Atomic theory and kinetic theory

Atomic theory

For thousands of years, the ancient Greeks believed that every pure substance (such as gold, iron, etc.) consisted of atoms. According to them, if a pure substance is cut into small pieces, then the tiny parts are cut again, then cut back… and so on, then there will be the smallest pieces that cannot be cut again. The smallest pieces that cannot be cut again are called atoms. Atom means “cannot be divided” (Greek language)

At that time the atom is considered to be no longer divided. But later some scientists discovered electrons and atomic nuclei (protons and neutrons) so that the assumption that atoms could not be subdivided was wrong. So atoms consist of electrons (negatively charged) and atomic nuclei. Electrons circle the nucleus. Inside the nucleus there are protons (positively charged) and neutrons (neutral or not charged).

There is another theory; the name is the continuous theory. This theory states that pure substances can be divided to infinity. According to this theory, there is no such thing as the smallest piece. The smallest pieces can still be cut into smaller pieces. So it becomes infinite. Which of these two theories is correct? The correct atomic theory or continuous theory?

In physics, every theory will be recognized scientifically if the theory can be proven in experiments. In the 18th, 19th and 20th centuries, through experiments conducted by scientists, the atomic theory proved to be true.

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First, understand the following reviews. Elements are pure substances that cannot be subdivided into other substances chemically, for example gold (Au), iron (Fe), copper (Cu), zinc (Zn), Sodium (Na), Calcium (Ca), chlorine (Cl) , Nitrogen (N), oxygen (O), hydrogen (H) etc. Besides the elements, there are also compounds. Compounds consist of elements. Because it consists of elements, the compound can still be divided into elements. Examples of compounds are water.

The smallest part of an element is an atom, while the smallest part of a compound is a molecule. Molecules consist of atoms sticking together. Pure gold is an example of an element. Pure gold consists of gold (Au) atoms. Iron pieces are also an example of elements. Iron consists of iron (Fe) atoms. So the element is a pure substance consisting of similar atoms.

The water that you often see, hold and drink is composed of water molecules (the chemical formula is H2O). Water molecules consist of two Hydrogen atoms (H) and one oxygen atom (O).

Here are some proofs of atomic theory:

First, the law of constant comparison.

The law of comparison states that if the elements combine to become compounds, the compounds formed have the same mass ratio. For example salt. The salt we see is a compound consisting of salt molecules (NaCl). Naturally, salt molecules are always formed from 23 parts of sodium (Na) and 35 parts of chlorine (Cl).

Continuous theory cannot explain it, but the atomic theory can explain. According to atomic theory, atoms are the smallest parts of an element so that atoms have mass. The mass ratio of the elements of a compound must be related to the relative mass of the atoms forming the element. Based on the number of each element that forms compounds, scientists determine the relative mass of atoms. It is said to be relative because the relative mass of an element is compared to the relative mass of other elemental atoms.

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Hydrogen is the lightest atom, so it is used as a benchmark. The relative mass of a hydrogen atom (H) is given a value of 1. By using the relative mass of a hydrogen atom as a benchmark, the relative mass of the carbon atom (C) is given a value of 12, the relative mass of the oxygen atom (O) is assigned a value of 16 and so on (see the periodic table of elements ). The relative mass of a carbon atom = 12 means that the mass of one carbon atom is 12 times greater than the mass of one hydrogen atom (H). The relative mass of an oxygen atom = 16 means that the mass of an oxygen atom is 16 times higher than the mass of one hydrogen atom (H).


In the International System (SI) we have a mass standard, namely iridium platinum which is stored in international weight and size institutions in French. Under the international agreement, iridium platinum mass is 1 kg, this is a standard kilogram. On an atomic scale, we also have a second mass standard, that is 12 C carbon atoms. Based on international agreements, the mass of 1 carbon atom 12 C is 12,0000 unified atomic mass units (abbreviated u).

1 u = 1.66 x 10-27 kg.

Mass of 1 Carbon atom (C) = 12,0000 u, mass of 1 Hydrogen atom (H) = 1,0078 u, mass 1 Oxygen atom (O) = 15,9994 u, mass 1 atom Sodium = 22,9897 u and so on . Regarding atomic mass, it can be seen in full on the periodic table of elements.

In addition to atomic mass, there is also a molecular mass. Molecular mass is the total mass of the atoms of a molecule. For example the mass of a salt molecule (NaCl) = the mass of one atom of sodium (Na) + the mass of one atom of chlorine (Cl). The mass of water molecules (H2O) = the mass of 2 hydrogen atoms (H) + the mass of one oxygen atom (O).

Second, brown motion

In ancient times, there lived an English biologist named Robert Brown. He was researching pollen put in water (1827). Water and pollen are seen using a microscope. Robert Brown found it strange after seeing the pollen move on his own. It’s weird because the pollen moves. The direction of pollen movement is arbitrary but continuously. He suspects that the movement is a life, where pollen lives so that it can move. But his guess is wrong because small organic particles like pollen also move when put in water. This movement is called Brownian motion.


This discovery cannot be explained until the development of kinetic theory.

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Kinetic theory

Kinetic means to move (Greek). The kinetic theory states that every substance consists of atoms or molecules and that atoms or molecules move continuously randomly.

When moving, atoms or molecules have to speed. Atoms or molecules also have mass. Because it has mass (m) and velocity (v), then the atom or molecule has kinetic energy (EK) and momentum (p). Kinetic energy: EK = 1/2 m v2. While momentum: p = m v. In addition to momentum, there are forces. When moving, there is a possibility of collisions. So forces arise because of changes in momentum when a collision occurs.

We can say that the kinetic theory is based on kinetic energy, momentum, and force. These three things are studied in the subject of motion dynamics (the laws of Newton, impulse, and momentum). The difference is that in kinetic theory we apply dynamics at the level of atoms or molecules. The kinetic theory was developed by Boyle (1627-1691), Daniel Bernoulli (1700-1782), Joule (1818-1889), Kronig (1822-1879), Rudolph Clausius (1822-1888) and Clerk Maxwell (1831-1879).

The existence of this kinetic theory could explain Brown’s discovery. According to kinetic theory, pollen moves because it is driven by fast-moving water molecules. The number of water molecules is enormous, so pollen is pushed from various directions.

Based on the law of constant comparison and the discovery of brown motion, atomic theory is increasingly believed by scientists. Atoms are the smallest pieces of any substance. Thus the atom has a size. What is the size of an atom?

In 1905, Einstein theoretically investigated the size of atoms. Based on atomic theory, kinetic theory and data obtained through experiments, he found that the atomic diameter was about 10-10 m. So the diameter of the atom is obtained through calculation. The size of atoms has been found, so the atomic theory is recognized, as well as kinetic theory.

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