Discovery of Molecules (English)

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1-3: Introduction of the Concept of the Molecule

[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

N_A,

which has formerly

been called

"Avogadro's number".

It has been determined

by various methods

and the result is

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