ATOMIC MODELS
Over the past two and a half thousand years, our view of the atom has changed
dramatically. Complete the questions below, and along the way you will be
experiencing the major stepping stones to our current atomic model.
Democritus (born 460BCE)
Democritus's beliefs:
(i) Everything is made of atoms
(ii) Atoms have different sizes
(iii) Atoms have different shapes and textures
(iv) Atoms can bond together because they have hooks or other linking structures on their
surfaces
(v) Different tastes occur due to different atom properties, for example if something is
bitter it is because its atoms are sharp and jagged
Question 1
Which of these do we still believe in today?
Question 2
What evidence could Democritus have had for his beliefs?
John Dalton (1803)
New evidence available: 36 elements were known; Law of Conservation of Mass;
reacting ratios of elements in compounds; rough relative atomic weights for certain
elements
Dalton believed:
(i) All matter is made of atoms
(ii) Atoms are solid, indestructible spheres
(iii) Atoms cannot be broken down further
(iv) Different elements have different types of atoms
(v) All atoms of the same element have the same mass and same properties
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(vi) Compounds are formed from two or more different elements combining in simple,
whole number ratios
(vii) Atoms cannot be made and they can't be destroyed
(viii) In a chemical reaction, atoms are re-arranged
Question 3
Which of these do we still believe today?
Question 4
Dalton erroneously thought the formula of water was OH and the formula of ammonia
was NH. What are the correct formulas of these compounds?
J. J. Thomson (1904)
New evidence available: 84 elements known; increased understanding about radiation
Thomson's experiment: Thomson studied cathode ray tubes, which were tubes
containing a near vacuum. He ran high voltages across the tube and found that 'rays'
were emitted from the cathode. The fact that the particles in the rays were bent
towards the positive plate showed him they were negatively charged. He called them
'corpuscles' (later, electrons) and concluded they were part of all matter.
Question 5
Describe Thomson's atomic model.
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Ernest Rutherford (1911)
New evidence available: The existence of electrons; greater understanding about
the nature of alpha and beta radiation
Rutherford's experiment: Firing positively charged alpha particles (helium nuclei) at a
very thin sheet of gold adhered to glass. This was surrounded by a circular, zinc sulfide
coated screen that flashed when hit by an alpha particle. This showed that most past
through, but some are deflected and a few bounced back. At the time he said this was
as surprising as shooting a cannonball at a piece of tissue paper and having it bounce
back at you. This contradicted Thomson's model because positive charges should have
been able to move straight through his atom.
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Question 6
Describe Rutherford's atomic model and explain how his experiment led to this model.
Niels Bohr (1922)
New evidence available: 86 elements known; existence of electrons and nucleus;
further data from radiation experiments; spectra studies; concept of atomic number
Bohr's experiment: Bohr studies the emission spectra of hydrogen and found that the
lines always appeared in the same places. From this he concluded that electrons
move in energy levels called shells. Matter can absorb or emit energy only by
electrons moving between shells and the shells are always the same size for any
particular atom.
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Question 7
How was Bohr's atomic model different from Rutherford's atomic model?
James Chadwick (1932)
New evidence available: 88 elements known, first particle accelerators had been built
Chadwick's experiment: Chadwick repeated other people's experiments firing alpha
particles at beryllium. He had created a new device for measuring radiation specifically
to search for the neutral particle he believed must exist, since atomic mass
measurements were always higher than the atomic number. Sure enough, he found it
and called it the 'neutron'. He calculated its mass to be 0.1% higher than a proton.
Question 8
How did Chadwick's model of the atom differ from Bohr's?
Wave-mechanical atom (current model)
New evidence available: 114 elements known; vast amounts of information
supplied from particle accelerator experiments
This model is based on quantum mechanics and is far too complicated to try to explain
simply. Suffice to say that it is a mathematically-described model that has been
frequently modified to incorporate new discoveries.
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Question 9
Do you think this model will ever change again? Explain your answer.
Question 10
On the timeline, sketch each of the major atomic models in the appropriate place.
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