2. Early philosophers thought that matter was
composed of tiny particles
They named the tiny particles atoms
Atoms—”cannot be divided”
They didn’t perform experiments, they reasoned,
debated, and discussed—not of evidence or
proof
3. Greek philosopher
Idea of atom
No experiments to support idea
No protons, electrons, or neutrons
Solid, indestructible
Hard substances have rough atoms that stick
together
Smooth atoms are in liquids and they slide over each
other
Sleep is caused by atoms escaping the brain
4. Scientists in laboratories began debating the
existence of atoms once more
They came to realize that all matter is made up of
elements
Element—matter made of atoms of only one kind
5. John Dalton proposed the following ideas about
matter:
1. Matter is made up of atoms
2. Atoms cannot be divided into smaller pieces
3. All the atoms of an element are exactly alike
4. Different elements are made of different kinds
of atoms
6. 1803 proposed that elements consist of
individual particles called atoms
He had the first atomic theory that had evidence
to support it
Called elements “pure”
7. Dalton pictured an atom as a hard sphere that was
the same throughout—like a tiny marble
8. Dalton’s theory was tested in 1870
William Crookes did experiments with a glass tube.
Electrode—a piece of metal that can conduct
electricity
Anode—an electrode with a positive charge
Cathode—an electrode with a negative charge
9. Crookes discovered that cathode rays are
negatively charged by studying deflection of
cathode rays by magnetic fields
10. Crookes discovered that cathode rays:
1. move in a straight line
2. cause glass to glow
3. carry negative charges
4. are capable of being deflected by electric
fields
11. J.J.Thomson placed a magnet beside the tube
from Crookes’s experiment.
He concluded that cathode rays are negatively
charged particles of matter—he knew that
opposite charges attract each other
The particles were traveling in a straight line
toward the anode
12. He discovered the electron when he was
experimenting with gas discharge tubes. He
noticed a movement in a tube. He called the
movement cathode rays.The rays moved from the
negative end of the tube to the positive
end.
He realized that the rays made of negatively
charged particles –electrons.
13. J.J.Thomson inferred that electrons are a part of
every kind of atom because they are produced by
every kind of cathode material.
The biggest surprise—particles smaller than the
atom exist!
This contradicted Dalton’s theory and was not
widely accepted by fellow physicists and
chemists
14. If atoms contain one or more negatively charged
particles, then all matter, which is made of
atoms, should be negatively charged as
well…..but…..all matter is not negatively
charged.
Does matter contain a positive charge?
15. Thomson’s model of the atom was a sphere of
positive charge with negatively charged
electrons spread evenly among the positive
charge
(Model looks like a chocolate chip cookie—also
called the plum pudding model)
The positive charge of the sphere is equal to the
negative charge of the electrons..therefore..the
atom is neutral.
16. It was later discovered that all atoms are not
neutral.
The number of elements within an element can
vary.
17. Rutherford began an experiment to find out if
Thomson’s model of the atom was correct.
He fired alpha particles (positively charged) at a
thin film of metal. Since alpha particles are
positively charged, then they should be repelled
by particles of matter which also have a positive
charge
18. Rutherford reasoned that the thin, gold film did not contain
enough matter to stop the speeding alpha particle or
change its path.
There wasn’t enough charge in one place
Actually….the alpha particles were veering off at large angles
Most particles passed straight through the foil like the foil
wasn’t even there…
Some particles went straight back or were deflected as if
they had hit something…
19. The experiment shows:
Atoms are made of a small, positive nucleus; the
positive nucleus repels (pushes away) positive
alpha particles
Atoms are mostly empty space
20. The positively charged alpha particles were moving
with such high speed that it would take a large
positive charge to cause them to bounce back.
Rutherford’s new model of the atom showed that
its positive electric charge and the majority of its
mass were concentrated in an almost point sized
nucleus.
21. Scientists identified the positive charges in the
nucleus as protons.
The rest of each atom is empty space occupied by
the atom’s almost massless electrons
Finally, since all the mass wasn’t accounted for in
Rutherford’s model, it was proposed that another
particle must be in the nucleus…the neutron.
22. The model was revised again to include the
neutron…now the model has a tiny nucleus
tightly packed with +protons and neutral
neutrons.
Negatively charged electrons occupy the space
surrounding the nucleus
The number of electrons=the number of protons
23. Now we have an understanding that the nucleus is
extremely small compared to the size of the
atom.
Now we understand Rutherford’s
experiment..most of the alpha particles went
directly through the gold foil without
interference from the gold atoms because there
is plenty of open space.
24. Neils Bohr—calculated exactly what energy levels
the orbit of the electrons would represent for the
hydrogen atom.
However…scientists determined that it was
impossible to know the precise location of an
electron at any particular moment
Bohr’s calculations didn’t work for every element
25. Small nucleus with electrons traveling in an
unpredictable pattern around it called the
electron cloud.
