Electron Number (English)

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2-2: The Number of Electrons in an Atom

As learned in Part 1,

it turned out

that an electron

exists in an atom.

Are there a fixed number

of electrons

in each atom?

The answer to this question

was given

by the experiment

of the scattering of X rays from an atom.

Let's explain

briefly below.

[The Property of X Rays]

In 1895,

X rays were discovered

by W. C. Roentgen

(Germany, 1845 - 1923),

while he was doing

experiment

on vacuum discharge.

He found

that a kind of "rays"

which had extremely

high penetrability

were emitted

from a Crookes tube

covered by a sheet

of thick black paper,

and it made some

of fluorescent materials

at a distant place

in the room fluoresce

(emit light),

and it exposed

photographic plates

in a drawer of a desk.

It was clear

that this phenomena

is not due to

the cathode rays,

because the cathode rays

could not

penetrate a thick

paper or a glass.

This "rays" were named

X rays for

the reason that

they were unknown.

The reason

why X rays come

out of a Crookes tube

is because the cathode

rays (electrons)

with a very high speed

collide with glass tube

or nearby metal.

According to the theory

of electromagnetism,

when a charged

particle suddenly accelerate

or decelerate,

it radiates

an electromagnetic

wave (or light).

In the vacuum discharge

tube, a large number of

high speed electrons

collide against

the glass tube or metal

and are enforced to stop.

This is sudden

deceleration.

Then an intensive

electromagnetic wave

is emitted.

This is the reason

of the radiation

of X rays.

The evidence that

X rays are electromagnetic waves

were shown by a

diffraction

phenomenon that occurred

when it passed

through a very thin slit.

This phenomenon

was quite similar

to the case of

an ordinary light.

A polarization phenomenon

was confirmed as well.

These phenomena showed

that X rays are

electromagnetic waves

like an ordinary light.

[Scattering of X Rays by an Atom, the Number of Electrons]

An atom is electrically

neutral.

We know on the other hand

that an atom contains

electrons

which are negatively charged.

Accordingly there must

exist "something"

with positive charge

which cancels the negative

charge of the electrons.

The mass of electron

is quite light,

i.e.

about 1/1800

of that of the lightest atom,

the hydrogen atom.

This means that

the above "something"

with positive charge

must carry almost all

of the mass of an atom.

You should keep

this in mind.

As mentioned above,

it was clarified

that X rays

are electromagnetic waves.

If X rays are

irradiated to an atom,

their electric fields

exert forces

on the charged objects

in it (electrons

or "something"

in the atom)

and oscillate them.

An oscillating charged

object

which repeats acceleration

and/or deceleration

rapidly emits electromagnetic

waves around.

Thus the incident

X rays are scattered

in various directions

as schematically shown

in the following figure.

According to the

theory of electromagnetism,

a charged particle

moving with an acceleration

emits an electromagnetic

wave whose strength

is proportional

to the square

of the acceleration.

The acceleration

of a particle

is obtained by

dividing the force acting

on the particle by the mass.

Therefore the strength

of the emitted

electromagnetic wave

is inversely proportional

to the square of the mass.

Hence the strength

of the electromagnetic wave

emitted from the "something"

is 1/1000000 less than

what is emitted

from an electron,

so that it

is negligible.

Namely, we can consider that

X rays are scattered only by the electrons in the atom.

[The Number of Electrons in an Atom]

Experiments to illuminate X rays

on various atoms

were carried out

as schematically drawn

in the above figure.

In these experiments,

a part of the incident

X rays was scattered.

The remaining X rays

would penetrate the atoms,

and the strength

of the penetrated X

rays would

be weakened

(or decreased)

by the scattered X rays.

This decreasing rate

depends on the number

of electrons

in the atom.

By investigating this

in detail,

the number of electrons

could roughly be estimated.

The results told

that the number

of electrons

in hydrogen atom

is 1 and in helium 2,

and so on.

People have known

that the number of electrons in an atom is about a half of the atomic weight

in general.

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