Part 1 - The Early YearsDemocritus was the first to believe the idea

of the atom. He was a Greek philosopher and believed that everything in the world must be made of tiny indivisible particles called “atomos.”

Some of his ideas did agree with later scientific theory, but did not explain chemical behavior, and were not based on the scientific method – but just

philosophy. Aristotle, who was more widely respected at the time, rejected this belief and the “atomic” view eventually faded.

Democritus’s atomic theory stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.” According to Democritus, atoms were miniscule quantities of matter. Democritus predicted that atoms cannot be destroyed, that they differ in size, shape, and temperature, that they are always moving, and that they are invisible. He believed that there were an infinite number of atoms. This hypothesis was created in 465BC.

Critical Thinking Questions1. Which of Democritus’s theories do you think still hold true

today?

2. Why were his theories rejected by the scientific community?

Part 2Dalton’s Experiment

0.6 L of hydrogen gas

+ 0.6 L chlorine gas

yields 1.2 L muriatic acid gas

1.2 L of hydrogen gas

+ 0.6 L chlorine gas

yields 1.2 L muriatic acid gas + 0.6 L hydrogen gas

0.6 L of hydrogen gas

+ 1.2 L chlorine gas

yields 1.2 L muriatic acidgas

+ 0.6 L chlorine gas

1.2 L of hydrogen gas

+ 1.2 L chlorine gas yields 2.4 L muriatic acid gas

Dalton’s 5 Part Atomic Theory (1803)

All elements are made of atoms.

Atoms of an element are identical.

Atoms of different elements are different.

Atoms combine in simple whole number ratios to make compounds.

Atoms may rearrange in chemical reactions, but are not changed in the reaction.

Dalton’s Model of the Atom: Dalton described the atom (his model) as a small hard sphere. All elements were made of many of these spheres. Since atoms of each element are different from each other, the sphere for each element would be slightly different.

Critical Thinking Questions3. Evaluate the information provided for Dalton’s experiment. Which one of the five parts of his theory is best supported by the experimental data? Use specific data from the model to explain how the data supports his theory.

4. What part of Dalton’s model of the Atom do we still agree with today? What part of the model has been discarded?

Part 3Thomson’s Experiments (1897)

Part 1: Thomson used a hollow glass tube with a metal plate on either end. Thomson removed the air from the tube, and connected the tube to an electricity source. He found that “rays” travelled across the tube from the cathode to the anode.

Part 2: Thomson exposed the ray to a magnet and found that the ray bent toward the positive end of a magnet, and away from the negative end of a magnet.

Part 3: Thomson used an electric field to measure the mass to charge ratio of the ray. He found this ratio to be almost 2000 times larger than the mass to charge ratio for a charged hydrogen atom. So.. either the ray particles had a HUGE charge, or a very small mass.

Critical Thinking Questions5. What technological advancement was required for Thomson to conduct

his experiments?

6. What part of the atom did Thomson discover? Give specific evidence from his experiments to support your claim.

Thomson’s Model of the Aton: Thomson held on to Dalton’s idea that atoms were solid objects. He modified Dalton’s model to add a positively charged “base” material that had small negatively charged particles imbedded. He called this the Plum Pudding model.

7. What part of Thomson’s model of the Atom do we still agree with today? What part of the model has been discarded?

Part 4Millikan’s Experiment - 1909

An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'. He received the Nobel Prize for his work. We're going to explain that experiment here, and show how Millikan was able to determine the size of a charge on a single electron.

What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he

noticed that the charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons carrying this unit charge.

Critical Thinking Questions8. What was Millikan’s main contribution to atomic theory? Give the value for this discovery.

9. What was the experiment he used to do make this discovery?

Part 5Rutherford’s Experiment (1911)

Rutherford set out to test Thomson’s model of the atom using alpha particles. Alpha particles are essentially a Helium nucleus that is emitted from a larger atom during radioactive decay. These small particles are able to pass through paper, and thin layers of other materials.

The experiment involved using a sheet of gold that was only 2-3 atoms thick. The alpha particles were released from a lead box through a slit. 99.999% of the particles passed directly through the gold foil as if it wasn’t there. A very small number of particles were deflected, some a little, some were deflected almost straight back.

Critical Thinking Questions10. What technological advancement was required for Rutherford to

conduct his experiment?

11. What part of the atom did Rutherford discover? Give specific evidence from his experiment to support your claim.

Rutherford’s Model of the Atom: Rutherford held on to Thomson’s idea that atoms were made up of positive and negative parts. He modified Thomson’s model to specify that the positive part of the atom was contained in a small, dense nucleus, and the electrons orbited the nucleus like planets orbit the sun.

12. What part of Rutherford’s model do we still agree with today? What part of the model has been discarded?

Part 6Bohr’s Experiment (1922)

Bohr observed that when a sample of hydrogen gas was exposed to electric current it released energy as light. When this light was viewed with a spectroscope, specific wavelengths of light were seen. Different wavelengths of light correspond to different levels of energy. Bohr used this same method to test other elements and found that each emitted its own distinct color of light, each made up of its own wavelength combinations.

Bohr realized that this phenomenon was not possible with Rutherford’s model of the atom, since electrons travelling in an elliptical orbit would produce a continuous rainbow of colors instead of the distinct lines seen.

high energy low energy

Bohr’s Model of the Atom: Bohr held on to Rutherford’s idea that atoms were made up of a positive nucleus with electrons orbiting around the nucleus. He modified Rutherford’s model to specify that the electrons travel in an orbit that is a set distance from the nucleus. This results in each electron orbit containing a specific amount of energy.

Critical Thinking Questions13. What property of atom behavior did Bohr discover? Give specific

evidence from his experiment to support your claim.

14. What part of Bohr’s model do we still agree with today? What part of the model has been discarded?

15. Our understanding of the atom has changed from Dalton to today. When changes were made to the model of the atom were all previous models thrown out, or were they modified? Why do you think this is the case?

Part 7Today’s Model…Quantum Mechanical ModelThe current model, the quantum mechanical model, is a mathematical model that predicts the probability of electron locations and paths within an electron cloud.  This model resulted from scientists such as de Broglie, Schrödinger, and Heisenberg.  

1925 - Louis de Broglie suggests that if light can act as a particle, then a particle might have wave properties

In 1926, Schrodinger further explained the nature of electrons in an atom by stating that the exact location of an electron cannot

be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found.

The Heisenberg uncertainty principle states that it is impossible to know both the momentum and the position of a particle at the same time.

James Chadwick realized that the atomic mass of most elements was double the number of protons. He discovered the neutron in 1932

These scientists started to look at electrons acting as both particles and waves, and determined that there can be no certainty in finding the position of an electron if you know the momentum at which it travels.  This led to the idea of electrons existing in an electron cloud moving as both a particle and a wave at the same time.  Hence came the quantum mechanical model

Quantum Mechanical Model:

Critical Thinking Questions16. List all the scientists, their discovering, and year of their

discovering for the Quantum Mechanical Model (use the picture above).

17. In the Quantum Mechanical model, how do electrons travel? Where are they located?