1. 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

2. Parts of a Chemical Formula 2 H 2 O 4 Pb(OH) 4 Coefficient Subscript Element Symbol

3. 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

4. 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. Group all like atoms!! 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.

5. Count the Atoms Oxygen – 1 atom Hydrogen – 2 atoms H 2 O Formula - H 2 O

6. Count the Atoms Calcium – 1 atom Oxygen – 2 atoms 2 atoms Hydrogen – 2 atoms Ca(OH) 2

7. Coefficients The number of water molecules is shown with a coefficient. Coefficients are written in front of the chemical formula. 3 H 2 O

8. Count all the atoms 3 H 2 O The coefficient multiplies all the atoms after it. Hydrogen 3 x 2 = 6 Oxygen 3 x 1 = 3

9. Test Yourself Formula Atom 1 present How Many ? Atom 2 present How Many? Atom 3 present How Many? Pb(OH) 4 Mg 3 (PO 3 ) 2 As(IO 4 ) 5 2Cu(ClO 2 ) 2 1 lead 4 Oxygen 4 Hydrogen

10. Counting atoms Formula` Atom 1 present How Many ? Atom 2 present How Many? Atom 3 present How Many? Pb(OH) 4 Mg 3 (PO 3 ) 2 As(IO 4 ) 5 2Cu(ClO 2 ) 2 44 3 magnesium 2 phosphorus 6 oxygen 1 arsenic 5 iodine 20 oxygen 2 copper 4 chlorine 8 oxygen lead 1 Oxygen Hydrogen

11. Try Grouping Like Atoms Given the formula CH 3 O(CH 2 ) 2 OH How many atoms of carbon in the formula? 3 How many atoms of oxygen in the formula? 2 How many atoms of hydrogen in the formula? 8