Transcript Regents Unit 4: Counting Atoms in Formulas

Chemical Formulas:
The Basics
Subscripts in Chemical Formulas
 Numbers
inside formulas.
 Smaller than the font size & lowered below
the line of type.
 Subscripts show the number of atoms of
each element in a representative unit.
Rules for Subscripts in Formulas
 No
subscript – it’s understood to be 1.
 Subscripts
refer to the element immediately
preceding them.
Remember – every element symbol starts
with an upper case letter.
 Subscripts
after parentheses refer to
everything inside ( ).
Chemical Formulas
 H2O
has 2 hydrogens and 1 oxygen
 NH3 has 1 nitrogen and 3 hydrogens
 CH4 has ? 1 carbon and 4 hydrogens
 CO2 has ? 1 carbon and 2 oxygens
 AgNO3 has ? 1 silver, 1 nitrogen, 3 oxygens
 Al(NO3)3 has ? 1 aluminum, 3 nitrogens, and
9 oxygens
No. of atoms in formulas
 Al(NO3)3
 Al
= 1
N = 1 X 3 = 3
O = 3 X 3 = 9
 Total = 13
Subscript outside ( ) X
subscript inside ( )
=
# of atoms
Hint: Look for uppercase letters!
No. of atoms in formulas
 Mg3(PO4)2
 Mg
= 3
P = 2 X 1 = 2
O = 4 X 2 = 8
 Total = 13
Coefficients
 Numbers
in front of formulas
 Refer
to everything in the formula
immediately following
 No
coefficient? It’s understood to be 1.
Coefficients
 2H2O
means 2(H2O)
 To count atoms take coefficient X
subscript


2 X 2 = 4 H’s
2 X 1 = 2 O’s
Hydrates
 A group
of salts that have water
molecules stuffed in the empty spaces
 Formulas are distinctive
 Ex: CuSO45H2O


 means “is associated with” or PLUS
Not a true chemical bond, but the structure is
definite so the number of empty spaces is
definite and the number of water molecules is
definite
CuSO45H2O
 Count
up the atoms!
1 X Cu
1XS
4XO
10 X H
5XO
5[CuSO45H2O]
 Count
up the atoms!
5 X Cu
5XS
20 X O
50 X H
25 X O
Coefficients X
Subscripts
Representative Unit
 Depends
on the structure of the
substance.
 For
covalent substances, the
representative unit is the molecule.
 For
ionic substances, which make a
crystal lattice, the representative unit is the
unit cell.
Covalently bonded
substances make
molecules.
If you have more raw
materials, you make
more molecules.
NaCl is an ionic
compound.
The ions are arranged
in a crystal lattice.
There is one Na+ for every Cl-.
They are arranged in a repeating pattern.
The more raw
materials you have,
the larger the crystal.
It is not useful to have
a “molecular” formula
for ionic compounds –
every crystal in the
world is different.
This diagram shows the
relative sizes of the two
ions.
Use empirical formulas
instead – give smallest
whole number ratio.
When all the ions are in the
correct location, you can
get a large “single crystal.”
This model shows the position of the ions but not
the relative sizes.
Covalent vs. Ionic

All Non-metals
 Molecules
 Definite size
 Molecular formula
gives exact
composition of
molecule

Metal & Nonmetal
 Crystal Lattice
 Indefinite size
 Empirical formula
gives smallest whole
number ratio of
elements