The Chemical Context of Life
Chapter 2
The Chemical Context of Life

What is an element?
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
What is a compound?


The basic form of matter that cannot be broken down
Two or more different elements combined in a fixed ratio
What is an atom?

The smallest unit of an element that still has the properties of
that element.
Subatomic particles

neutrons




protons




no charge
found in the nucleus
mass of 1 amu
positive charge
found in the nucleus
mass of 1 amu
electrons



negative charge
found outside the nucleus
little mass
Characteristics of Elements



Atomic number: Number of protons in the nucleus of
an atom
Mass number: Number of protons plus the number of
neutrons in the nucleus
Isotope: An element that has a different mass than
normal due to a change in the normal number of
neutrons
Electrons




Located outside of the nucleus
Unless otherwise indicated the number of electrons is
the same as the number of protons
Potential energy of electrons increases as their distance
from the nucleus increases
Electrons orbit the nucleus in energy levels
Electron Energy Levels
Electron Energy Levels

The first energy level contains how many electrons?


The second energy level contains how many
electrons?


2
8
The third energy level contains how many electrons?

8
An element whose valence shell is complete is said to
be what?

Inert or unreactive
Electron Orbitals
Electron Configurations of First 18
Elements
Chemical Bonds

Covalent bond


Non-Polar Covalent bond


Sharing of a pair of valence electrons by two atoms
Equal sharing of electrons
Polar Covalent bond

Unequal sharing of electrons

What sort of
covalent bonds
are seen here?
Ionic Bonds

Ion - transfer of electrons between atoms

Ionic bond – attraction between two oppositely charged
ions

Cation – positively charged ion (lost electron)

Anion – negatively charged ion (gained electron)
Hydrogen Bonds



Oxygen is one of the most electronegative elements
In a water molecule electronegative oxygen “hogs”
electrons.
This “hogging” of electrons results in which type of bond
between oxygen and hydrogen within the water
molecule?

Polar covalent
Hydrogen Bond


Hydrogen bonds occur
between two polar
molecules, or between
different polar regions
of one large macromolecule.
One “relatively”
negative region is
attracted to a second
“relatively” positive
region.
Van der Waals Interactions


Weak attractions that occur when atoms and molecules
are very close together
May occur in regions of a single large molecule such as a
protein
Shape and function



Molecular shape determines how molecules recognize
and respond to one another
Molecules with shapes similar to brain signal molecules
can affect mood and pain perception
Ex: morphine and heroin mimic endorphins in the brain
Chemical Reactions

Starting materials are known as


Ending materials are known as



reactants
products
Matter can only be ______________ not created or
______________
When forward and reverse reactions occur at the same
rate, ______________ has been established
Ionic bond, cation, anion