Ch 2 The chemical
context of life
1.
2.
3.
4.
5.
6.
7.
Chemistry Definitions
Essential Elements of Life
Subatomic particles
Isotopes
Electrons
Bonds
Chemical Reaction
Bombardier Beetle
Definitions
1.
Matter
2.
Elements
3.
Compounds
Compound
*Has characteristics different from those
of its elements (covalent bonds)
+
Sodium
Chloride
Sodium Chloride
Essential Elements of Life

Include
– Carbon, hydrogen, oxygen, and nitrogen
– Diatomic elements
– Make up 96% of living matter
Trace Elements

Essential to life
Required in minute quantities
 Frequent cofactors to enhance enzyme
activities


Make up remaining 4% of living
matter
Trace Elements
Basic Structures of Matter

Each element
– Has certain kind of atom that is different
from those of other elements

An atom
– Smallest unit of matter that still retains
the properties of an element
Subatomic particles
Smaller parts of an atom
3 types (charge & location):
1. Neutrons
2. Protons
3. Electrons
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
(a) This model represents the
electrons as a cloud of
negative charge, as if we had
taken many snapshots of the 2
electrons over time, with each
dot representing an electron‘s
position at one point in time.
(b) In this even more simplified
model, the electrons are
shown as two small blue
spheres on a circle around the
nucleus.
Atomic Number

Atoms of the various elements
Differ in their number of subatomic
particles
*Atomic number of an element
1. is the number of protons
2. unique to each element
Atomic Mass

Mass number of an element:
1. Sum of protons and neutrons in the
nucleus of an atom
2. Is only an approximation of the atomic
mass of an atom
Isotopes

Atoms of a given element:
1. May occur in different form
2. Differ in the number of neutrons
3. Have the same number of protons
Radioactive Isotopes



Spontaneously give off particles and
energy
Have more neutrons than atomic
nucleus can accommodate
Undergoes spontaneous
decomposition
Types of Radioactive
Emission
1.
Alpha particle
2.
Beta particle
3.
Gamma radiation
Radioactive Decay


Occurs at constant rate, specific to
radioactive isotope
Rate is called half life
– Time required for quantity of radioactive
substance to be reduced by ½
– Ex. 10g to 5g to 2.5g
Electrons





Located outside the atomic nucleus in
cloud
Mass in negligible (9.1 x 10-28 amu)
Electrical Charge
Function = neutralize proton charge
Directly participates in formation of
chemical bonds
Electron details


Location determination depends on
energy level & spin
Energy Level
– Average distance from atomic nucleus =
shell
Electrons in Shell
Formula
2n2= #electrons in shell
Shell rules

Named as numbers (1,2,3..)

Lower number = closer to nucleus

Lower number = lower potential
energy of electrons

Electrons differ in amounts of PE
Figure 2.7A
(a) A ball bouncing down a flight
of stairs provides an analogy
for energy levels of electrons,
because the ball can only rest
on each step, not between
steps.
Third energy level (shell)
Second energy level (shell)
Energy
absorbed
First energy level (shell)
Energy
lost
Atomic
nucleus
Figure 2.7B
(b) An electron can move from one level to another only if the energy
it gains or loses is exactly equal to the difference in energy between
the two levels. Arrows indicate some of the step-wise changes in
potential energy that are possible.
Orbitals

Region in space in which the electron
has a high probability of being found
at any given time
Orbital Type
1.
2.
3.
4.
S
P
D
F
Electron orbitals.
Each orbital holds
up to two electrons.
x
Y
Z
1s orbital
2s orbital
Three 2p orbitals
1s, 2s, and 2p orbitals
Electron-shell diagrams.
Each shell is shown with
its maximum number of
electrons, grouped in pairs.
Figure 2.9
(a) First shell
(maximum
2 electrons)
(b) Second shell
(maximum
8 electrons)
(c) Neon, with two filled shells
(10 electrons)
Electron Locations
Rules:
1. electrons are always located in
lowest available energy location
2. Lowest energy shell = 1
3. Lowest shape orbital =1
Examples
1.
1H
2.
2He
3.
3Li
4.
6C
5.
7N
6.
8O
Chemical Behavior of
Atom

Defined by its electron configuration
and distribution
Atomic number Helium
Hydrogen
1H
Atomic mass
First
shell
2
He
4.00
2He
Element symbol
Electron-shell
diagram
Lithium
3Li
Beryllium
4Be
Boron
3B
Carbon
6C
Nitrogen
7N
Oxygen Fluorine
8O
9F
Neon
10Ne
Second
shell
Sodium Magnesium Aluminum Silicon Phosphorus Sulfur
13Al
16S
11Na
12Mg
14Si
15P
Third
shell
Figure 2.8
Chlorine
17Cl
Argon
18Ar
Valence Electrons



Valence = outermost shell
Determine the chemical behavior of an
atom
Responsible for forming chemical
bonds between atoms