Modern Theories of the Atom

Modern Atomic Theory states that matter is composed of small, indivisible particles called atoms. 

Three Laws of Modern Atomic Theory

The Law of Conservation of Mass states that matter can neither be created nor destroyed, all that happens is that it changes forms. 

No matter where you obtain a compound, whether it’s from a lab experiment or from collection out in the field, the ratio of elements in it will remain constant. 

When element A and element B combine they can form different compounds in different ratios to one another. Dividing these different ratios should generate whole number answers. 

Example: A 15.39 g sample of iodine reacts with 62.92 g of chlorine to form iodine pentachloride, ICl5. If iodine pentachloride is the only product formed calculate its mass. 


Example: Two samples sodium fluoride decompose into their constituent elements. The first sample produces 15.8 kg of sodium and 20.1 kg of fluorine. If the second sample produces 192.0 g of sodium, how many grams of fluorine were also produced? 


Example: Which of the following is an example of the law of multiple proportions?

a) A sample of bromine (Br) is contains equal amounts of its two isotopes.

b) Two different samples of H2O have the same mass ratio.

c) The atomic mass of sodium (Na) is 22.99 amu.

d) Two different compounds composed of sulfur (S) and oxygen (O) have different mass ratios: 2.48 g O: 1 g S and 1.24 g O: to 1 g S.