The History of the
Atom
Atomic Model, Law of Conservation, Law of Definite
Composition, Atomic Theory
The Atomic Model
Greek philosopher Leucipus was first person who believed that mater was
made of tiny particles called atoms.
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Matter is made of empty space through which atoms move.
Atoms are tiny, indestructible objects.
Different atoms have different sizes and shapes.
Changes in matter are due to changes in combinations of atoms.
Law of Conservation of Mass
Lavosier was the scientist who developed this.
The mass of products in a chemical reaction will be the same as the mass of
the reactants.
• This was a revolutionary idea, because if you burn wood the ashes weigh less
thatn the original wood did.
• Lavosier was able to figure this out because he did a lot of work with gases
and realized that they actually have the same mass.
Law of Definite Composition
Joseph Proust developed this.
A chemical compound will always have the same composition, regardless of
how it was made.
• This was revolutionary because impurities made it so this didn’t always seem
true.
• For example = the chemical definition of water is H₂O. There are always 2
atoms of hydrogen to one of oxygen in our definition of water. Or, 8g of
Oxygen for every gram of Hydrogen.
John Dalton’s Atomic Theory (1808)
(Dalton’s 5 Postulates)
• All matter is made of small, indescribable particles called atoms.
• False: Atoms can be broken. A better statement that’s true: Matter cannot be broken by chemical means.
• Atoms of the same element have identical properties.
• False: The presence of isotopes disproves this.
• Atoms of different elements have different properties.
• True
• Atoms obey the law of conservation of mass – the mass of what we make is equal to the
mass of what we started with.
• True
John Dalton’s Atomic Theory (1808)
• Atom’s obey the law of multiple proportions (this is known as Dalton’s Law): If 2
elements can make more than one chemical compound, the ratios of the masses of one of
the elements that combine with a given amount of the second element will be a small, whole
number.
• Ex. – Hydrogen and Oxygen make 2 chemical compounds. In one compound, 2 grams of
hydrogen combine with 16 grams of oxygen. In another compound, 2 grams of hydrogen
combine with 32 grams of oxygen. The ratio of oxygen in one compound is 32:16 = 2:1
• Why is this important? It leads to the far more important understanding of atoms
always combine in whole number ratios to form chemical compounds.
• This law is true.