1. Models of the Atom a Historical Perspective

2. Early Greek Theories
Democritus
400 B.C. - Democritus thought matter could not be divided indefinitely.
This led to the idea of atoms in a void.
fire
air
water
earth
Aristotle
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.

3. John Dalton 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).

4. 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.
Dalton’s “Billiard ball” model ( )
Atoms are solid and indivisible.
Thompson “Plum pudding” model (1900)
Negative electrons in a positive framework.
The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.

5. Ernest Rutherford (movie: 10 min.)
Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive substance
path of invisible -particles
Most particles passed through. So, atoms are mostly empty.
Some positive -particles deflected or bounced back!
Thus, a “nucleus” is positive & holds most of an atom’s mass.

6. Bohr’s model Electrons orbit the nucleus in “shells”
Electrons can be bumped up to a higher shell if 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).

7. 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 protons
Elements are often symbolized with their mass number and atomic number
E.g. Oxygen: O 16 8
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.

8. Atomic
Mass p+ n0 e– Ca 20 40 20 20 20 Ar 18 40 18 22 18 Br 35 80 35 45 35

9. 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 He Li
3 p+
4 n0
2e– 1e–
Li shorthand
3 p+
4 n0
2 p+
2 n0
Draw Be, B, Al and shorthand diagrams for O, Na

10. Be B Al O Na 8 p+ 11 p+ 8 n° 12 n° 4 p+ 5 n° 5 p+ 6 n° 13 p+ 14 n°
2e– 8e– 1e– Na
8 p+
8 n°
2e– 6e– O

11. 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 radioisotopes
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, atoms have definite numbers of protons.

12. For more lessons, visit www.chalkbored.com
6Li 7Li
3 p+
3 n0
2e– 1e–
3 p+
4 n0
2e– 1e–
For more lessons, visit

13. Atomic Mass The mass of a proton or a neutron is:
1.67 x g
The mass of an electron is negligible compared to mass of proton or neutron :
9.11 x g

14. Atomic Mass Unit
Relative masses of atoms are compared using a reference isotope as a standard.
Carbon-12 was chosen as the reference isotope.
Carbon-12 was assigned exactly 12 atomic mass units
Atomic mass unit (amu): one twelfth the mass of a carbon-12 atom
In nature, most elements occur as a mixture of 2 or more isotopes.
Each isotope of an element has fixed mass & a natural percent abundance.

16. Atomic Mass Atomic mass is NOT the same thing as nuclear mass
Atomic mass is the mass of the atom complete with its electrons
Nuclear mass is the mass of the nucleus alone.
To get nuclear mass, one must remove the mass of the electrons.

19. Convert percentage to a decimal by dividing by 100