Notes on the timeline of the atomic model beginning with Democritus and going through Rutherford.

1. Defining the Atom
Chapter 2 Notes

2. Democritus,
a Greek Philosopher, was
among the first to suggest the existence
of the atom.
He
believed atoms were indivisible and
indestructible.
Though
true, these thoughts lacked
experimental support.
Early Models of the Atom

3. 2000
years later, John Dalton transformed
Democritus’s ideas on atoms into scientific
theory using experimental methods.
He
studied the ratios in which chemicals
combine in reactions.
The
result was his atomic theory
John Dalton

4. 1.
All elements are composed of tiny,
indivisible parts called atoms.
2.
Atoms of the same element are
identical. The atoms of any one element
are different from those of any other
element
Dalton’s Atomic Theory

5. 3.
Atoms of different elements can
physically mix or chemically combine in
simple whole-number ratios to form new
compounds.
4.
Chemical reactions occur when atoms
are separated, joined, or rearranged.
Atoms of one element, however, are
never changed into atoms of another
element as a result of a chemical reaction.
Dalton’s Atomic Theory (cont)

6. One
part of Dalton’s theory does not
hold true today:
◦ Atoms are not indivisible; they can be
broken down into smaller, more fundamental
particles.
Three types of subatomic particles.
Subatomic Particles

7. Discovered
in 1897 by J.J. Thomson
Electrons
are negatively charged
subatomic particles.
His
discovery involved the use of the
cathode ray tube.
Robert
Millikan found the quantity of
charge and mass of the electron to be 1
unit of negative charge
◦ and 1/1840 the mass of a Hydrogen atom,
respectively.
Electrons

8. Involved
passing electric current through
gases at low pressure in a sealed glass
tube.
Electrodes
attached to a source of
electricity were placed at each end of the
tube.
The
positive end was the anode and the
negative end was the cathode
The
result was glowing beam that moved
from the cathode to the anode
Cathode Ray Tube

9. Eugene
Goldstein found that in a cathode
ray tube, there were rays moving in the
opposite direction of the cathode ray
These
positively charged particles are
called protons
James
Chadwick later discovered a final
subatomic particle known as a neutron.
◦ A neutron is electrically neutral but has a mass
nearly equal to that of a proton.
Protons and Neutrons

10. Rutherford’s Gold Foil Expt.

11. The
nucleus is the central core of the
atom
It
is composed of protons and neutrons
The
nucleus is the region of the atom that
has an overall positive charge.
The Atomic Nucleus

12. Most
of the mass of the atom is
concentrated in the nucleus.
The
rest of the atom is assumed to be
mostly empty space; other than small
negatively charged electrons
The Atomic Nucleus