This document discusses the development of atomic models from ancient Greek ideas to Rutherford's gold foil experiment and Bohr's model of the hydrogen atom. It introduces Dalton's atomic theory and developments like the plum pudding model, discovery of the electron, Rutherford's nuclear model, and Bohr's explanation of emission spectra. Key concepts covered include atomic structure, isotopes, atomic notation, and forces that hold the nucleus together.

1. IB Physics Power Points
Topic 07 and 13
Atomic and Nuclear
www.pedagogics.ca
Physics
Atomic
Structure

2. Part 1
Development of an Atomic Model
Democritus to Niels Bohr

3. Early ideas about atoms
Democritus (~ 400 BC) proposed
that matter was composed of
indivisible particles he called
“atomos”.
This idea was largely ignored until
John Dalton (1766-1844) proposed
his atomic theory as a model for
matter
While this theory was not completely
correct, it revolutionized how
chemists looked at matter and
brought about chemistry as we
know it today instead of alchemy

4. Dalton’s Atomic Theory - Summary
1. matter is composed, indivisible
particles (atoms)
2. all atoms of a particular element
are identical
3. different elements have different
atoms
4. atoms combine in certain whole-
number ratios
5. In a chemical reaction, atoms are
merely rearranged to form new
compounds; they are not
created, destroyed, or changed
into atoms of any other
elements.

5. Dalton’s theory states
1. matter is composed of
indivisible particles called
atoms. (Dalton visualized
atoms as dense spheres –
not unlike billiard balls)
However, the discovery of
the electron by J.J. Thomson
in the late 1800’s introduced
the idea that there might be
something smaller than
atoms.

6. To include
electrons, scientists had
to modify the Dalton
atomic model. The new
vision was a positive
sphere that had the
newly discovered
negative electrons
embedded in it. This was
called the plum
pudding model.

7. In the early 1900’s, Ernest Rutherford tested
the plum pudding model with his famous
gold foil experiment.
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More great info @
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

8. Predicted results of
gold foil experiment if
the Plum Pudding
model had been
correct.
This is NOT what was
observed.

9. What was observed?
To explain the results, Rutherford
proposed a new model that
included a positive center for the
atom called the nucleus.

10. Adding the evidence from emission spectra
Colored lights do not emit all the wavelengths of the visible
light spectrum. For example, a red light emits mostly
wavelengths from the red end of the spectrum.
An energized gas sample will emit light of specific
wavelengths characteristic of the gas. This is called a line
spectrum

11. Emission spectra are unique for each element

13. The Bohr model of the atom
• developed using information from hydrogen
emission spectrum studies.
• a central dense positive
nucleus composed of protons
and neutrons.
• negative electrons orbit the
nucleus like planets around
the Sun (but not flat like the
solar system)
• mostly empty space. Nucleus
is 10-5 times smaller than
atom.

14. Orbiting electrons occupy discrete energy levels!
Electrons can only “jump” between energy levels if
they absorb or emit a specific amount of energy.
Atoms have quantized energy states!

15. The line spectrum of hydrogen as a direct result
of energized electrons releasing a specific
amount of energy by emitting a photon of light
of a certain wavelength.
The different lines in the hydrogen spectrum
where evidence for a number of different energy
levels.

16. lower energy higher energy
longer wavelength shorter wavelength
Visible spectrum
for
hydrogen atom
convergence

17. Limitations of the Rutherford Bohr
Nuclear Model
Why do electrons orbit the nucleus? i.e. model does
not explain why the electrons orbit rather than spiral
towards the nucleus.
Why is there one mass of positive charge in the
nucleus? i.e. or if there are multiple positive particles
why does the nucleus not fly apart?

18. Part 2 Atomic Structure – The Basics
Nuclide A specific atomic structure as determined
by the number of protons and neutrons in
the nucleus
Nucleon A nuclear particle (protons and neutrons)
Mass The total number of nucleons (protons +
Number neutrons)
(A)
Atomic The number of protons in the nucleus
Number (Z)
Isotope Two atoms of the same element (same Z)
but with different masses (A). The mass
difference between isotopes is due to
varying numbers of neutrons.

19. Atomic Notation
Show the name of the element, and the mass
number in hyphen notation
sodium-23
Show the mass number and atomic number in A-Z
notation (atomic number often omitted)
mass number A 23
atomic number Z 11
Na

20. Forces in the Nucleus