Discovery of Atoms (English)

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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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