2. Purpose
• Up to this point we have mainly dealt with individual atoms. We will now
apply some of what we have learned to compounds. Specifically, we will
learn what the formula tells us and how to determine and use the masses
of compounds.
3. Atoms combine to form compounds
• Hydrogen and oxygen can combine to form either water or hydrogen
peroxide
H2O H2O2
Water Hydrogen Peroxide
O
H H
O O
H
H
4. Subscripts in formulas tell how many of each
atom is present.
• H2O
– Subscripts belong to the atom in front of the subscript.
– The 2 subscript belongs to hydrogen; therefore, there are two
hydrogens
– There is no subscript on oxygen. It is understood that this is a 1
meaning there is one oxygen
5. Subscripts outside of parentheses apply to
everything in the parentheses.
• (NH4)2SO4
– The two outside the parentheses means that there are two NH4 units
in this substance
• Each NH4 contains 1 nitrogen; therefore, there are a total of 2
nitrogens in the substance.
• Each NH4 contains 4 hydrogens; therefore, there are a total of 8
hydrogens in the substance.
– There is 1 sulfur since it has a subscript of 1.
– There are 4 oxygens according to its subscript of 4.
6. Pause and Practice
• How many of each atom are present in the following substances?
– P4O10
– H2CO3
– SiCl4
– Mg(NO3)2
– Cu3(PO4)2
7. Pause and Practice Answers
• How many of each atom are present in the following substances?
– P4O10
• 4 phosphorus, 10 oxygen
– H2CO3
• 2 hydrogen, 1 carbon, 3 oxygen
– SiCl4
• 1 silicon, 4 chlorine
– Mg(NO3)2
• 1 magnesium, 2 nitrogen, 6 oxygen
– Cu3(PO4)2
• 3 copper, 2 phosphorus, 8 oxygen
