Molecular Formulas
Deals with letters/symbols from the
periodic table to describe substances
instead of words
EXAMPLE:

Molecular
formulas
Words
Molecular Formulas:



Recall elements can combine to
form compounds
But when atoms combine, they do
so in specific ratios
For example:


Water, H2O
Combines 2 Hydrogens for every 1
Oxygen
Molecular Formulas:


An atom wants to be stable
To be stable, an atom must have a full
outer electron shell (8 electrons)
Octet Rule

Atoms combine together, forming
bonds, to fill their outer electron shell
Molecular Formulas:

The number of bonds that an atom
can form is termed the valence
Atoms in Family Valence
1
1
2
2
3
3
4
4
5
3
6
2
7
1
Number
of
bonds
each
atom
can
make
How many bonds would an element
in the families below make?
a) Family 2
2 bonds
b) Family 5
3 bonds
c) Family 7
1 bond
d) Family 8
none
Molecular Formulas:


To determine a molecule’s formula, use
the atoms valences & the cross over
rule
Ex.
So,
So,each
the O
H
needs
needs &&
makes
makes 12
bond
bonds
with
– 1O
with each H
Water – why H2O?
1
2
Family 1
Valence 1
Makes 1 bond
2
1
Family 6
Valence 2
Makes 2 bonds
Cross Over Rules:
Write
If needed,
symbols for
the
elements,
simplify
the with
the
metal first
valences
2
1
Eliminate
Write
the valence
any
of each atom
“1”s
Not needed here
Cross over
valences
What is the molecular formula
when Li & Br combine?
Write
If needed,
symbols for
the
elements,
simplify
the with
the
metal first
valences
1
1
Eliminate
Write
the valence
any
of each atom
“1”s
Not needed here
Cross over
valences
What is the molecular formula
when Be & Cl combine?
Write
If needed,
symbols for
the
elements,
simplify
the with
the
metal first
valences
2
1
Eliminate
Write
the valence
any
of each atom
“1”s
Not needed here
Cross over
valences
What is the molecular formula
when Al & O combine?
Write
If needed,
symbols for
the
elements,
simplify
the with
the
metal first
valences
3
Eliminate
Write
the valence
any
of each atom
“1”s
Not needed
Cross over
valences
2
What is the molecular formula
when Mg & O combine?
Write
If needed,
symbols for
the
elements,
simplify
the with
the
metal first
valences
Eliminate
Write
the valence
any
of each atom
“1”s
Cross over
valences
2
1
2
1
Both “2”s cancel
each other out
What is the molecular formula
when Ca & S combine?
Write
If needed,
symbols for
the
elements,
simplify
the with
the
metal first
valences
Eliminate
Write
the valence
any
of each atom
“1”s
Cross over
valences
2
1
2
1
Both “2”s cancel
each other out
Atom “X” from family II combines with
atom “Y” from family VII.
What is the compound’s molecular
formula?
2
1
Atoms in Family
Valence
1
1
2
2
3
3
4
4
5
3
6
2
7
1
Given the compound,
B2xO3y
What would be the numerical value of
“x” & “y”? Why?
B is from family 3, so it forms 3 bonds
O is from family 6, so it forms 2 bonds
Apply cross-over rule, x = 2 & y = 3
Naming Molecules:


Scientists need a standardized way
to name molecules.
Two methods:
1.
For molecules with a metal
& a non-metal
2.
For molecules with 2 nonmetals
Naming Molecules:
Steps to naming molecules with
a metal & non-metal:
metal non-metal
Identify the metal
& the non-metal
Name the metal
sodium chloride
Name the non-metal but
change ending to “ide”
Name Al2O3
Identify the metal
& the non-metal
metal non-metal
Name the metal
aluminum oxide
Name the non-metal but
change ending to “ide”
Name CaF2
Identify the metal
& the non-metal
metal non-metal
Name the metal
calcium fluoride
Name the non-metal but
change ending to “ide”
“mono” never
placed on 1st
Naming Molecules: non-metal!!!
Steps to naming molecules with
a 2 non-metals:
# of
Prefix
Atoms
Name
first non1 the
mono
metal
2
di
Name
3 the
trisecondmonocarbon di oxide
non-metal, change
4
ending
totetra
“ide”
5
penta
Add prefixes to each non6
hexa
metal
Name P2O5
Steps to naming molecules with
a 2 non-metals:
# of
Prefix
Atoms
Name the first non1
mono
metal
2
di
5
penta
Name the second
di phosphorus pentaoxide
3
tri
non-metal, change
4
tetra
ending
to “ide”
Add prefixes to each non6
hexa
metal
Name CBr4
Steps to naming molecules with
a 2 non-metals:
# of
Prefix
Atoms
Name the first non1
mono
metal
2
di
Name the second carbon
3
tri
non-metal, change
4
tetra
ending
to “ide”
5
penta
Add prefixes to each non6
hexa
metal
tetra bromide
Name the following
compounds:
a) CF4
carbon tetrafluoride
b) MgCl2
magnesium chloride
c) Na2O
sodium oxide
d) N2S3
dinitrogen trisulfide


Elements combine to form molecules
in specific ratio to become stable (fill
outer electron shell)  Octet Rule
Use Cross Over Rule & Valences to
determine a molecule’s formula
Atoms in Family
Valence
1
1
2
2
3
3
4
4
5
3
6
2
7
1

Naming compounds with a metal & nonmetal:
name of metal + name of non-metal – ide

Naming compounds with 2 non-metals:
prefix-name of non-metal + prefix-name of non-metal – ide
Activities:

436 Worksheet #3