1. History of the Atom

2. The Greek philosopher Democritus (460–370 B. C
The Greek philosopher Democritus (460–370 B.C.) was the first person to
propose the idea that matter was not infinitely divisible. He believed matter was made up of tiny individual particles called atomos, from which the English word atom is derived. Democritus believed that atoms could not be created, destroyed, or further divided. Democritus and a summary of his ideas are shown below.

3. Evidence for atoms
One of the first pieces of direct evidence for the existence of atoms actually came from a Botanist, Robert Brown. In 1827, whilst studying pollen grains floating on the surface of water, he noticed that the pollen moved in independent and random directions. By doing the same experiments with particles of dust, he was able to rule out that the motion was due to pollen being "alive", but it remained to explain the origin of the motion. This motion was called “Brownian Motion” and remained unexplained for almost 80 years.
Although not the first to give weight to this theory, Einstein observed that the a small particle would receive a random number of impacts of random strength and from random directions in any short period of time. This would cause a sufficiently small particle to move in exactly the way described by Brown.

4. John Dalton – Atomic Theory
Work done in the 19th century by John Dalton (1766–1844), marks the beginning of the development of modern atomic theory.
Dalton revived and revised Democritus’ ideas based upon the results of scientific research he conducted. The main points (postulates) of Dalton’s atomic theory are shown below.
More on John Dalton’s Postulates

5. J.J. Thomson – Discovery of the electron
At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than anything else.
Thomson was experimenting by applying high voltages to gases at low pressure in a device known as a Cathode Ray Tube. He noticed an interesting effect. This is shown in the animation below and on the next slide:

8. JJ Thomson identified the electron by measuring the cathode rays response to both electric and magnetic fields. By balancing the forces on the beam Thomson worked out the specific charge to mass (q/m) ratio of the electron beam. The ratio for the electron beam was nearly 1800 times greater than the q/m ratio of a hydrogen ion, from which Thomson concluded that:
The charge on the electron was much greater than the hydrogen ion
-or-
the mass of the electron was much smaller than the hydrogen ion.
Thomson went on to determine that the amount of charge on the electron and the hydrogen ion was the same. Therefore, the mass of the electron had to be very small.

9. Where had these tiny particles come from
Where had these tiny particles come from? Since they were so small, Thomson suggested that they could only have come from inside atoms. So Dalton's idea of the indestructible atom had to be revised.
Thomson proposed a different model for the atom. He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding – but with lots more space in between).
Thomson’s Plum Pudding Model

10. Ernest Rutherford – Discovery of the nucleus
The next development came about 10 years later. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil.
They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described.
However, they got a big surprise. Look at their experiment here:

11. Alpha particles (named after and denoted by the first letter in the Greek alphabet, α) consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as He2+ or 42He2+. They are a highly ionizing form of particle radiation, and have low penetration.
Alpha particles are emitted by radioactive nuclei such as uranium, thorium, actinium, or radium in a process known as alpha decay.

12. How this experiment works is similar to this below

14. Another example of Rutherfords experiment is shown below

15. Turn up the volume to listen to what rutherford did.

17. Thompson Model Hypothesis If the positive charge were scattered throughout the atom, the a particles would have been scattered through a very small angle.
Rutherford Model Results The alpha particles are scattered through a large angle due to the dense and positively charged nucleus. From this,
Rutherford deduced that the positive charge is concentrated in the centre of the atom, and called this concentration of positive charge the nucleus.
It was furthermore experimentally found that although the nucleus only occupies approximately of the total volume of the atom or less, it contains all the positive charge and accounts for at least % of the atom's mass.

19. In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right.
“I was told long afterwards by a distinguished physicist who had been present at my lecture that he thought I had been pulling their leg.” -JJ Thomson

20. Niels Bohr’s Energy Levels
The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.

23. Dalton studied electromagnetic spectra to develop his theories
Dalton studied electromagnetic spectra to develop his theories. Each element has its own distinct line spectra.  For example:
                                                                                                                      
Hydrogen line spectra
                                                                                                                                                                                                                      
Helium line spectra
                                                                                                                                                                                                                      
Neon line spectra

24. How spectra originate

27. Summary - So Far Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today). In John Dalton introduced a new form of the ancient Greek idea of atoms. In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom. In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun. In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels. We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).

29. Questions
1. Which of Dalton's ideas below do we no longer believe?
a.Elements contain only one type of atom.
b.Atoms rearrange in chemical reactions.
c.Atoms are solid masses that can't be split into smaller particles.
2. Which statement about Thomson's model of the atom is true?
a.The charge on the electrons is far greater than the positive charge in the atom.
b.The total charge carried by the electrons equals the positive charge in the atom.
3. In which two ways did Rutherford change Thomson's model of the atom?
a.He said that the electrons were concentrated in the nucleus of the atom.
b.He said that the positive charge was concentrated in the nucleus of the atom.
c.He said that the electrons were orbiting nucleus of the atom.
4. How did Bohr change Rutherford's model of the atom?
b.He said that electrons could only occupy specific energy levels as they orbit.
c.He said that the electrons could orbit the nucleus in a completely random way.
5. Which of the following ideas of John Dalton are still believed to be correct?
a.Atoms are solid. b.Atoms in an element are all the same type of atom. c.Atoms cannot be split into smaller particles.
d.Compounds contain different types of atom.
6. Who proposed the 'Plum Pudding' model of the atom?
a.Dalton b.Thomson c.Rutherford d.Bohr

30. 7. What is the charge on an electron?
a.Neutral b.Positive c.Negative
8. Who performed the alpha particle scattering experiment as part of Rutherford's team?
a.Dalton b.Thomson c.Geiger and Marsden d.Bohr
9. In the experiment carried out by Rutherford's team, what type of radiation was used?
a.Gamma radiation b.Beta radiation c.Alpha radiation
10. Which statement below is correct?
a.Most of the volume of an atom is taken up by a large nucleus.
b.Atoms are mostly space.
11. How did Rutherford revise ideas about the distribution of positive charge in an atom compared with the 'plum pudding' model?
a.Rutherford suggested that the charge was evenly spread throughout the atom.
b.Rutherford suggested there was no positive charge in the atom.
c.Rutherford suggested that the positive charge was concentrated in the nucleus.
12. Who first suggested that the electrons orbited the nucleus in fixed energy levels (shells)?
a.Dalton b.Thomson c.Rutherford d.Bohr
13. Which of these 3 sub-atomic particles is yet to be discovered? (more than one)
a.Electrons b.Protons c.Neutrons

31. 8Nitrogen → 7Oxygen + Hydrogen Nuclei
Discovering the proton
Continuing research based on the work of others, Ernest Marsden wanted to find out what would happen if he bombarded light nuclei with alpha rays. Marsden, however, was not able to complete the experiment. So in 1918 Rutherford decided to do it himself.
When Rutherford shot alpha particles into nitrogen gas, he noticed something strange. Some of the scintillations in his detector did not seem to be coming from either alpha particles or nitrogen atoms. They looked like scintillations made by hydrogen nuclei.
At first, Rutherford thought the nuclei were coming from his radioactive (radium) source.
Further experiment showed that they were coming from the nitrogen gas. Rutherford concluded that some of the nitrogen atoms were disintegrating when hit by alpha particles. Rutherford was changing atoms of nitrogen into atoms of oxygen.
8Nitrogen → 7Oxygen + Hydrogen Nuclei
His experiment convinced Rutherford that the nitrogen nucleus had hydrogen nuclei in it.
That meant the hydrogen nucleus was an elementary particle.
Rutherford named it the proton, from the Greek word "protos," meaning "first."

32. In 1932, Rutherford’s co-worker, English physicist James Chadwick (1891–1974), showed that the nucleus also contained another subatomic particle, a neutral particle called the neutron. A neutron has a mass nearly equal to that of a proton, but it carries no electrical charge. Thus, three subatomic particles are the fundamental building blocks from which all atoms are made—the electron, the proton, and the neutron. Together, electrons, protons, and neutrons account for all of the mass of an atom. The properties of electrons, protons, and neutrons are summarized in the table below.

33. Summary

34. Want to know more about the discovery of the neutron
Want to know more about the discovery of the neutron? Put your pens down and turn your brains on ……

35. Discovering the neutron
By 1910, measurements of atomic mass indicated that all nuclei must contain integer numbers of some other particle. Rutherford thought that the nucleus consisted of protons and 'neutral doublets' formed from closely bound protons and electrons.
Rutherford described his 'neutral doublet', or neutron, in The particle would be uncharged but with a mass only slightly greater than the proton. Because it was uncharged there would be no electrical repulsion of the neutron as it passed through matter, so it would be much more penetrating than the proton. This would make the neutron difficult to detect.

36. In 1930, the German physicists Walther Bothe and Herbert Becker noticed something odd. When they shot alpha rays at beryllium (atomic number 4) the beryllium emitted a neutral radiation that could penetrate 200 millimeters of lead. In contrast, it takes less than one millimetre of lead to stop a proton. Bothe and Becker assumed the neutral radiation was high-energy gamma rays.
In 1932 Irène and Frédéric Joliot-Curie investigated this radiation in France. They let the radiation hit a block of paraffin wax, and found it caused the wax to emit protons.
They measured the speeds of these protons and found that gamma rays would have to be incredibly energetic to knock them from the wax.

37. Chadwick reported the Joliot-Curie's experiment to Rutherford, who did not believe that gamma rays could account for the protons from the wax. He and Chadwick were convinced that the beryllium was emitting neutrons. Neutrons have nearly the same mass as protons, so should knock protons from a wax block fairly easily.
Chadwick worked day and night to prove the neutron theory, studying the beryllium radiation with an ionisation counter and a cloud chamber. He found that the wax could be replaced with other light substances, even beryllium, and that protons were still produced.
Within a month Chadwick had conclusive proof of the existence of the neutron. He published his findings in the journal, Nature, on February 27, 1932.
We now know it is not a combination of an electron and a proton.

39. What are protons and neutrons made of? Quarks
By the early '60's, physicists had gathered sufficient data to indicate the presence of quarks. The first people to interpret this data in the form of a quark theory were Murray Gell-Mann and George Zweig.
Quarks were discovered in 1968 at SLAC by Jerome Friedman, Henry Kendall and Richard Taylor, for which they won the Noble Prize in In their experiment they bounced electrons off protons (Hydrogen atom nuclei). They observed that the electrons were not deflected in a manner consistent with a point-like proton. James Bjorken and Richard Feynman analysed their data and found that the scattering of the electrons was consistent with a proton consisting of smaller particles - quarks.

40. Quarks are one type of matter particle
Quarks are one type of matter particle. Protons and neutrons are composed of quarks.
 There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom.
Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively.
The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years.

41. Quark Structure of the Proton, and Neutron
What do the charges add up to?

43. So……..What do we know now?
Right click above and select play

44. Summary

45. Summary