Counting Atoms in Compounds or Molecules Big Idea: • Given a chemical formula be able to count the number of atoms for each element in the compound or molecule.
Parts of a Chemical Formula 2 H2O 4Pb(OH)4 Element Symbol Subscript Coefficient
3 Parts of a Chemical Formula 1. 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. • A subscript shows how many atoms of that element are bonded in the molecule. • If there is no subscript then one atom of the element is present. • If the subscript is outside the brackets, all the atoms inside the brackets are multiplied by the subscript outside the brackets. • 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 H2O The coefficient multiplies all the atoms after it. Hydrogen 3 x 2 = 6 Oxygen 3 x 1 = 3
Test Yourself Pb 1 O 4 H 4
Counting atoms Pb 1 O 4 H 4 Mg 3 P 2 O 6 As 1 I O 20 5 Cu 2 8 4 O Cl