Download - Counting atoms
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