
 Differentiate between elements and compounds.
 Identify the basic principles of atomic structure and
how they determine the behavior of an element.
 Identify the main types of chemical bonds.
 Discuss the relative strength of different types of
chemical bonds.
 Recognize that chemical reactions make and
break chemical bonds.
