Models of the
Atom
John Dalton’s Model of the Atom (1827)
• Dalton like the Greeks thought that atoms were hard
indivisible, uncuttable particles, solid like billiard balls.
Electricity and Vacuum Tubes Shed Light on Atoms
• Michael Faraday (1838) found that if a tube with positive
and negative electrodes had the air pumped out, it began
to glow at the negative electrode end. With a better
vacuum pump, Heinrich Geissler (1857) found that a
glowing beam formed between the two electrodes.
The Laws of Electric Charges
• Like charges repel each other away.
• Unlike charges attract each other.
Where is the glowing beam coming from?
• Experiments by William Crookes (1870s) and J.J.
Thompson (1897) showed that the glowing beam was
coming from the negative electrode (cathode) and that the
glowing beam was negatively charged. The glowing beam
was called cathode rays because it came from the cathode.
Cathode rays found to be in all elements (all atoms)
• The exact same glowing beam formed no matter what
element was used for the cathode. This indicated that
matter (atoms) was made up of negative things and
because matter overall is neutral, it must also have
something positive to neutralize the negative. A new
model of the atom, the raisin bun or plum pudding model
was proposed by J.J. Thompson (1904).
The Gold Foil Experiment and The Nuclear Atom
• Ernest Rutherford designed an experiment in which a beam
of heavy positive alpha particles was shot at a very thin
gold foil sheet that was only a few atoms thick. Rutherford
expected that the alpha particles would plow directly
through the gold atoms (supposedly soft plum pudding
atoms) like an artillery shell would plow through a piece of
tissue paper.
Detecting Alpha Particle Motion
• A circular screen coated with zinc sulphide was placed
around the gold foil sheet. The zinc sulphide glowed
whenever an alpha particle crashed into it.
Gold Foil Experiment : Unexpected Results
• Rutherford found that 99 % of the alpha particles went
straight through the gold foil but a small number of
particles were scattered way off track and some even
bounced back!
Rutherford was Astonished
• The results of the Gold Foil Experiment astonished
Rutherford. He wrote, “… It was almost as incredible as if
you fired a 15 inch shell at a piece of tissue paper and it
cam back and hit you.”
A New Model of the Atom: The Nuclear Atom
• Rutherford’s experiment showed him that there was a
small positive centre in the atom that he called the nucleus.
Around this centre, the negative electrons were thought to
rotate much like the planets rotate around the sun.
Rutherford’s atom was mostly empty space with a small
positive centre and moving negative electrons around the
centre.
What is in the Nucleus?
• More experiments convinced Rutherford that the nucleus
must have both positive and neutral particles in it. The
positive particles were called protons. In 1932, an
associate of Rutherford, James Chadwick, discovered the
neutron, a particle that is neutral and weighs the same as a
proton.
Atom Composition: Three Particles
1.
2.
3.
4.
5.
6.
Atoms are composed of
protons, neutrons and
electrons.
The nucleus contains protons
(+) and neutrons (0).
Electrons (-) are moving
rapidly around the nucleus.
The number of electrons
equals the number of protons
in a neutral atom.
The charge on one electron
exactly balances the charge
on one proton. 1+ added to 1gives a charge of 0.
A proton and neutron weigh
1832 X as much as an
electron.
What About the Electrons of an Atom?
• Rutherford’s planetary model of the electrons was shown
to be incorrect since the laws of physics predicted that a
negative charge orbiting a positive nucleus would lose
energy, fall inward and crash into the nucleus, producing a
flash of violet light called the violet catastrophe. This does
not happen in nature so Rutherford’s model was wrong.
The Bohr Model of the Atom : Electron Energy Levels
• In 1913, Niels Bohr proposed that an atom’s
electrons are arranged in energy levels (shells)
around the nucleus. The electrons moved not in
circling orbits but within the shells. The inner
shell holds 2 electrons maximum, the second shell
holds 8 electrons maximum and the third shell 18.
The Bohr Model of the Atom : Electron Energy Levels
• This model helped to explain the spectra of elements.
The Bohr Atom and Spectra
• Electrons can move up to higher energy levels if energy is added to
them. These electrons are said to be excited.
• When electrons drop to lower energy levels (due to their attraction to
the nucleus {- and + charges}), they release or give off energy.
Falling Electrons Give off Specific Energies of Light
• Bright line spectra are
produced when
energized electrons
fall back to lower
levels giving off
specific energies, the
specific colours of
light in the bright line
spectrum.
Element Spectra : Unique Like Fingerprints
• Each element has its own unique bright line and
absorption spectrum which identify it like different
fingerprints identify different people.
The Bohr Model of the Atom : Main Ideas
1.
2.
3.
4.
5.
The nucleus is just
1/10,000 of the atom’s
volume. Most of the
atom is empty space.
The nucleus contains
99.9% of an atom’s
mass/weight.
The nucleus contains
positive protons and
neutral neutrons which
are 1832 times heavier
than electrons.
The negative electrons
are found in energy
shells around the
nucleus.
The number of
electrons is always
equal to the number
of protons
The Sodium Atom has 11 electrons (and protons)
1.
2.
3.
The first shell (n=1) can hold
only 2 electrons (2*(1)2.
The second shell (n=2) can
hold 8 electrons (2*(2)2.
The third shell (n=3) can hold
18 electrons (2*(3)2 .
Visualizing an Atom
If an atom were blown up in size to the size of a football field, the nucleus
(protons and neutrons) would be the size of a single pea on the 50 m
(yd) line and the electrons would be like salt grains buzzing around the
field in their shells.
How Small is an Atom?
• In an “average” grain of salt from a salt shaker, there are 1.2 X 1018
atoms or 1,200,000,000,000,000,000 atoms.
How do the Atoms of Different Elements Differ?
• The number of positive protons in the nucleus (the atomic number) of
an atom determines what kind of atom it will be (what kind of element it
will be).
The Quantum Mechanical Model of the Atom
• A number of persons working from 1925 - 1928 eventually produced
the modern highly mathematical and complex model of the atom called
the Quantum Mechanical Model of the Atom. This is the model still
accepted today. Electrons move rapidly within three-dimensional
shapes called orbitals. Many features of the Bohr atom are kept in the
Quantum Mechanical Model of the atom.
Review of the Models of the Atom from 1800 - 1940
End of Presentation