This document outlines the development of atomic theory from ancient Greek philosophers to modern physics. It describes key contributors such as Democritus, Dalton, Thomson, Rutherford, Bohr, and Schrodinger who proposed models of the atom based on experiments. The modern atomic theory is that atoms are composed of a nucleus of protons and neutrons surrounded by electrons in electron clouds, and that elements are distinguished by their atomic number. Isotopes are variants of the same element that differ in neutron number. Forces like electromagnetic, strong, and weak govern atomic structure.

2. Atomic Theory
• Democritus (440 BCE)-
realized that if you continued
to cut something, eventually
you would end up with
something that couldn’t be
cut anymore, “atomos” –
meaning not able to divide
• Atoms smallest particle
that an element can be
divided & still be the same
substance
• All matter is made of atoms

4. John Dalton (1803)
• realized that atoms combine
in very specific proportions
(ratios) based on mass
• all substances are made of
atoms and they cannot be
created, divided or destroyed
(made of a single substance)
• All atoms of the same
element are exactly alike and
different from other elements,
(unique)
• Atoms join with other atoms
to form new substance

6. J. J. Thomson (1897)
• discovered that there were
small particles inside the
atom, meaning that atoms
can be divided into smaller
substances
• Electrons – negatively
charged particles attracted to
positively charged particles
• Plum pudding model –
electrons are mixed
throughout the atom, soft
blobs of matter

8. Ernest Rutherford (1909)
• Discovered that an
atom contains a
nucleus with
positively charged
particles and that the
electrons must be
“floating” around the
nucleus
• Most of an atom is
empty space

11. Niels Bohr (1913)
• Proposed that electron
moved around the
nucleus in energy levels
(shells), but no electrons
between the energy level
(think ladder)
• Electrons can jump from
one level to another
• Travel in a definite path

13. Modern Atomic Theory
• Erwin Shrodinger &
Werner Heisenberg
• Electrons have no
predictable pattern
and move in a region
where electrons are
likely to be found
called the electron
cloud

16. Atoms
• All atoms have a nucleus
– protons (+),
– neutrons (no charge)
– electrons (-)
• Same number of protons and electrons an
atom has no charge
• More protons (+) than electrons (-) the atom
has a positive ion form (more positives than negatives)
• More electrons (-) than protons (+) a negative
ion is formed (more negatives than positives)

17. • 117 different elements that are unique and
all things known to exist come from a
combination of these elements in specific
mass ratios
• Simplest atom is made of one proton, and
1 electron – hydrogen (has no neutrons)

18. • All additional element will have protons,
neutrons and electrons
• The atomic number of an element is
determined by the number of protons,
– 1 is hydrogen, 6 is carbon, hydrogen has 1
proton, carbon has 6 protons (you cannot
change the number of protons)
• To find neutrons take the mass number
(rounded) and subtract the protons.

19. Isotopes
• Isotopes have the same number of protons but
additional neutrons which causes the atomic
mass to be different
• Isotopes can be stable (maintain their structure)
and unstable (fall apart over time)
• Unstable isotopes are radioactive and will decay
over time giving off particles and energy
(radioactive)

20. • Mass number
determines the
isotope, the number
of protons and
neutrons added
together
• Most elements have
isotopes
• All isotopes of an
element have the
exact properties of
the element

21. Forces in
atoms
• Gravitational force –
pulls objects toward
each other—depends on
mass and distances
between the objects—
very small force in
atoms
• Electromagnetic force
–– proton (+) and
electrons (-) have strong
attraction which keeps
the electrons in motion
around the nucleus of

22. • Strong force – force
which keeps protons
from flying apart due
to close distance
between protons and
neutrons
• Weak force – relevant
to radioactive atoms-
allows neutrons to
change into proton
and electron