Counting Atoms Every chemical formula tells you how many atoms of each element there are in a compound. Important because if you can count atoms, you can balance equations.

H2O Here’s an example How many H? How many O? SUBSCRIPT How many H? How many O? SUBSCRIPTS only refer to the atom that they are BEHIND.

LET’S PRACTICE! MgCl2 Atoms of Magnesium: Atoms of Chlorine: Al2S3 Atoms of Aluminum: Atoms of Sulfur:

What if you have 2 MOLECULES of water? 2H2O What is this?

2H2O This is a COEFFICIENT COEFFICIENTS apply to the ENTIRE formula. The 2 in front means that there are 2 molecules of water present! You MULTIPLY the coefficients and SUBSCRIPTS.

2H2O How many hydrogen atoms are present? How many oxygen atoms are present?

Let’s practice 3H2SO4 4CH3OH Atoms of Hydrogen: Atoms of Sulfur: Atoms of Oxygen: 4CH3OH Atoms of Carbon: Atoms of Hydrogen: Atoms of Oxygen:

What about this? Ba(OH)2 What does this mean? Ba O O H H

PARENTHESES If elements or compounds are inside of PARENTHESES, then the SUBSCRIPT behind the parentheses applies to everything inside. Ba(OH)2 ATOMS OF BARIUM: ATOMS OF OXYGEN: ATOMS OF HYDROGEN:

Let’s Practice Ca3(PO4)2 Al2(SO4)3 Atoms of Calcium: Atoms of Phosphorus: Atoms of Oxygen: Al2(SO4)3 Atoms of Aluminum: Atoms of Sulfur: Atoms of Oxygen:

What about this BAD BOY??? 2Ca3(PO4)2 Atoms of Calcium: Atoms of Phosphorus: Atoms of Oxygen: