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5.1 Evolution of the Atomic
Model
Learning Goals
• I can describe the components of atoms and
their locations within atoms.
• I can describe how the work of Dalton,
Thomson, Rutherford and Bohr contributed to
the development of the atomic model.
Atomic Theory
• John Dalton’s atomic theory described elements in
terms of atoms which he believed to be small,
indivisible particles that make-up all matter.
• Atomic Theory
• He stated that all the atoms of the same element are
identical in mass and size, but different elements are
made up of different atoms.
• Joseph John Thomson determined that atoms contain
negatively charged particles, which are now called
electrons.
• He developed a model of the atom that shows
electrons inserted throughout a mass of positively
charged material.
• Thomson model
• Ernest Rutherford updated the model of the atom as
mostly empty space, with a small, dense, positively
charged nucleus at the centre.
• His continued work, as well as that of others,
eventually led to the identification of the proton and
neutron in the nucleus.
• Rutherford model
• Neils Bohr revised Rutherford’s model of the atom by
stating that electrons are stable in specific energy
levels around the nucleus.
• Bohr Model
1. What is an atom? What are the three subatomic
particles of an atom?
• An atom is the smallest particle of an element that has the same
properties of the element.
Protons – positively charged
found in the nucleus
1 amu (atomic mass unit)
Neutron – neutral (no charge)
found in the nucleus
1 amu (atomic mass unit)
Electron – negatively charged
found in energy levels surrounding the nucleus
0 amu (atomic mass unit)
2. What is Dalton’s description of an
atom? List Dalton’s Atomic theory.
1.
2.
3.
4.
Elements are made of tiny particles called atoms.
Atoms cannot be created nor destroyed
All atoms of a given element are identical.
The atoms of a given element are different from
those of any other element.
5. Atoms of one element can combine with atoms of
other elements to form chemical compounds.
Thomson’s Discovery of electrons:
3. Thomson’s experimental evidence indicated that all
atoms contain the same negatively charged particles.
Describe Thomson’s experimental evidence.
• When a ray was emitted from the negatively
charged cathode it travelled to the positively
charged anode.
• Thomson discovered that the same kind of
negative ray was always emitted for all elements.
• These particles are now called electrons and are
found in all atoms.
4. What is the difference between Thomson’s model
of the atom and Dalton’s model of the atom?
• Dalton stated that different elements are
composed of different atoms and that atoms cannot
be divided into smaller particles.
• Thomson built on Dalton’s model by stating that
the atom is a lump of positively charged materials
with negative electrons inserted throughout.
5. Rutherford's experiment video
6. What are alpha particles? What is the source
of these alpha particles?
• Alpha particles are positively charged particles
• The source is a very small amount of
radioactive element called radium.
7. Describe Marie Curie’s contribution to
science.
• Marie Curie was a polish born physicist who
received two Nobel Prizes (one for physics and
one for chemistry)
• She discovered and isolated the radioactive
element radium.
8. What is the result of Rutherford’s gold foil
experiment?
• Some of the particles were repelled backward
instead of passing through the foil.
• Since the alpha particles are positively
charged, Rutherford proposed that all the
positively charged materials in an atom
formed a small dense centre and that the
electrons would have to be separated from it.
• He named this centre the nucleus
9. What did Bohr discover about the movement
of electrons?
• Bohr proposed that electrons could only move
within fixed regions (energy levels), rather
than being able to move anywhere around the
nucleus.
10. How can an electron move from one energy
level to a higher energy level?
• For an electron to move from one energy level
to a higher one, it must absorb a specific
amount of energy (quantum).
11. Describe the nucleus of an atom. What is it
made of?
• The nucleus is composed of two types of
particles: the positively charged proton and a
neutral neutron.
12. Draw the four models of an atom using
diagrams.
John Dalton
Joseph John Thomson
12. Draw the four models of an atom using
diagrams. (continued)
Ernest Rutherford
Niels Bohr
Practice
• Page 186 # 1, 2, 5, 6, 8