Counting Atoms in Chemical Formulas

Hooray

Counting Atoms

1) Start at the left side of the formula and underline the capital letter plus any lower case letter that occurs before the next capital letter in the formula. Continue with the remaining elements in the formula.

• NaCl = Sodium (Na)• NaCl = Chlorine (Cl)

Do this one!

• NH4SO4

What elements are contained in this compound? Write out the names, not the symbols!

Counting Atoms

2. Now, look at the subscript numbers. This tells you how many atoms there are of each element. If there is no subscript number, then there is only one atom of that element.

• Fe2S3 = 2 Iron atoms and 3 Sulfur atoms

Do this one!

• NH4SO4

Next to the elements you wrote for this compound, write how many atoms of each element are contained in the compound.

Polyatomic Ions3. Compounds with more than two elements

almost always have polyatomic ions. Ca(OH)2

Polyatomic ion (hydroxide)

• This is important when counting atoms.Ca(OH)2 = 1 Calcium, 2 Oxygen,

2 Hydrogen atoms

• The subscript number only applies to the elements in parentheses.

Do these!

Count the number of atoms of each element in a chemical compound.

• NaCl # of Na atoms:# of Cl atoms:

• H2O # of H atoms:

# of O atoms:

Do these!• Ca(COз)2 # of Ca atoms:

# of C atoms:# of O atoms:

• Caffeine – C8H10N4O2

Write down which atoms are in the compound and how many there are of each.

Do these!• Fartox – C6Cl5NO2

Write down which atoms are in the compound and how many there are of each.

• Arsole – C4H5As

Write down which atoms are in the compound and how many there are of each.

Do these!• (NH4)3PO4

Write down which atoms are in the compound and how many there are of each.

• B(Ar(CF3)2)4

Write down which atoms are in the compound and how many there are of each.

Do these!• Mg(OH)2

Write down which atoms are in the compound and how many there are of each.

• Al2(SO4)3

Write down which atoms are in the compound and how many there are of each.