Big Idea:

Given a chemical formula be able to count the number of atoms for each element in the compound or molecule.

Counting Atoms in Compounds or Molecules

Parts of a Chemical Formula

2 H2O

4 Pb(OH)4

Coefficient Subscript

Element Symbol

3 Parts of a Chemical Formula1. Chemical symbols

- Written in the order they appear on the periodic table, from left to right.

2. The subscript - Gives the number of atoms

- Must be written after the symbol, half size

3. The coefficient- Gives the number of compounds or molecules- Must be written regular size before the formula

Counting Atoms

Rules for counting atoms in a molecule or compound.

1. A subscript shows how many atoms of that element are bonded in the molecule.

2. If there is no subscript then one atom of the element is present.

3. If the subscript is outside the brackets, all the atoms inside the brackets are multiplied by the subscript outside the brackets.

4. Coefficients are multiplied last. The total number of atoms in the formula is multiplied by the coefficient.

Count the Atoms

Oxygen – 1 atom

Hydrogen – 2 atoms

Formula - H2O

Count the Atoms

Calcium – 1 atom

Oxygen – 2 atoms

Hydrogen – 2 atoms

Ca(OH)2

Coefficients

The number of water molecules is shown with a coefficient.

Coefficients are written in front of the chemical formula.

3 H2O

Count all the atoms

3 H2OThe coefficient

multiplies all the atoms after it.

Hydrogen 3 x 2 = 6

Oxygen 3 x 1 = 3

Test YourselfFormula Atom 1

presentHow Many?

Atom 2present

How Many?

Atom 3present

How Many?

Pb(OH)4

Mg3(PO3)2

As(IO4)5

2Cu(ClO2)2

1Pb 4O 4H

Counting atomsFormula Atom 1

presentHow Many?

Atom 2present

How Many?

Atom 3present

How Many?

Pb(OH)4

Mg3(PO3)2

As(IO4)5

2Cu(ClO2)2

1Pb 4O 4H

3Mg 2P 6O

1As 5I 20O

2Cu 4Cl 8O