An introduction to the atom and early atomic theory.

1. Chapter 5

2. Democritus – thought
atoms were indivisible &
indestructible
 Lacked experimental
support
4th
century B.C.

3.  John Dalton – (1766-1844)
Dalton’s Atomic Theory
1. All elements are composed of tiny,
indivisible atoms
2. Atoms of the same element are
identical. Atoms of 1 element are
different from another element

4. Dalton’s Atomic Theory (cont)
3. Atoms of different elements
can either physically mix or
chemically combine in
simple, whole number ratios
to form compounds

5. Dalton’s Atomic Theory (cont)
4. Chemical Reactions occur
when atoms are separated,
joined or rearranged.
Atoms of 1 element cannot
change into another element
by a chemical reaction.

6. Dalton’s Theory is mostly
accepted today
Except that we now know
atoms CAN be divided - into
subatomic particles

7. JJ Thompson (1856-1940)
discovered electrons using a
device called a cathode ray
tube

8. •He sealed gases in a tube
fitted at both ends with
electrodes (metal disks)
•Connected to a source of
high voltage electricity

9. A glowing beam formed
between the 2 electrodes
He called it the
cathode ray

10. Cathode Ray Tube

11. The cathode ray is attracted
to metal plates that have a
positive charge
It is repelled by negatively
charged plates

12. The ray must be composed of
negatively charged particles
moving at a high speed
He called these particles
electrons

13. JJ Thomson’s model of the atom is
called the plum-pudding model
He thought the electrons were
randomly placed throughout the
atom, like the currants in a plum
pudding

14. Ernest Rutherford discovered the
nucleus
He used a thin sheet of gold foil and
bombarded it with alpha particles
(helium nuclei)

15. Most particles passed through the
foil as he expected
However, some were deflected
slightly and came through at an
angle
Some particles bounced back at him

16. He hypothesized that there must
be a dense positive part of the
atom, but most of the atom is
empty space
Rutherford’s
Gold Foil Experiment

17. Bohr – proposed that electrons
have a fixed energy and move in
energy levels around the nucleus
– which is why they don’t fall into
the nucleus

18. The energy levels are like the
rungs of a ladder – electrons
cannot be in between levels, and
need a specific amount of energy
to move from one to another

19. Dalton – thought atoms were
solid and indivisible
JJ Thomson – discovered the
electron, & made the plum-pudding
model

20. Rutherford – discovered the
nucleus
Bohr – proposed electrons in
orbitals around nucleus