The indivisible atom

The Indivisible Atom
Dr. K. Shahzad Baig
Memorial University of Newfoundland
(MUN)
Canada
Petrucci, et al. 2011. General Chemistry: Principles and Modern Applications. Pearson Canada Inc., Toronto, Ontario.
Tro, N.J. 2010. Principles of Chemistry. : A molecular approach. Pearson Education, Inc.

Law of Conservation of Mass
In a chemical reaction, matter is neither created nor destroyed.
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠 = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
The total mass of substances present after a chemical reaction is the same as the total
mass of substances before the reaction.

EXAMPLE
A 0.455 g sample of magnesium is allowed to burn in 2.315 g of oxygen gas. The sole
product is magnesium oxide. After the reaction, no magnesium remains and the mass of
unreacted oxygen is 2.015 g. What mass of magnesium oxide is produced?
First, determine the total mass before the reaction.
mass before reaction = 0.455 g magnesium + 2.315 g oxygen
= 2.770 g mass before reaction
2.770 g mass after reaction = ? g magnesium oxide after reaction+ 2.015 g oxygen after
reaction
? g magnesium oxide after reaction = 2.770 g mass after reaction - 2.015 g oxygen after
reaction
= 0.755 g magnesium oxide after reaction

PRACTICE EXAMPLE A:
A 0.382 g sample of magnesium is allowed to react with 2.652 g of nitrogen gas. The
sole product is magnesium nitride. After the reaction, the mass of unreacted nitrogen is
2.505 g. What mass of magnesium nitride is produced?
PRACTICE EXAMPLE B:
A 7.12 g sample of magnesium is heated with 1.80 g of bromine. All the bromine is used
up, and 2.07 g of magnesium bromide is the only product. What mass of magnesium
remains unreacted?

Law of definite proportions
All samples of a given compound, regardless of their source or how they were prepared,
have the same proportions of their constituent elements.
EXAMPLE
Two samples of carbon dioxide are decomposed into their constituent elements. One
sample produces 25.6 g of oxygen and 9.60 g of carbon, and the other produces 21.6 g of
oxygen and 8.10 g of carbon. Show that these results are consistent with the law of
definite proportions
𝐹𝑜𝑟 𝑡ℎ𝑒 𝑓𝑖𝑟𝑠𝑡 𝑠𝑎𝑚𝑝𝑒:
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑂𝑥𝑦𝑔𝑒𝑛
𝑀𝑎𝑠𝑠 𝑜𝑓 𝐶𝑎𝑟𝑏𝑜𝑛
=
25.6 𝑔
9.60 𝑔
= 2.67 𝑜𝑟 2.67 ∶ 1
𝐹𝑜𝑟 𝑡ℎ𝑒 𝑠𝑒𝑐𝑜𝑛𝑑 𝑠𝑎𝑚𝑝𝑒:
𝑀𝑎𝑠𝑠 𝑜𝑓 𝑂𝑥𝑦𝑔𝑒𝑛
𝑀𝑎𝑠𝑠 𝑜𝑓 𝐶𝑎𝑟𝑏𝑜𝑛
= 8.10 = 2.67 𝑜𝑟 2.67 ∶ 1

EXAMPLE
Using the Law of Constant Composition
When 0.455 g of magnesium reacted with 2.315 g of oxygen, 0.755 g of magnesium
oxide was obtained. Determine the mass of magnesium contained in a 0.500 g sample
of magnesium oxide.
= 0.301 g magnesium

For Practice
Two samples of carbon monoxide were decomposed into their constituent elements.
One sample produced 17.2 g of oxygen and 12.9 g of carbon, and the other sample
produced 10.5 g of oxygen and 7.88 g of carbon. Show that these results are
consistent with the law of definite proportions.
PRACTICE EXAMPLE A:
What masses of magnesium and oxygen must be combined to make exactly 2.000 g of
magnesium oxide?
PRACTICE EXAMPLE B:
What substances are present, and what are their masses, after the reaction of 10.00 g
of magnesium and 10.00 g of oxygen?

Law of multiple proportions
If two elements form more than a single compound, the masses of one element combined with
a fixed mass of the second are in the ratio of small whole numbers.
the mass ratio of oxygen to carbon in carbon dioxide is 2.67:1
the mass ratio of oxygen to carbon in carbon monoxide is 1.33:1
The ratio of these two masses is itself a small whole number.
Mass oxygen to 1 g carbon in carbon dioxide
Mass oxygen to 1 g carbon in carbon monoxide
=
2.67 𝑔
1.33 𝑔
= 2.00

For Practice
Hydrogen and oxygen form both water and hydrogen peroxide. A sample of water is
decomposed and forms 0.125 g hydrogen to every 1.00 g oxygen. A sample of hydrogen
peroxide is decomposed and forms 0.250 g hydrogen to every 1.00 g oxygen. Show that
these results are consistent with the law of multiple proportion
PRACTICE EXAMPLE A:
What masses of magnesium and oxygen must be combined to make exactly 2.000 g of
magnesium oxide?
PRACTICE EXAMPLE B:
What substances are present, and what are their masses, after the reaction of 10.00 g of
magnesium and 10.00 g of oxygen?

Dalton’ s Atomic Theory
His theory involved three assumptions:
1. Each chemical element is composed of minute, indivisible particles called atoms.
Atoms can be neither created nor destroyed during a chemical change.
2. All atoms of an element are alike in mass (weight) and other properties, but the atoms
of one element are different from those of all other elements.
3. In each of their compounds, different elements combine in a simple numerical ratio,
for example, one atom of A to one of B (AB), or one atom of A to two of B

The indivisible atom

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