Chapter 3 Notes
Atomic Theory – Part 1
Learning Targets: I can summarize and compare the main ideas of atomic theory.
Democritus:
Ideas about atoms have been hypothesized since 400 BC. Democritus called them an
atom based on the Greek word meaning “indivisible”. Aristotle did not believe in
atoms. He thought all matter was continuous. Aristotle’s opinion was accepted for
almost 2000 years. Neither Democritus’ nor Aristotle’s ideas were supported by
experimental evidence, so each remained speculation until the 18th century.
John Dalton:
Dalton’s Atomic Theory
In 1808, an English schoolteacher named John Dalton proposed an explanation for
the above laws. He reasoned that elements were made up of atoms and that only
whole numbers of atoms can combine to form compounds.
His theory is outlined below:
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 can not 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.
Dalton’s theory explains the law of conservation of mass and the law of definite
proportions.
Modern Atomic Theory
Not all of Dalton’s atomic theory has proven to be correct.
 Atoms are divisible (although the law of conservation of mass holds true for
chemical reactions)
 Atoms of a given element can have different masses (isotopes)
Dalton’s atomic theory has not been discarded, only modified to reflect these
discoveries.
J.J. Thomson – Discovery of the __________________________.
In the late 1800s experiments were conducted using cathode ray tubes. The glass
tube contained gases at low pressures and an electric current was passed through
them.
(Draw a cathode ray tube and label, page 72, figure 4)
Cathodes and Electrons
When an electric current is passed from the cathode to the anode a glowing cathode
ray is produced. The following observations were made:
1. When a magnetic field was applied the cathode ray was deflected proving
that the ray is negatively charged.
2. The rays were deflected away from a negatively charged object.
These observations led to the hypothesis that the particles that compose cathode
rays are negatively charged. Joseph John Thomson in 1897 carried out experiments
that strongly supported this hypothesis. Thomson concluded that all cathode rays
are composed of identical negatively charged particles, which were named
electrons.
Charge and Mass of the Electron
Cathode rays provided evidence that atoms are divisible and that one of the atom’s
basic constituents is the negatively charged electron.
Thomson’s experiments also determined that electrons have a very large charge-tomass ratio.
In 1909 Robert A Millikin measured the charge of the electron.
From these 2 pieces of information the mass of the electron is 1/1837 the mass of
the simplest type of H atom.
2 other inferences were made because of the electron:
1. Because atoms are electrically neutral, they must contain a positive charge to
balance the negative electrons.
2. Because electrons have so much less mass than atoms, atoms must contain
other particles that account for most of their mass.
Thomson proposed the plum pudding model: negative electrons are spread out
evenly throughout the positive charge of the rest of the atom (watermelon). New
experiments disproved this model.
Ernest Rutherford – Discovery of the _______________________.
In 1911 Ernest Rutherford provided more detail of atomic structure.
Rutherford and his associates performed the “Gold Foil Experiments”. They shot
alpha particles (positively charged particles about 4 times the size of a hydrogen
atom) at a thin piece of gold foil. A screen detected that most of the particles passed
through the foil with little or no reflection. A small amount of the particles were
deflected at wide angles (they were shocked by this).
Rutherford’s conclusions:
 Deflected alpha particles hit a very small densely packed bundle of matter
with a positive charge.
 The volume of the nucleus was very small compared to the total volume of
the atom.