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1-2: The Discovery of the Atom

[The Atomism of Demokritos]
The idea that the matter consists of minimum units, i.e. atoms, was proposed by an ancient Greek philosopher, Demokritos (Ancient Greece, ~ B.C. 500). "Atom" means "unable to be divided".
Demokritos thought that this world is made of the atoms which are moving in the "empty" which spread infinitely.
On the other hand, Aristoteles (Ancient Greece, B.C. 384 - 322) thought that the world is filled with continuous substances.
The view of the Aristoteles style was dominant in a period from the ancient to the medieval times.

[Existence of the Chemical Elements]
In the 18th century, the experimental chemistry became precise, and then oxygen and hydrogen were discovered for example. Thereby Aristoteles' four-element theory in which the world was thought to be made of the four elements, i.e., fire, water, earth, and air, was denied.
A. L. Lavoisier (France, 1743 - 94) elucidated that there exist some elements which could not be disintegrated into any fragment by usual chemical means. He defined this type of thing as "chemical element" or simply "element" (1789). He therefore thought that every matter should be made of a combination or a compound of various elements.

[The Law of Constant Proportions]
In the 18th and 19th centuries, the scientific atomism based on the experimental facts was established.
First, the law that
"the composition of a pure chemical compound is independent of its method of preparation"
was made clear. This is called the law of constant proportions.
For example, although water is a compound of two kinds of elements, hydrogen and oxygen, the ratio of the weight of hydrogen to that of oxygen in the water is fixed at the value 1 : 8, independently of how it is formed.

[The Law of Multiple Proportions]
Furthermore, an important law, the law of multiple proportions, was proposed.
It states that,
"When two elements A and B combine to form more than one compound, the weights of B which combine with a fixed weight of A are in the proportion of small whole numbers (integers)".
For example, let's consider the case where carbon and oxygen are combined to form carbon dioxide and carbon monoxide. While 12 g of carbon is combined with 32 g of oxygen to form carbon dioxide, the same weight of carbon is combined with 16 g of oxygen to form carbon monoxide. Therefore the ratio of the weights of oxygen combined with 12 g of carbon is 32 : 16 = 2 : 1.

[The Atomism of Dalton]
The above-stated two laws may easily be understood if we assume that every element consists of basic units called atoms.
This is the atomism (theory of atom) proposed in 1808 by J. Dalton (UK, 1766 - 1844). It is the first scientific atomism.
Let's explain the theory taking the case as an example where carbon combines with oxygen to form carbon dioxide and carbon monoxide. They combine with each in the following two ways:

As shown in the illustration below, one carbon atom (C) combines with two oxygen atoms (O) in the reaction [1], on the other hand one carbon atom (C) combines with one oxygen atom (O) in the reaction [2]. Accordingly, if we assume that the atomic weight (relative atomic mass) of carbon is 12, then that of oxygen is thought to be 16.

Dalton clarified the combination of atoms in various compounds as playing a jigsaw puzzle or a patchwork.
Consequently, it is considered that one oxygen atom and two hydrogen atoms are combined with each other to form water as shown in the following figure.

Accordingly, if we assume the atomic weight of oxygen to be 16, then that of hydrogen is considered to be 1.
Thus the idea that the most fundamental units of matter are atoms has been established. This is nothing else than the foundation of the scientific atomism.

[Atomic Weight]
The atomic mass measured in suitable units is called atomic weight.
As clearly seen in Dalton's atomism, if we assume the mass of hydrogen atom to be 1, then that of carbon is about 12, and oxygen to be about 16. The atomic weight of a comparatively light element is close to an integer, but that of a heavier element deviates from an integer.
At present, the atomic mass is expressed in units of atomic mass unit (u).

An element in nature is usually a mixture of isotopes of the element. The number of atoms of a given isotope in the mixture is called abundance; usually expressed as a percentage of the total number of the atoms of the element.
For example, the carbon element contains two kinds of carbon isotopes, i.e., 12C and 13C; their abundances are 98.9% and 1.1%, respectively.
The atomic mass unit is defined as one-twelfth of the mass of a carbon atom of the isotope 12C which is the most abundant carbon isotope in nature. The atomic weights are therefore the masses relative to 12C as 12 [u].
The atomic weight of an element in nature is the mean value obtained by taking the average of the atomic masses being multiplied by the corresponding abundances. For example, the atomic weight of natural carbon is

Similarly, that of hydrogen is 1.008 [u], helium 4.003 [u], oxygen 16.00 [u] and sodium 22.99 [u].
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