Counting Elements and Atoms Unit 6-1

I. Counting Elements When counting the elements in a compound, count only the capital letters. Example – H2O has two elements- Hydrogen and Oxygen Example – CO2 has two elements- Carbon and Oxygen Try This – How many elements are in NaCl? 2 elements What are they? Sodium and Chlorine How many elements are in C12H22O11? 3 elements What are they? Carbon, Hydrogen and Oxygen

II. Counting Atoms When counting atoms, count the subscripts for each element. Subscripts are the small numbers that are behind the chemical symbol. Example – H2O When there is no subscript after the element, it’s assumed that the subscript is a 1! Example – H2O – There are TWO atoms of Hydrogen and only ONE atom of Oxygen. Example – NaCl – There is only ONE atom of Sodium and only ONE atom of Chlorine.

d. To determine the TOTAL number of atoms, add up all atoms for all elements in the molecule. e. Let’s try some examples: 1. How many TOTAL atoms are in H2O? Answer- 2 atoms of Hydrogen + 1 atom of Oxygen = 3 atoms total 2. How many TOTAL atoms are in C12H22O11? Answer- 12 atoms of Carbon + 22 atoms of Hydrogen + 11 atoms of Oxygen = 45 total atoms

III. Coefficients Sometimes there will be coefficients in front of the entire compound/molecule. Example- 3 H2O If there is NO coefficient it is assumed to be 1 just like with subscripts. The coefficient tells you how many molecules (groups) there are of a specific compound. Example- 3 H2O means that there are 3 molecules (groups) of H2O.

d. To determine the number of atoms there are when there is a coefficient, you multiply the coefficient and subscript. Example – 3 H2S Atoms of Hydrogen = 3 x 2 (coefficient x subscript) Atoms of Sulfur = 3 x 1 (coefficient x subscript) ii. Try this- 2NaCH Atoms of Sodium= 2x 1 =2 Atoms of Carbon =2 x 1= 2 Atoms of Hydrogen= 2x1 = 2 Total Atoms in 2NaCH= 2 +2 +2 =6 iii. Try This – 2Ca3 Atoms of Calcium = 2 x 3 = 6 Total atoms of 2Ca3=6

IV. Putting it All Together Try the following examples: Material # of Elements # of Molecules (groups) Total # of Atoms   KCl4 2 1 5 NaCHO3 4 6 4H2O 12 2KCl4 10