Molecules and
Compounds
Chapter 19
What we will Learn
• Relate chemical behavior of an element, including bonding,
to its placement on the periodic table
•Explain how elements form chemical bonds and identify the
role of electrons in bonding
•Predict chemical formulas of compounds made up of two
different elements
•Write chemical formulas for compounds made up of many
different elements
•Calculate the formula mass of a compound and compare
different compounds based on their formula masses
The Octet Rule
When an atom has eight valence
electrons, it is said to have an octet
of electrons.
Atoms form chemical bonds with
other atoms by either sharing
electrons, or transferring them in
order to complete their octet and
move to a lower energy state.
This is known as the octet rule.
Predicting Atomic
Behavior
Looses 1
electron to
satisfy Octet
Rule
Octet Rule
Satisfied
Looses 1
electron to
satisfy Octet
Rule
Types of Bonds
Chemical Bonds fall into two categories,
depending on whether the valence electrons
are transferred or shared
Ionic Bonds: When electron(s)
are transferred from one atom
to the other.
Covalent Bonds: When
electron(s) are shared between
atoms.
Ionic Bonds
Atoms that either gain or lose an
electron become ions. Ions may have
either positive or negative electric
charge.
The positive and negative ions
are attracted to each other,
creating the bond. Ionic bonds
tend to form between more than
one pair of atoms at a time. The
bond between sodium (Na) and
chlorine (Cl) in sodium chloride
(salt) is a good example of an
ionic bond.
Ionic Bonds
If you put an alkali (Na)
with a halogen (Cl), you
get an ionic bond because
one atom strongly wants
to lose an electron and
the other strongly wants
to gain one.
Alkali Metals
Halogens
Ionic Bonds
When sodium, with its +1
charge, comes into contact
with chlorine, with its -1
charge, they become
electrically neutral as long
as they are together. This is
because +1 and -1 cancel each
other out.
This also explains why sodium
and chlorine combine in a 1:1
ratio to make sodium chloride
IONIC BONDING
PROPERTIES OF IONIC COMPOUNDS
• Form between a metal and a nonmetal
• Brittle/crystalline
• High melting/boiling points
• Dissolve (ions come apart) in water
IONIC BONDING
NAMING IONIC COMPOUNDS
• The metal (positive ion) goes first in the
formula
• The nonmetal (negative ion) goes last in the
formula
• The name of the metal stays the same, and the
name of the nonmetal is changed so it ends
in –ide.
For example: Oxygen becomes Oxide
NAMING IONIC
COMPOUNDS
• NaCl
• Sodium Chloride
• HF
• Hydrogen Fluoride
• MgI2
• Magnesium Iodide
• KBr
• Potassium Bromide
This is called Binary Nomenclature
Oxidation Numbers
A sodium atom always ionizes
to become Na+ (a charge of
+1) when it combines
with other atoms to make a
compound. Therefore, we say
that sodium has an
oxidation number of 1+. An
oxidation number indicates
how many electrons are
lost, gained, or shared when
bonding occurs.
Oxidation Numbers
Writing a Chemical Formula
Using Oxidation Numbers
1. Find the oxidation numbers of each
element in the compound
2. Determine the ratios of each element
and then write the chemical formula
Oxidation Numbers
Some elements
have more than
one oxidation
number. In this
case, roman
numerals are
used to
distinguish the
oxidation
number.
Element
Copper (I)
Copper (II)
Iron (II)
Iron (III)
Chromium (II)
Chromium (III)
Chromium (VI)
Lead (II)
Lead (IV)
Oxidation Number
1+
2+
2+
3+
2+
3+
6+
2+
4+
Polyatomic Ions
Polyatomic Ions: contains more than one type
of atom (poly means many)
Name
Ammonium
Formula
NH4+
Oxidation #
1+
Acetate
Hydroxide
Phosphate
C2H3O2OHPO43-
113-
Nitrate
Hydronium
Sulfate
NO3H3O+
SO42-
11+
2-
Writing Chemical formulas with
polyatomic Ions
1. Find the formula and charge of the positive Ion
2. Find the formula and charge of the negative ion
(for poly atomic ions you will not have to memorize
them they will be given)
3. Determine how many of each ion are needed so the
charges are equal to zero and write the chemical
formula (Hint: Least common multiple)
Example:
Al2(SO4)3
Writing Chemical formulas with
polyatomic Ions
Example: Write the formula for Aluminum
Sulfate
Al3+
SO421. Find Aluminum’s
Oxidation Number
(it is 3+)
2. Sulfate’s oxidation
number will be given (it
is 2-)
3. How many of each do we need
to satisfy the charges? The
common denominator between 3
and 2 is 6.
6/3 = 2 Aluminums
6/2 = 3 Sulfates
Al2(SO4)3
Covalent Bonds
In a covalent bond the electrons
are shared between atoms.
An important difference
between covalent and ionic bonds is
that covalent bonds act only
between the atoms in a single
molecule, while ionic bonds act
between all adjacent atoms
Covalent Bonds
Elements that have two to six valence electrons tend
to form covalent bonds with each other since the
tendency to take or receive electrons is more
matched.
Covalent bonds can form between
two different types of atoms, or
between two or more atoms of
the same type. The gases
nitrogen (N2), oxygen (O2) and
hydrogen (H2) are a few
examples. We call these
covalently bonded atoms of the
same type diatomic molecules.
Covalent Bonds
• This is how you DRAW a SINGLE COVALENT
BOND
• A single line means each atom shared one
electron each to get full.
Covalent Bonds
• Some atoms need more than one electron to
become stable.
• What type of element is oxygen?
• What kind of bond will two oxygen atoms
make?
Covalent Bonds
– Draw the Electron Dot Diagram for
Oxygen.
– How many electrons does each oxygen
need to become stable?
– How many TOTAL electrons are shared?
– FOUR
Covalent Bonds
• How would you draw this?
O O
• This is a DOUBLE COVALENT BOND.
• 2 pairs of electrons are shared
Covalent Bonds
– What kind of bond would 2 nitrogen
atoms form?
– COVALENT
– How many electrons does each atom
need?
– Three
– How many total electrons are shared?
– Six, 3 pairs of electrons
Covalent Bonds
– How would you draw this?
– This is a TRIPLE COVALENT BOND 3
pairs shared
Covalent Bonds
– Sometimes, one atom is not enough to fill all
the “empty” spots another atom has.
– How many electrons does Carbon need to be
stable?
– How many electrons does Hydrogen need to
be stable?
Covalent Bonds
How do you write the Chemical Formula
for a MOLECULE?
H2
O2
F2
CH4
N2
Covalent Bonds
• Naming Covalent Molecules
– The element with the lowest ionization energy
goes first (farthest from Fluorine).
– Each element gets a prefix to identify how many
there are.
– Exception: If there is only one of the first
element, no prefix is used.
– The second element changes so that it ends
with –ide.
Covalent Bonds
Prefixes for covalent bonding
nomenclature
Number
Prefix
Number
Prefix
1
Mono-
5
Penta-
2
Di-
6
Hexa-
3
Tri-
7
Septa-
4
Tetra-
8
Octa-
Covalent Bonds
Name the following molecules
• H2O
• CO2
• SF6
• B2Si
• SO3
P2O5
• Dihydrogen monoxide
• Carbon dioxide
• Sulfur hexafluoride
• Diboron monosilicide
• Sulfur trioxide
• Diphosphorus pentoxide
Covalent Bonds
Molecular Weight
Does a molecule of water (H2O) have the same
mass as a group of atoms (also called a formula
unit) that make up the ionic compound calcium
carbonate (CaCO3)?
Molecular Weight
Carbon atoms are used as a
standard for determining the
atomic mass units for the
other elements on the periodic
table. One carbon atom is equivalent
to 12.01 atomic mass units.
Molecular Weight
A chemical Formula does 3 things for us:
1. Tells us the types of atoms in the
compound
2. Lets you know if polyatomic ions
are present
3. Allows you to calculate the Molecular
Weight (MW) or Formula Mass
Molecular Weight
Lets go back to our first example: Water vs Calcium
Carbonate.
Water has a chemical formula of H2O
Hydrogen has an atomic mass of 1.01 amu
Oxygen has an atomic mass of 16.00 amu
So one molecule of water has:
2(1.01 amu) + 1(16.00 amu) = 18.02 amu
Molecular Weight
Lets go back to our first example: Water vs Calcium
Carbonate.
Calcium Carbonate has a chemical
Formula Of CaCO3
Calcium has an atomic mass of 40.08 amu
Oxygen has an atomic mass of 16.00 amu
Carbon has an atomic mass of 12.01 amu
So one molecule of Calcium Carbonate has:
1(40.08 amu) + 1(12.01 amu) + 3(16.00 amu) = 100.09 amu
Molecular Weight
The molecular weight comparison:
18.02 amu for Water vs. 100.09 amu for
Calcium Carbonate
Molecular Weight
Molecular Weight
Rules for finding Molecular Weight:
1. Write the chemical formula for the
compound.
2. List the atoms, number of each atom,
and the atomic mass of each atom
3. Add up the values for each type of
atom
Isomers
Isomers: Compounds that have the same
molecular formula but different structural
formulas.
Isomers
Example: C3H6O
How many ways can we bond these
atoms? (hint there are 7 ways
minimum)