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1-3: Introduction of the Concept of the Molecule

The smallest unit particle of matter which has a specified chemical property is called molecule.
For example, if we divide the water into fine portions, we reach the smallest unit particles possessing the property of water. This is the molecule of water (H2O). If we divide this molecule of water, we have two hydrogen atoms and one oxygen atom, but they no longer have the chemical property specifying the water.
The general idea of molecules was not yet clear in the atomic theory (atomism) of Dalton. It was after the following Avogadro's law, in which the concept of atom and that of molecule were clearly distinguished.

[Gay-Lussac's Law, Avogadro's Law]
J. L. Gay-Lussac (France: 1778 - 1850) found the following law on gaseous reaction (1809):
"When two kinds of gases combine completely, the volumes of the gases are in a simple integer ratio to each other".
For example, one volume of nitrogen combines with three volumes of hydrogen to form two volumes of ammonia.
A. Avogadro (Italy, 1776 - 1856) considered that this simple integer ratio is just the ratio of the numbers of molecules contained in these gases. Thus he proposed the following Avogadro's law (1811).
"Under the same pressure and the same temperature, even if the kind of gas differs, the same volume of gas contains the constant number of molecules in it"
This law was later confirmed experimentally.
On the basis of this Avogadro's law, it became possible to compare the relative weights of various molecules and atoms.
The molecule of water is expressed as H2O, and the molecule of carbon monoxide is CO and that of carbon dioxide CO2. Following the above-mentioned Avogadro's law, we can consider that a gas molecule consisting of a single element, for example like a hydrogen molecule or an oxygen molecule, is a composite of two atoms which could be expressed as H2 or O2. This type of molecule is called diatomic molecule.
Therefore the reaction in which hydrogen and oxygen combine to form water can be displayed as the following figure.

Since the molecule of the noble gases (helium, neon, argon, krypton, xenon and radon) consists of single atoms, it is called monoatomic molecule. "Inert gases" is a semi-obsolete name for the noble gases.

[Molecular Weight]
The mass of a molecule, namely the molecular weight, is equal to the sum of the atomic weights of the constituent atoms. For example, the molecule of water is H2O, and the atomic weight of hydrogen H is about 1 [u] and that of oxygen O is about 16 [u]. Accordingly, the molecular weight of water is 1 x 2 + 16 = 18 [u].

[Avogadro's Constant]
The amount of a compound or an element which is numerically equal to the molecular weight in grams is called mole expressed by the symbol "mol". The mole is also called gram molecule.
Since the molecular weight of carbon 12 (12C) is, for example, 12 [u], 1 mole (= 1 gram molecule) of carbon 12 is 12 grams of it.
At "stp" that denotes the "standard temperature and pressure", the volume of gas of 1 mol is 22.414 liter being independent of the kind of gas.
"stp" denotes the condition that the temperature is 0 dgree Celcius and the pressure is 1 atmospheric pressure, i.e., 1013.25 hPa (hectopascal) or 760 mmHg.
The number of molecules in one mole of any pure substance is a constant called Avogadro's constant NA, which has formerly been called "Avogadro's number". It has been determined by various methods and the result is

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