Counting Atoms in a Compound

Located on the extra page of notes. Copy what is in red.

Counting Atoms• Chemical formulas are made of two parts:– 1. Atomic symbols- one or two letter abbreviation

for an element Each symbol starts with a capital letter, so a new

symbol starts when you see another capital letter.

– 2. Subscripts- numbers behind a symbol that tell how many atoms are present

Ex. NaClO3 3 is the subscript of O, showing there are 3 O atoms in the compound

Counting Atoms

• When you count the number of atoms for an element in a compound, you use the subscripts.

• Ex. NaCl- The subscript of Na is 1, so there is 1 Na atom. The subscript of Cl is 1, so there is 1 Cl atom.

Na- 1Cl- 1

Counting Atoms

• Ex. AlPO4 Al has a subscript of 1, so there is 1 Al atom. P has a subscript of 1, so there is one P atom. O has a subscript of 4, so there are 4 O atoms.

Al- 1P- 1O- 4

Counting Atoms• When there are parentheses in a compound,

multiply the subscript inside by the number outside to find the total number of atoms.

• Ex. Ca(NO3)2 Ca has a subscript of 1, so there is 1 Ca atom. N has a subscript of 1 x the 2 outside the parentheses, so there are 2 N atoms. O has a subscript of 3 x 2, so there are 6 O atoms.

Ca- 1 O- 6N- 2

Counting Atoms

• NOW…. Do the worksheet, both sides. Makes sure you write out the name of the element, or you will not get credit. The first one is done for you as an example.