1.3 Early Models of the Atom
ANCIENT MODELS
Empedocles, a Greek philosopher, proposed that all matter is made up of four basic substances: earth,
water, air, and fire. Each of these substances has different combinations of four specific qualities: dry,
moist, cold, and hot. Aristotle, another well-respected philosopher, supported this four element theory of
matter and, as a result, this model was believed to be true for almost 2000 years.
Empedocles’ Model of Matter
Watch the video and answer the following questions:
1. What did Democritus propose?
2. What does the Greek word “atomos” mean?
3. Why did people not believe in atomism?
4. What do alchemists do?
5. How was Gilbert’s work important to the structure of the atom?
6. What law did Lavoisier propose?
7. What law did Proust propose?
The Laws of Chemical Change
1. Law of Conservation of Mass: During a chemical reaction, the mass of the reactants must equal the
mass of the products. Matter cannot be created nor destroyed.
2. Law of Definite Composition: The percentage of the elements that make up a particular compound
never varies. eg. hydrogen and oxygen can be mixed in any proportion, but the resulting water will always
have the same composition. The formula will always be H2O and by mass, the water will contain 11.19%
hydrogen and 88.81% oxygen. Any excess hydrogen or oxygen will remain unreacted.
Dalton’s Atomic Theory
In 1803, John Dalton, an English schoolteacher, came up with his atomic theory. His theory consists of
the following ideas:
1. Matter consists of definite particles called atoms.
2. Each element is made up of its own type of atom.
3. Atoms of different elements have different properties.
4. Atoms of two or more elements can combine in constant ratios to form new substances.
5. Atoms cannot be created, destroyed, or subdivided in a chemical change.
Dalton’s Billiard Ball Model
The Subatomic Particles
Dalton’s theory failed with the discovery of positive and negative electric charges, In 1897, JJ
Thomson developed a model of the atom that included these charges. In his model, the negatively charged
particles were called electrons.
Thomson’s Raisin Bun Model
In 1911, Thomson’s model of the atom failed when Ernest Rutherford performed his famous Gold Foil
Experiment. He showed that the atom was made up of mostly empty space with a small, positively
charged core (which he called the nucleus). He later named the positively charged particles found in the
nucleus protons.
Rutherford’s Nuclear Model
In 1932, James Chadwick discovered neutrons within the nucleus of the atom. Neutrons are similar in
mass to protons, but carry no charge. Some atoms of an element were found to be heavier than others.
This was due to the fact that they had more neutrons in their nucleus. Elements that have the same number
of protons but different number of neutrons are called isotopes.
The atomic number is what defines the element. It is the number of protons found in the nucleus. The
mass number represents the number of protons and neutrons found in the nucleus. An atom is electrically
neutral having the same number of protons and electrons. An ion is formed when an atom gains or loses
electrons. The charge on the ion represents the number of electrons gained or lost. A positively charged
ion has lost electrons, while a negatively charged ion has gained electrons.
Eg.
PEN CHART
Example
Protons
Electrons
Neutrons