1. 1.3 Atomic Theory

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Early ideas about matter
Greek philosophers thought matter was made of atomos which would be the smallest pieces of matter.
Aristotle believed matter was made of
different combinations of earth, air, fire, and water.
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Alchemists tried to turn common metals into gold.
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4. Development of Atomic Theory I
John Dalton ( )
He studied gases that make up Earth’s atmosphere.
He suggested that matter is made of small, hard spheres
This is the basis for Dalton’s Atomic Theory.
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5. Dalton’s Atomic Theory:
All matter is made of small particles called atoms.
Atoms cannot be created, destroyed, or divided into smaller particles.
All atoms of the same element are identical in mass and size, but they are different in mass and size from the atoms of other elements.
Compounds are created when atoms of different elements link together in definite proportions.
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Atomic Theory II
J. J. Thomson ( )
Studied electric currents in gas discharge tubes
Determined that the currents were streams of negatively charged particles
Hypothesized that atoms are made of smaller particles.
Proposed the “raisin bun” model of the atom:
- positively charged bun with negatively charged particles
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Atomic Theory III
Ernest Rutherford ( )
Found that some particles were deflected in directions not originally predicted.
Suggested that the deflection of the charged particles was because the atom had a tiny dense centre called a nucleus, & electrons moved around the nucleus.
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Atomic Theory IV
Niels Bohr ( )
Studied gaseous samples of atoms, which were made to glow by passing an electric current through them.
Based on his observations, Bohr proposed
that electrons surround the nucleus in specific
“energy levels” or “shells.”
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Inside the Atom
Atom = smallest particle of an element that retains the properties of the element
made up of three kinds of particles called subatomic particles:
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10. LecturePLUS Timberlake
Subatomic Particles
Particle Symbol Charge Relative
Mass
Electron e
Proton p
Neutron n
LecturePLUS Timberlake

11. Atomic number = number of protons Atomic mass = mass of an atom
Expressed in amu (atomic mass units)
Equal to the number of protons + neutrons
In a neutral atom, # of protons = # of electrons

12. For example: