Atoms, Molecules,
and Ions
Laws of Chemical Changes
Law of Conservation of Mass
Law of Definite Proportion
Law of Multiple Proportion
Law of Conservation of Mass
In a chemical reaction, no change in mass
takes place. The total mass of the products
is equal to the total mass of the reactant.
Antoine Lavoisier
Law of Conservation of Mass
Antoine Lavoisier
 formulated this law by describing one of his experiments involving
mercuric oxide. He placed a small amount of mercuric oxide, a red
solid, inside a retort and sealed the vessel tightly.
 He weighed the system, and then subjected it to high temperature.
During the heating, the red solid turned into a silvery liquid. This
observation indicated that a chemical reaction took place. After
which, the setup was cooled and then weighed. The weight of the
system was found to be the same as before heating.
Law of Conservation of Mass
 How many grams of water will be formed if 1.00 g hydrogen gas
reacts with 8.00 g oxygen?
The reaction can be represented by the following word equation:
hydrogen + oxygen
water
 5.58 g iron reacted with 3.21 g sulfur. How many grams of iron (II)
sulfide were produced?
The reaction involved was:
iron + sulfur
iron(II) sulfide
Law of Conservation of Mass
 Ammonia is produced by the reaction of nitrogen with
hydrogen:
nitrogen + hydrogen
ammonia
Law of Definite Proportion
A compound always contains the same
constituent elements in a fixed or definite
proportion by mass.
Law of Multiple Proportions
 If two elements can combine to form more than one
compound, the masses of one element that will
combine with a fixed mass of the other element are in a
ratio of small whole numbers.
 application of this law using the example of carbon
which reacts with oxygen to form carbon monoxide and
carbon dioxide.
a. In carbon monoxide, 1.00 g carbon combines with 1.33
g oxygen; whereas, in carbon dioxide, 1.00 g carbon
combines with 2.66 g oxygen.
a. It can be seen that the ratio is 1:2.
Dalton’s Atomic Theory
 proposed by John Dalton
 can be used to explain the laws of chemical change.
 based on the following set of postulates:
1. Elements are made up of very small particles known
as atoms.
2. All the atoms of an element are identical in mass and
size, and are different from the atoms of another
element. Dalton used the different shapes or figures
to represent different elements, as follows:
Dalton’s Atomic Theory
3. Compounds are composed of atoms of more than
one element, combined in definite ratios with whole
number values.
4. During a chemical reaction, atoms combine,
separate, or rearrange. No atoms are created and no
atoms disappear.
During the time of Dalton, the atom was
believed to be the smallest particle
comprising substances. However,
before the end of the 19th century,
experiments provided proof of the
existence of smaller particles within the
atom.
Subatomic Particles
particles contained in an atom