Download Counting Atoms Notes


Elements
› Must be on periodic
table
› Represented by one
chemical symbol
only, can be one or
two letters
› Pure substance
› Atom is smallest unit





Compound
NOT on the periodic
table
Represented by a
chemical formula of
2 or more elements
Combination of
elements creates a
new substance with
different properties
Molecule is smallest
unit.

The force of attraction that holds two
atoms together

A chemical formula is a shorthand
notation that uses chemical symbols and
numbers to represent a compound.
A
subscript is a number written below
and to the right of a chemical symbol
in a formula.
H2O

A coefficient is the large number in front
of the chemical formula. It means there
are that many molecules of the
compound
4H2O

Start by listing all the elements
› Remember, look for the capital letters!!

Start by listing all the elements
› Remember, look for the capital letters!!

Then, look to the right of each element.
› Subscript = how many atoms it has
› no subscript = it is understood to be 1 atom.
› If there is a coefficient, multiply
your count by that number.
 H2 O
›Hydrogen = 2
›Oxygen = 1
If there are parenthesis in the formula,
whatever is inside the parenthesis gets
multiplied by the subscript.
 For example:

Mg(SO4)2
2x1=2 2x4=8

Carbon dioxide = 1 atom of carbon + 2
atoms of oxygen
CO2

Calcium carbonate = 1 atom of calcium
+ 1 atom of carbon + 3 atoms of oxygen
CaCO3

Sulfuric acid = 2 atoms of hydrogen + 1
atom of sulfur +4 atoms of oxygen
H2SO4

Hydrogen peroxide = 2 atoms of
hydrogen + 2 atoms of oxygen
H2O2
C12H22O11 12 C + 22 H + 11O = 45

Number of elements? Count the capital
letters…….
Number of atoms? Count the little numbers
(subscripts). Don’t the forget the “1”.