The Historical Development of the Atomic Model

• From ancient Greece philosophy, to today’s model of the atom –

• The model of the atom demonstrates the purpose of science:

“The purpose of science is to create models that explain

natural phenomena.”

Scientific ModelsA model is the best possible explanation which

accounts for all observed phenomenon and has predictability.

1) Best means there can only be ONE model.2) An unanswered question means change the

model.3) Predictability is the test of a good or “true”

model.4) Occham’s Razor – of 2 possible explanations,

choose the simpler.

The ancient Greeks were the first to postulate that matter consists of indivisible

constituents.

The laws of conservation of mass and definite proportions provided the experimental foundation for the

atomic theory

• Law of Conservation of Mass:– No detectable gain or loss of mass occurs

in chemical reactions. Mass is conserved.

• Law of Definite Proportions:– In a given chemical compound, the

elements are always combined in the same proportions by mass.

Dalton’s Atomic Theory:1. Matter consists of tiny particles called atoms.

2. Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not themselves break apart.

3. In any sample of a pure element, all the atoms are identical in mass and other properties.

4. The atoms of different elements differ in mass and other properties.

5. In a given compound the constituent atoms are always present in the same fixed numerical ratio.

Rephrased John Dalton:•Each element is composed of atoms

•All atoms of an element are identical.

•In chemical reactions, the atoms are not changed.

•Compounds are formed when atoms of more than one element combine.

•Dalton’s law of multiple proportions: When two elements form different compounds, the mass ratio of the elements in one compound is related to the mass ratio in the other by a small whole number.

• It follows from Dalton’s Atomic Theory that atoms of an element have a constant, characteristic atomic mass or atomic weight

• For example, for any sample of hydrogen fluoride:

• F-to-H atom ratio: 1 to 1• F-to-H mass ratio: 19.0 to 1.00

– This is only possible if each fluorine atom is 19.0 times heavier than each hydrogen atom

Later scientists realized that the atom consisted of charged entities.

•The voltage causes negative particles to move from the negative electrode to the positive electrode.

*The path of the electrons can be altered by the presence of a magnetic field.

•Consider cathode rays leaving the positive electrode through a small hole.

•If they interact with a magnetic field perpendicular to an applied electric field, the cathode rays can be deflected by different amounts.

The Discovery of Atomic The Discovery of Atomic StructureStructure

Cathode Rays and Electrons

•The amount of deflection of the cathode rays depends on the applied magnetic and electric fields.

•In turn, the amount of deflection also depends on the charge to mass ratio of the electron.

•In 1897, Thomson determined the charge to mass ratio of an electron to be 1.76 108 C/g.

•Goal: find the charge on the electron to determine its mass.

From the separation of radiation we conclude that the atom consists of neutral, positively, and negatively charged entities.

Thomson assumed all these charged species were found in a sphere.

Thomson’s Plum-Pudding Model

Consider the following experiment by Robert Millikan :

•Oil drops are sprayed above a positively charged plate containing a small hole.

•As the oil drops fall through the hole, they are given a negative charge.

Goal: find the charge on the electron to determine its mass.

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Cathode Rays and Electrons

Gravity forces the drops downward. The applied electric field forces the drops upward.

When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate.

The Discovery of Atomic The Discovery of Atomic StructureStructure

Cathode Rays and Electrons• Using this experiment, Millikan determined the charge on

the electron to be 1.60 10-19 C.• Knowing the charge to mass ratio, 1.76 108 C/g,

Millikan calculated the mass of the electron: 9.10 10-28 g.

• With more accurate numbers, we get the mass of the electron to be 9.10939 10-28 g.

Ernest Rutherford’s Gold Foil Experiment

• Results of the Rutherford experiment

(a) The results that the metal foil experiment would have yielded if theplum pudding model had been correct

(b) Actual results

Rutherford’s Nuclear Model of the atom

•Small, dense, positively charged nucleus

•Contains protons (+1 charge)

•Contains neutrons (no charge)

•Remainder of the atom is mostly empty space

•Contains electrons (-1 charge) in the “empty space”