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6 Development of Atomic Theory – History – Chemists – Timeline

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The atomic theory is a theory that states that matter is composed of discrete units called atoms. The term atom comes from the ancient Greek term atomos which means “indivisible.” According to its development of atomic theory until the 20th century, the theory that stated atom was indivisible was denied because the atom is actually a combination of various subatomic particles. The subatomic particles are electrons, protons, and neutrons which then found may be separated from each other. The findings on these atoms were previously expressed by several theories in different times. In general, we recognize 6 theories about atomic development, and we will explain them one by one below.


You may also want to read: Scientists Who Contributed to The Atomic Theory

1. The Early Greeks Theory

The first atomic theorist was Democritus, a Greek scientist and philosopher who lived in the fifth century BC. At that time, Democritus found that if a stone was divided in half, the two halves would have essentially the same properties as the whole. And after that, he tried to cut the stone continually into smaller and smaller pieces up to some point where there would be a piece that would be so small as to be indivisible. He called these small pieces of matter “atomos”, in Greek it means indivisible.

Democritus stated that atoms were specific to the material which they composed. And what is more, Democritus also believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other and during these collisions, could rebound or stick together. Therefore, changes in matter were a result of dissociation or combinations of the atoms as they moved throughout the void. Democritus’ theory was remarkable during that time.

But not long after, Aristotle, the most influential philosopher of Ancient Greece argued that a material could be divided infinitely without changing the original properties of the material. At that time the theory of Aristotle was trusted by many people because Aristotle had done many experiments using the scientific method.

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2. Dalton Atomic Model

John Dalton developed the atomic theory around the 1800s. He developed the atomic theory because he disagreed with the theory of atoms that Aristotle had previously proposed. He passed through several experiments and discovered several atomic weights and created symbols for atoms and molecules. From the findings, these are the conclusion of Dalton atomic theory:

  • Atoms are the smallest particles or substances that cannot be subdivided into smaller parts.
  • If it can be described, then the atom will look like a simple ball that is very small.
  • An element composed of the same atoms, while the compound is composed of different atoms according to their constituent elements.
  • Atoms can combine to form compounds with simple and integer ratios.
  • Chemical reactions are the separation, merging, or rearrangement of atoms so that atoms cannot be created or destroyed.

John Dalton’s atomic theory was generally accepted because it explained the laws of conservation of mass, definite proportions, multiple proportions, and other observations. However, the theory also has several weaknesses, there are:

  • No explanation about the difference between one element atoms with another element.
  • No explanation about the electrical properties of matter
  • No explanation about how atoms are related

However, from the theory put forward by Dalton, many researchers began to have an interest in research on the atomic model. Then many researchers began to put forward his theory of the atomic model and many of the changes that were added in the development of atomic theory because much of the new evidence was found.

One of the further experiments were conducted by Thomson and was successfully overcame the weakness of the previous theory of Dalton. Thomson’s theory showed that atoms are not undivided, but are composed of several types of subatomic particles. The three subatomic particles are protons, electrons, and neutrons.

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3. Thomson Atomic Model

In 1897, there was a scientist named J.J. Thomson who did a research to refine Dalton’s atomic theory. Joseph John Thomson was born in Cheethan Hill, England and was a professor of experimental physics at the Carendish Laboratory, Cambridge, London. He was successful for developing the atomic theory and received a Nobel Prize in physics in 1906 for his discovery of atomic theory.

At that time, Thomson experimented using a cathode ray tube or also called an electron gun. Thomson used a cathode ray tube with a magnet and find that the resulting green beam is made of negatively charged material. In addition, he also did a lot of research and found that the mass of these particles is almost 2000 times lighter than the hydrogen atom.

From this study, Thomson suggested that Dalton’s theory of atoms which said that atom could not be divided into smaller parts was wrong. After that, Thomson conducted a follow-up study and determined that the negative charge of the electron requires a positive charge that can balance both. Thus, he concludes that this negative charge is surrounded by a positively charged material.

The theory of Thomson is then known as the “plum pudding” model of the atom which stated that the negatively charged plums were surrounded by positively charged pudding. Also noted that the atomic theory according to Thomson also had a weakness, he was not explain the electron movement that occurs in the atom. For that, the theory needed to undergo further development.

4. Rutherford Atomic Model

A few years later, more precisely in 1911, Ernest Rutherford, one of Thomson’s disciples, did some further research on Thomson’s plum pudding model. The study was conducted by firing beam from positively charged particles called alpha particles against a very thin layer of gold foil. Since alpha particles had a lot of mass, Rutherford thought that all alpha particles would penetrate directly to the gold foil. At that time Rutherford argued that the alpha particle would penetrate the positively charged material and then would hit the screen detector on the other side.

But that hypothesis did not match with what Rutheroford had predicted. Some of the alpha particles penetrated the gold layer, but some of them were deflected by the gold foil and then hit a detector at another location. Some of them even returned straight back to the path they took.

After this research, Rutherord argued that this alpha particle must hit something that was very small, dense, and positively charged so that there were some alpha particles that went straight back. From this experiment, Rutherford also concluded that atoms were composed mostly of empty spaces and the existing positive charge is not evenly distributed within the atom but squished into a tiny nucleus in the center of the atom.

Although Rutherford’s theory can show that atoms have a nucleus that is positively charged and surrounded by a negative electron, this theory also has a weakness. The weakness is that Rutherford’s theory cannot explain why electrons do not fall into the nucleus.


5. Bohr Atomic Model

In 1913, the Danish physicist and also a student of Ruhterford, Neils Bohr repaired Rutherford’s atomic theory through his experiments on the spectrum of hydrogen atoms. This experiment managed to give a picture that electrons are occupying the area around atomic nucleus. Bohr’s explanation of a hydrogen atom involves a combination of Rutherford’s classical theory and the quantum theory of Planck, expressed by four postulates, as follows:

  • There is only a certain set of orbits that is allowed for one electron in a hydrogen atom. This orbit is known as a stationary motion (settling) electron and is a circular path around the nucleus. The path, which is also called the atomic shell, is a circular orbit with certain radius. Each path is marked by an integer called the principal quantum number (n), starting from 1, 2, 3, 4, 5, and so on and denoted by the symbols K, L, M, N, O, and so on. The first path with n = 1 is named shell K. The second path with n = 2 is named shell L, and so on. The larger the n value it means that is farther from the nucleus hence the greater the electron’s energy orbiting the skin.
  • As long as the electron is in the stationary path, the energy of the electron is still remaining so there is no energy in the form of radiation that is emitted or absorbed.
  • Electrons can only move from one stationary path to another stationary path. In this transition, a certain amount of energy is involved, the magnitude corresponding to the Planck equation, E2 – E1 (ΔE) = hf
  • The allowed stationary path has a magnitude with certain properties; particularly the property is called angular momentum. The magnitude of the angular momentum is a multiple of h / 2p or nh / 2p, where n is an integer and h is the Planck constant.

According to the Bohr’s atomic model theory, electrons surround the nucleus at certain paths called electron shells or energy levels. The lowest energy level is the deepest electron shell; the outer layer has the bigger shell number and higher energy level.

The summary of Bohr’s atomic model theory is that atom consists of atomic shells as a place for electron to move. Despite this new invention, this theory also had a weakness that is this atomic theory could not able to explain the colors spectrum of atom that consisted of many electrons.

6. Modern Atomic Model

In 1927, Erwin Schrodinger, an Austrian scientist, put forward an atomic theory called quantum mechanical model of the atom. This theory uses mathematical equations to explain the possibility of finding electrons in certain positions. The quantum mechanical model has similarities with the Niels Bohr atomic theory in terms of energy levels or atomic skins, but differs in terms of their shape or orbit.

In the atomic theory of quantum mechanical model, the position of electrons is uncertain. The thing that can be determined is about to predict the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud. Where in the most dense clouds, there is the greatest possibility for the discovery of electrons, and vice versa, the least electrons are found in less dense cloud regions. Thus, this theory model also explains the concept of sub-energy level.

The formulation of Erwin Schrodinger was very difficult to understand by scientists at that time. Schrodinger’s theory is similar to the solar system whose orbits are erratic and the sun is at its core. For the discovery of the atomic theory of quantum mechanics, Erwin Schrodinger received physics Nobel Prize in 1933.

After understanding the atomic development theory, we also have to understand more about the subatomic substances invention. The invention of subatomic substances was a great discovery to help scientists developed the atomic model theory. Hence, we will explain more about the history of subatomic substances below.

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History of Subatomic Substances

This time will discuss about the subatomic substances in the nucleus. As already mentioned above,
The subatomic substances are the neutrons and protons. In science nowadays, proton determines the atomic number. Meanwhile, atomic nucleus which has a very large weight compared to electron will determine the mass of an atom.

The weight of neutrons and protons usually has the same amount. Protons have a positive charge while neutrons are uncharged (neutral). In the history of chemistry we know that atoms have substances, especially parts of protons and neutrons that are interlocking with each other. At the beginning of the discovery of protons and neutrons were obtained by various experiments by scientists. To understand the detail about the experiments, we will explain it one by one as below.

1. Proton

In 1886 there was a German physicist named Eugen Goldstein. He invented the anode or proton rays. At that time, the essence of the cathode ray had not determined yet, but Goldstein conducted an experiment with a cathode ray tube and found the following facts.

  • If the cathode is not hollow, it turns out that gas behind the cathode remains dark. However, when the cathode is given a gas hole behind, the cathode will emit light.
  • This indicates the presence of radiation coming from the anode, which passes through the hole at the cathode and permits the gas behind the cathode.
  • The radiation is called an Anode ray or positive ray or canal ray. The experimental results show that the canal rays are radiation particles that are positively charged.

The radio-graphic particles turn out to depend on the type of gas in the tube. It means, if the gas in the tube is replaced, it will produce particles of light canal with different size. The smallest irradiation particles are obtained from hydrogen gas. These particles are then called protons.
Mass of 1 proton = 1.6726486 x 10-24 grams = 1 sma
The charge of 1 proton = +1 = +1.6 x 10-19 C

The charge and mass of the light particles composed of other gases and is always in a spherical multiplier of the mass and the charge of the protons, so it is suspected that the particles consist of protons. Then in 1919, Rutherford discovered that protons formed when alpha particles were fired at the nucleus of a nitrogen atom. The same thing happened to the firing of another atomic nucleus. This proves that the atomic nucleus consists of protons as alleged by Goldstein.

You may also want to read: Proton, Electron, Neutron

2. Neutron

Neutron was invented by James Chadwick along with Ernest Rutherford in 1932 using alpha rays, but that is still only an assumption, and its existence has been suspected by Aston since 1919. In that year, Aston invented the mass spectrometer, which is the instrument that can be used to determine mass Atoms or molecules.

With the instrument, Aston discovered that atoms of the same element can have different masses. This phenomenon is called isotope. It was also found that the mass of an atom was not the same as the number of protons. Many atom mass are about twice of the proton mass. Based on these two facts, Aston suspected that the existence of neutral particles in atoms with different amount even though the elements are the same. Later in 1930, W. Bothe and H. Becker fired at beryllium nuclei with alpha particles and discovered a high-penetrating particle radiation.

In 1932, James Chadwick proved that the radiation consists of neutral particles whose mass is almost equal to the mass of protons. Because they are neutral, they are called neutrons. Further experiments prove that neutrons are also fundamental particles of the atomic constituents.
Mass 1 neutron = 1.6749544 x 10-24gram = 1 sma
Neutrons are uncharged (neutral)

You may also want to read: Hydrogen Uses – Uses of Hydrogen Peroxide

Those are the explanation about the development of atomic theory. We hope that this article will help you to understand more about atom and its developing theory. Please stay update for our next article about chemistry. Thank you.

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Post Date: Wednesday 03rd, May 2017 / 06:01 Oleh :
Kategori : Molecules