Chapter 6
Molecules and Compounds
Physical and Chemical Properties
Physical properties:
distinguishes one kind of matter from
another
they can be observed or measured
without changing a substance into another
substance
example: boiling point, color, odor,
hardness, density, solubility, and electrical
conductivity
Physical and chemical properties
Chemical properties:
describes how a substance unites with
another substance to form a new substance
two substance unite or two substances break
apart
new substances from a chemical change
have new physical and chemical properties
example: rusting, burning
Elements and Compounds
Elements are composed of
only one type of
atom…they are therefore
the most basic
component of the
physical world.
Several elements exist
naturally as molecules –
groups of two or more
atoms linked by chemical
bonds - an example of
this is oxygen
Elements and Compounds
Compounds are composed of different types of
atoms linked together – compounds are
combinations of two or more kinds of elements
Example: water (H2O) Composed of two
hydrogen atoms and one oxygen atom
Compounds can be simple (like water) or complex
(like sugar – C12H22O11)
Elements and Compounds
Molecular mass:
the sum of all the masses of all the
atomic masses (atomic mass units –
amus) in the compound
So…what is the molecular mass of sugar?
Chemical Formulas
• Molecular formula:
gives the name and number of each
atom in the molecule
example: H2O
• Structural formula:
Shows the
arrangement of the
atoms in the molecule
Chemical Formulas
• Empirical formula:
shows only the simplest ratio of the
atoms in the molecule
one empirical formula can represent
dozens of different molecules
Chemical Bonding with Electrons
When two elements join, their chemical
union determines the resulting
characteristics of the compound.
Bonding is the joining together of
elements…
it involves mainly the Valence
Electrons.
Chemical Bonding with Electrons
With the exception of some (i.e. hydrogen)
atoms are generally most stable when
they have a full octet (8 electrons) in their
valence level.
Chemical Bonding with Electrons
Graphite and diamonds
look very different and
yet are both made of
carbon!
Chemical Bonding with Electrons
They illustrate the fact
that the same
element can often
bond in more than
one way.
Chemical Bonding with Elements
Atoms fill their valence by bonding with other
atoms to form compounds.
Atoms react in several ways to achieve this
greater stability…
Chemical Bonding with Elements
Atoms fill their valence by bonding with other
atoms to form compounds.
Atoms react in several ways to achieve this
greater stability…
Chemical Bonding with Electrons
Two ways they achieve stability:
1. Atoms will gain or lose electrons.
Some atoms forcibly remove
electrons from other atoms.
The atom that loses
electrons is now stable and the atom that
gained is now stable.
Chemical Bonding with Electrons
2. Atoms will share electrons.
The goal is the same: a
more stable atom with an octet of
electrons in their valence shells.
Types of Bonding
1. Covalent Bonding
Two atoms have electronegativity
close to the same.
Also called: Electron Sharing
2. Ionic Bonding
One atom has a much greater
electronegativity than another
The electron is taken away!
Ionic Bonding
Formed when one or more electrons are
transferred from one atom to another.
When an atom gains or loses an electron it
is no longer neutral…
it is an Ion.
Ions
If an atom has more
electrons than
protons… it is an
anion (negative
charge)
If it has more protons
than electrons…it is a
cation (positive
charge)
Forces between Molecules
• There are forces between molecules
called intermolecular forces.
• The forces of adhesion and cohesion
between the molecules that bind them
• Differing substances have differing
molecular forces – at room temp.
molecules of water do not stick together as
well as water molecules at zero degrees.
Types of Intermolecular Forces
There are three types of intermolecular
forces:
1. Dipolar forces
2. London forces
3. Hydrogen forces
Dipolar forces
• Dipoles are molecules that have positively
and negatively charged ends due to the
unequal sharing of electrons.