This document discusses the early history and development of atomic theory. It describes how Democritus first proposed the idea of atoms in ancient Greece. John Dalton later transformed these ideas into a scientific theory through experimental evidence showing atoms combine in simple whole number ratios. Dalton proposed atoms as the fundamental units of elements and that they combine or separate in chemical reactions. Later, subatomic particles like electrons, protons, and neutrons were discovered inside atoms through experiments. The nucleus, composed of protons and neutrons, was found to be at the center with most mass concentrated there.

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. 200
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