Counting AtomsCounting Atoms

Obviously, there are more Obviously, there are more materials in the world than materials in the world than just 100 or so elements on just 100 or so elements on

the periodic table.the periodic table.

Individual atoms Individual atoms can combine with can combine with each other to form each other to form a group of atoms a group of atoms called a called a molecule.molecule.

1.1. The symbol of an element The symbol of an element represents one atom of that represents one atom of that element.element.

ExampleExample Na = Na = 1 Sodium atom1 Sodium atom

S = S = 1 Sulfur atom1 Sulfur atom

2.2. A A subscriptsubscript is a is a number written at the number written at the lower lower rightright corner corner behindbehind the symbol of the symbol of an element.an element.

If there is more than If there is more than one atom ‘stuck’ one atom ‘stuck’ together (in chemistry together (in chemistry we call this we call this bondedbonded), a ), a subscript is used to subscript is used to indicate the number of indicate the number of atoms.atoms.

Example)Example)

FF22 = = two atoms fluorine stuck togethertwo atoms fluorine stuck together

HH22O = O = 22 atoms of Hydrogen atoms of Hydrogen

11 atoms of Oxygen atoms of Oxygen

Total # of atoms Total # of atoms 33

CC66HH1212OO6 6 = = 66 atoms of Carbon atoms of Carbon

1212 atoms of Hydrogen atoms of Hydrogen

66 atoms of Oxygen atoms of Oxygen

Total # of atoms Total # of atoms 2424

3.3. A A coefficientcoefficient is a number written in is a number written in front of a chemical formula. It tells you front of a chemical formula. It tells you how many how many moleculesmolecules you have. you have.

Example)Example) 2 H2 H22O = O = 22 molecules of water molecules of water

44 total atoms of total atoms of HydrogenHydrogen

22 total atoms of total atoms of OxygenOxygen