Atomic Theory

Atomic Theory
John Dalton (1766-1844) is the scientist credited for proposing the atomic theory. This theory
explains several concepts that are relevant in the observable world: the composition of a pure
gold necklace, what makes the pure gold necklace different than a pure silver necklace, and
what occurs when pure gold is mixed with pure copper. Before discussing the atomic theory,
this article explains the theories that Dalton used as a basis for his theory: the law of
conservation of mass and the law of constant composition.
Law of Conservation of Mass: (1766-1844)
The law of conservation of mass states that the total mass present before a chemical reaction is
the same as the total mass present after the chemical reaction; in other words, mass is
conserved. The law of conservation of mass was formulated by Antoine Lavoisier (1743-1794)
as a result of his combustion experiment, in which he observed that the mass of his original
substance—a glass vessel, tin, and air—was equal to the mass of the produced substance—the
glass vessel, “tin calx”, and the remaining air.
Historically, this was a difficult concept for scientists to grasp. If this law was true, then how
could a large piece of wood be reduced to a small pile of ashes? The wood clearly has a greater
mass than the ashes. From this observation scientists concluded that mass had been lost.
However, the illustration below shows that the burning of word does follow the law of
conservation of mass. Scientists did not take into account the gases that play a critical role in
this reaction.
Law of Constant Composition
Joseph Proust (1754-1826) formulated the law of constant composition (also called the law of
definite proportions). This law states that if a compound is broken down into its constituent
elements, the masses of the constituents will always have the same proportions, regardless of
the quantity or source of the original substance. Joseph Proust based this law primarily on his
experiments with basic copper carbonate. The illustration below depicts this law; 31 grams of
H2O and 8 grams of H2O are made up of the same percent of hydrogen and oxygen.
Dalton's Atomic Theory
1. Each chemical element is composed of extremely small particles that are indivisible and
cannot be seen by the naked eye, called atoms. Atoms can neither be created nor
destroyed. Pictured below is a helium atom. The purple and red dots represent the
neutrons and protons in the nucleus. The black area around the nucleus represent the
electron cloud. The following sections discuss this further.
2. All atoms of an element are alike in mass and other properties, but the atoms of one
element differ from all other elements. For example, gold and silver have different atomic
masses and different properties.
3. For each compound, different elements combine in a simple numerical ratio. The
illustration below describes this rule. The second equation for the reaction is incorrect
because half of an atom does not exist.
Atomic theory can be used to answers the questions presented above. A pure gold necklace
is made up of atoms. A pure gold necklace and a pure silver necklace are different because
they have different atoms. Pure gold mixed with pure copper forms rose gold. The gold and
copper atoms combine in a simple numerical ratio.
Dalton's theory has not proven to be correct under all circumstances. The first rule was
proven incorrect when scientists divided atoms in a process called nuclear fission. The
second rule was proven incorrect by the discovery that not all atoms of the same element
have the same mass; there are different isotopes. However, these failures do not justify
discarding the atomic theory. It correctly explains the law of conservation of mass: if atoms
of an element are indestructible, then the same atom must be present after a chemical
reaction as before and, and the mass must constant. Dalton’s atomic theory also explains
the law of constant composition: if all the atoms of an element are alike in mass and if
atoms unite in fixed numerical ratios, the percent composition of a compound must have a
unique value without regards to the sample analyzed. The atomic theory led to the creation
of the law of multiple proportions.
Law of Multiple Proportions
The law of multiple proportions states that if two elements form more than one compound
between them, the masses of one element combined with a fixed mass of the second
element form in ratios of small integers. The illustration of the third rule of the atomic
theory correctly depicts this law.

Atomic Theory

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Atomic Theory

Law of Conservation of Mass: (1766-1844)

Law of Constant Composition

Dalton's Atomic Theory

Law of Multiple Proportions

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