Chemistry Chapter III: Atoms: The Building Blocks of Matter
The Atom: From Philosophical Idea to Scientific Theory
The particle theory of matter was supported as early as 400 B.C. by Greek philosophers such as
Democritus. He called the basic particle an atom, based on the Greek word meaning
“indivisible”. Atom is from Greek words, a, meaning “not”, and tomos, meaning “cutting”. The
idea of matter is conveyed as atoms cannot cut as small as an atom.
Democritus and Leucippus were the first to think of the idea of the atom. But Aristotle did not
believe in atoms as he believed that matter was continuous. His opinion was accepted for
nearly 2000 years until scientists began to gather evidence of the atomic theory in the
eighteenth century.
❖ Law of Conservation of Mass-the law that states that mass cannot be created or
destroyed in ordinary chemical and physical changes
❖ Law of Definite Proportions-the law that states that a chemical compound always
contains the same elements in exactly the same proportions by weight or mass
❖ Law of Multiple Proportions-the law that states that when two elements combine to
form two or more compounds, the mass of one element that combines with a given mass
of the other is in the ratio of small whole numbers
In 1808, John Dalton proposed an explanation for the law of conservation of mass, the law of
definite proportions, and the law of multiple proportions. His theory can be summed up by the
following statements;
1. All matter is composed of extremely small particles called atoms
2. Atoms of a given element are identical in size, mass, and other properties; atoms of
different elements differ in size, mass, and other properties
3. Atoms cannot be subdivided, created, or destroyed
4. Atoms of different elements combine in simple whole-number ratios to form chemical
compounds
5. In chemical reactions, atoms are combined, separated, or rearranged
Relating atoms to the measurable property of mass, Dalton turned Democritus’s idea into a
scientific theory that could be tested by experiment.
The Structure of the Atom
❖ Atom-the smallest unit of an element that maintains the chemical properties of that
element
❖ Proton-positively charged particles found in the nucleus
❖ Neutron-neutral charged particles found in the nucleus
❖ Electron-negatively charged particles found surrounding the nucleus
, ➢ Discovery of the Electron
Discoverer: J.J. Thomson (1897)
Experiment: Cathode Ray Tube
Claim: All elements have negative particles in
them.
Evidence: All elements gave off the same beam.
Magnets bend the beam. The negative wire
repelled the beam
Reasoning: Same little particles in all atoms,
same charges repel.
➢ Discovery of the Nucleus
Discoverer: Ernest Rutherford (1911)
Experiment: Gold Foil Experiment
Claim: The nucleus is tiny compared to the size of
the atom.
Evidence: It went straight to the foyer and some
deflected and some bounced back.
Reasoning: They either hit nothing, repelled by
something positive, or hit a dense positive center.
➢ Composition of the Atomic Forces
Protons have a positive charge and can repel each other. To keep the nucleus from not falling
apart, protons and neutrons are held closely together in the nucleus by the strong nuclear
force.
Subatomic Symbol Charge Mass Actual Mass Location in the
Particle Number (kg) atom
Proton p+,
1
𝐻 Positive (+1) 1 amu 1.673 × 10-27 nucleus
1
Neutron n°,
1
𝑛 Neutral (0) 1 amu 1.675 × 10-27 nucleus
0
The Atom: From Philosophical Idea to Scientific Theory
The particle theory of matter was supported as early as 400 B.C. by Greek philosophers such as
Democritus. He called the basic particle an atom, based on the Greek word meaning
“indivisible”. Atom is from Greek words, a, meaning “not”, and tomos, meaning “cutting”. The
idea of matter is conveyed as atoms cannot cut as small as an atom.
Democritus and Leucippus were the first to think of the idea of the atom. But Aristotle did not
believe in atoms as he believed that matter was continuous. His opinion was accepted for
nearly 2000 years until scientists began to gather evidence of the atomic theory in the
eighteenth century.
❖ Law of Conservation of Mass-the law that states that mass cannot be created or
destroyed in ordinary chemical and physical changes
❖ Law of Definite Proportions-the law that states that a chemical compound always
contains the same elements in exactly the same proportions by weight or mass
❖ Law of Multiple Proportions-the law that states that when two elements combine to
form two or more compounds, the mass of one element that combines with a given mass
of the other is in the ratio of small whole numbers
In 1808, John Dalton proposed an explanation for the law of conservation of mass, the law of
definite proportions, and the law of multiple proportions. His theory can be summed up by the
following statements;
1. All matter is composed of extremely small particles called atoms
2. Atoms of a given element are identical in size, mass, and other properties; atoms of
different elements differ in size, mass, and other properties
3. Atoms cannot be subdivided, created, or destroyed
4. Atoms of different elements combine in simple whole-number ratios to form chemical
compounds
5. In chemical reactions, atoms are combined, separated, or rearranged
Relating atoms to the measurable property of mass, Dalton turned Democritus’s idea into a
scientific theory that could be tested by experiment.
The Structure of the Atom
❖ Atom-the smallest unit of an element that maintains the chemical properties of that
element
❖ Proton-positively charged particles found in the nucleus
❖ Neutron-neutral charged particles found in the nucleus
❖ Electron-negatively charged particles found surrounding the nucleus
, ➢ Discovery of the Electron
Discoverer: J.J. Thomson (1897)
Experiment: Cathode Ray Tube
Claim: All elements have negative particles in
them.
Evidence: All elements gave off the same beam.
Magnets bend the beam. The negative wire
repelled the beam
Reasoning: Same little particles in all atoms,
same charges repel.
➢ Discovery of the Nucleus
Discoverer: Ernest Rutherford (1911)
Experiment: Gold Foil Experiment
Claim: The nucleus is tiny compared to the size of
the atom.
Evidence: It went straight to the foyer and some
deflected and some bounced back.
Reasoning: They either hit nothing, repelled by
something positive, or hit a dense positive center.
➢ Composition of the Atomic Forces
Protons have a positive charge and can repel each other. To keep the nucleus from not falling
apart, protons and neutrons are held closely together in the nucleus by the strong nuclear
force.
Subatomic Symbol Charge Mass Actual Mass Location in the
Particle Number (kg) atom
Proton p+,
1
𝐻 Positive (+1) 1 amu 1.673 × 10-27 nucleus
1
Neutron n°,
1
𝑛 Neutral (0) 1 amu 1.675 × 10-27 nucleus
0
