1. Models of the Atom a Historical Perspective

2. Size of Atom: Models Needed
Since atoms are smaller than the wavelength of visible light ->
Scanning tunneling microscope (STM) allows individual atoms to be detected and moved!
Physical model- objects in a convenient scale to observe
Conceptual model- explanation of a system and how it interacts- like predicting a weather system

3. Early Greek Theories fire air water earth
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.

4. 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
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).

5. 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.

6. Quantum Hypothesis Light is zillions of particles
Particles are known as photons
The smallest unit of something: quantum
Quantum means that it is a whole unit and groups can be multiplied by that single whole number to determine mass.
Mass and electric charge on subatomic particles-like an electron are quantum values.
When energy is absorbed, the electron moves to a higher energy level. When energy is released, the electron moves to a lower energy level and release light.

7. 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).

8. Atoms change energy levels
When atoms decrease energy levels they release a characteristic color of light
Neon releases a red-orange light
Fireworks use the colors to light up the night
Colors given off by elements

9. Implications of Atom’s “Light”
Spectroscope can be used to identify atoms
Atom’s color’s are called that element’s atomic spectrum-it’s fingerprints!
Identify make-up of stars light years away by examining the light they give off.

10. Principal Quantum Number
Each energy level in an atom has a Principal Quantum Number corresponding to its distance from the nucleus.

11. 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.

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

13. 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

14. 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

15. 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.

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6Li 7Li
3 p+
3 n0
2e– 1e–
3 p+
4 n0
2e– 1e–
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