COUNT ATOMHow to count atoms

How many elements?

• Remember the symbol for an element is made up of one or two letters.

• It is easy to figure out because they are capitalized.

• HF – (Hydrofluoric acid) is made up of H (hydrogen) and F (fluor)

• NaCl (sodium chloride) is made up of Na (sodium) and Cl (chlorine)

What about these?

• H2Se H – hydrogen and Se - selenium

• Na4O7P2

Na – sodium O - oxygen P - phosphorus

• HCl H – hydrogen Cl - Chlorine

• K2Te K – Potassium Te - Tellurium

• C232 N92O139P22C – carbon N – nitrogenO – oxygen P - phosphorus

Subscripts

• Subscripts tell us how many atoms of the element BEFORE them there are.

H2OSUBSCRIPT

No subscript• If there are no subscripts it means

there is only one atom of the element.

• HF – Hydrogen = 1 Fluorine = 1

• CaCl2 – Calcium = 1 Chlorine = 2

• H3PO4 – Hydrogen = 3 Phosphorus = 1

Oxygen = 4

What about these?

•  C3H4O3

C = 3 H = 4O = 3

•  H2OH = 2O = 1

•  C232 N92O139P22

C = 232 H = 92O = 139P = 22

But it has a parenthesis

• Parenthesis group atoms into polyatomic ions.

• Don’t worry, the only thing you have to

do is multiply the number of atoms inside the parenthesis by the subscript outside the parenthesis.

Say what?

• Think distributive property:

Zn (MnO4)2

Zn = 1 atomMn = 1 x 2 = 2 atoms

O = 4 x 2 = 8 atoms

What about these:

•  Fe(OH)3Fe = 1O = 1 x 3 = 3H = 1 x 3 = 3

•  Sr3(PO4)2Sr = 3P = 1 x 2 = 2O = 4 x 2 = 8

Finally the Coefficient

2H2OCOEFFICIENT SUBSCRIPT

2H2OThe coefficient gets applied to the

ENTIRE formula

-this means that there are 2 molecules of water present!

-you MULTIPLY this number by every subscript in the formula

2H2O

2 MOLECULES!!!!

2H2OHow many hydrogen atoms are present?

4

How many oxygen atoms are present?

2

Let’s try an example:

4CaCO3

1. Write down symbols

• 4CaCO3

Ca

C

O

Write down the coefficient next to each symbol

4CaCO3

Ca

C

O

4

4

4

Multiply by the subscript

4CaCO3

Ca

C

O

4

4

4

X

X

X

1

1

3

Multiply…

4CaCO3

Ca

C

O

4

4

4

X

X

X

1

1

3

=

=

=

4

4

12

Add totals together to get the total amount of atoms

4CaCO3

Ca

C

O

4

4

4

X

X

X

1

1

3

=

=

=

4

4

1220

Now with your table

• 3CO2

• 2C6H12O6

• 6K2SO4

• CaMg(CO3)2

• 3Pb(NO3)2