Lesson 5: The Discovery and Development of the Concept of Atom and its Subatomic Parts

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THE DEVELOPMENT OF THE STRUCTURE OF AN ATOM

(excerpt from “Atoms”; World Book International, 1999) 

 

                     The idea that everything is made up of a few simple parts originated during the 400’s B.C. in the philosophy of Atomism. Atomism was founded by the Greek philosopher Leucippus of Miletus, but his disciple Democritus of Abdera was the one who developed the philosophy more fully. In 465 B.C., Democritus gave his basic principle the name Atomos (now called Atom), meaning “uncuttable”. He imagined atoms as small, hard particles, all composed of the same substance but of different sizes and shapes. For example, atoms of water are round so that they can very easily slide past each other, atoms of salt are pointy and sharp that’s why it feels pointy to the tongue, and atoms of iron are like hooks so that they can attach to each other and be hard and strong.

Atoms of Iron are like hooks so that they can
attach to each other and be hard and strong.

                                During the Middle Ages (around 340 B.C.) however, the idea of atoms was largely ignored. This neglect resulted partly from the fact that this idea had been rejected by Aristotle, also a Greek philosopher, whose ideas dominated medieval philosophy and science. His idea of the composition of matter is that it consists of a little of each of the four elements: Earth, Water, Air and Fire. For the next 2000 years, this is what people would choose to believe in.

Aristotle believed that substances are made of a little
of each of the four elements Earth, Water, Air and Fire.

                          The development of the Atomic Theory advanced greatly when chemistry became an exact science during the late 1700’s. It was around this time that chemists discovered that there are in fact more elements than Aristotle believed, and that these elements could be combined to form different compounds. In 1808, John Dalton, a British Chemist, developed an Atomic Theory to explain this discovery. He conducted the first scientific experiment on atoms and eventually proved that Democritus was in fact correct. There are such things as atoms although he corrected that atoms DO NOT come in different sizes and shape. Atoms are all IDENTICAL, and they can be arranged in different combinations to make different compounds.

Dalton's Model of an Atom

                     In 1904, Joseph John Thomson, a British physicist, discovered that atoms are actually “cuttable”. He discovered electrons via his Cathode-Ray Tube experiment and immediately realized that electrons must be a part of the atom. He then proposed a model of an atom in which negatively charged electrons were embedded in a positively charged “pudding” or sphere to balance out the charges. This model became famous as the Plum-Pudding Model. Although Thomson’s description was far from correct, his work encouraged other scientists to investigate the structure of the atom.



J.J. Thomson's Plum-Pudding Model of an Atom

                           In 1911, the British physicist Ernest Rutherford presented his theory of atomic structure. Rutherford, a former student of Thomson’s, discovered the existence of nucleus via his gold foil experiment. He declared that all the positive charge of an atom is concentrated in its nucleus, and this tiny nucleus is surrounded by electrons travelling at tremendous speeds through the atom’s outer regions. And besides the atom’s nucleus and electrons, it’s pretty much empty space. This model became widely known as the Nuclear Model and although it did support the existence of electrons, it did not explain much about its arrangement in the atom.

Rutherford revised Thomson's Model
following his discovery of the nucleus.

                         In 1913, a description of the electron structure was proposed by Niels Bohr, a Danish physicist who had worked with Rutherford. He suggested that electrons could travel only in a certain set of orbits around the nucleus and each orbit/level has a limit as to how many electrons it can carry. Although Bohr’s model of an atom was inadequate, many of the ideas behind it proved correct and this model is still used in the present to explain many chemical concepts.


Bohr suggests that electrons have "orbits"

                          Rutherford would come out again with his proposition of the protons. Protons had been identified in 1902, but it was Rutherford who proved that protons form part of the nucleus since the nucleus has a positive charge. And so in 1914, he officially declared that protons exist in the nucleus, and not anywhere else in the atom.


Rutherford discovered that within the nucleus are protons.

                     Further studies about the structure of the atom was conducted by various scientists and by 1928, a correct description of the arrangement of electrons had been obtained with the help of Austrian physicist Erwin Schrodinger. He built upon the thoughts of Bohr and proposed the Quantum Mechanical Model of an Atom. This model states that, contrary to Bohr’s “orbit”, the exact path of an electron cannot be determined. However, we can determine where the electron has probably been and where it is likely to be going. So atoms basically have a tiny nucleus surrounded by a sphere of electron cloud – a region where electrons are widely spreading and moving around.

Erwin Schrodinger's Electron Cloud;
notice that the nucleus still didn't contain neutrons

                      Although physicists have fully understood the motions of electrons by 1928, the nucleus remained largely a mystery. They realized that the nucleus could not possibly consist of only protons because like-charges (positive and positive) repel. Then in 1932, a British physicist named James Chadwick discovered that the nucleus also contains uncharged particles called neutrons. He made this discovery via an experiment he conducted which is similar to that of Rutherford’s gold foil; but instead of using gold foil, he used boron foil.

Chadwick discovers neutrons

                      And so Atomic models often seen in present-day textbooks are somewhat a crossover between that of Bohr’s and Schrodinger’s, where a nucleus is surrounded with electrons in different levels, moving around in an imaginary region called the electron cloud.

"Textbook" Atom

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The Discovery and Development of the Concept of Atom and its Subatomic Parts (Handouts)

 

Activity # 7: Discovery and Development of the concept of atom and its Subatomic Parts

 Activity # 8: Discovery and Development of the Concept of Atom and its Subatomic Parts