Early Greek Theories
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
• All matter is made of atoms.
• Atoms of an element are identical.
• Each element has different atoms.
• Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
Adding Electrons to the Model
Materials, when rubbed, can develop a charge
difference. This electricity is called “cathode rays”
when passed through an evacuated tube (demos).
These rays have a small mass and are negative.
Thompson noted that these negative subatomic
particles were a fundamental part of all atoms.
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thompson “Plum pudding” model (1900)
Negative electrons in a positive
The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive
Ernest Rutherford (movie: 10 min.)
• Rutherford shot alpha (α) particles at gold foil.
Zinc sulfide screen
Thin gold foil
substance path of invisible
Most particles passed through.
So, atoms are mostly empty.
Some positive α-particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
• Electrons orbit the nucleus in “shells”
•Electrons can be bumped up to a higher shell i
hit by an electron or a photon of light.
There are 2 types of spectra: continuous spectra &
line spectra. It’s when electrons fall back down that
they release a photon. These jumps down from
“shell” to “shell” account for the line spectra seen in
gas discharge tubes (through spectroscopes).
Atomic numbers, Mass numbers
• There are 3 types of subatomic particles. We
already know about electrons (e–) & protons (p+).
Neutrons (n0) were also shown to exist (1930s).
• They have: no charge, a mass similar to
• Elements are often symbolized with their mass
E.g. Oxygen: 8 O
number and atomic number
• These values are given on the periodic table.
• For now, round the mass # to a whole number.
• These numbers tell you a lot about atoms.
# of protons = # of electrons = atomic number
# of neutrons = mass number – atomic number
• Calculate # of e–, n0, p+ for Ca, Ar, and Br.
Bohr - Rutherford diagrams
• Putting all this together, we get B-R diagrams
• To draw them you must know the # of protons,
neutrons, and electrons (2,8,8,2 filling order)
• Draw protons (p+), (n0) in circle (i.e. “nucleus”)
• Draw electrons around in shells
Draw Be, B, Al and shorthand diagrams for O, Na
8 p+ 2e– 6e–
11 p+ 2e– 8e– 1e–
Isotopes and Radioisotopes
• Atoms of the same element that have different
numbers of neutrons are called isotopes.
• Due to isotopes, mass #s are not round #s.
• Li (6.9) is made up of both 6Li and 7Li.
• Often, at least one isotope is unstable.
• It breaks down, releasing radioactivity.
• These types of isotopes are called
Q- Sometimes an isotope is written without its
atomic number - e.g. 35S (or S-35). Why?
Q- Draw B-R diagrams for the two Li isotopes.
A- The atomic # of an element doesn’t change
Although the number of neutrons can vary,
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