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Sezione C

L’atomo da Dalton a Bohr

40

Chemistry Readings

X-ray tube.

Why are the electronic shells named K, L, M…?

X-rays were discovered In the early twentieth century during the studies of

the cathodic ray. Their names were of capital importance for attributing

the names to the electronic shells.

What are X-rays?

When a metal plate is bombarded with a beam of fast-moving electrons,

the atoms of the target emit X-rays. They are high frequency electromag-

netic waves and their origin is due to the particular interaction between the

electronic beam and the atoms that are hit. In the impact an inner electron

is excited far from the nucleus and when it is recaptured, a great deal of

energy is released in the form of X-rays, whose frequency is related to the

element involved.

Different types

The first X-rays discovered seemed to be of two types. One type was able to penetrate sheets

of metal of a certain thickness; the other wasn’t. Initially the scientist who found this named

the more penetrating type A and the less penetrating type B. But they expected that even

more penetrating types of X-ray radiation would be discovered for other elements, so they

renamed them K and L to leave room for more penetrating types A through J. Anyhow these

more penetrating radiations were never observed, and the names remained unchanged.

Letters and shells

It became clear that an X-ray of K type was produced when electrons knocked out of the

n

=

1 shell are recaptured. L radiation is produced when electrons knocked out of the

n

=

2 shell are recaptured. For this reason the electronic levels took name from the types

of X-rays that were generated during the bombardment. Since the n

=

2 shell has higher

energy than the n

=

1 shell, less energy is released and L radiation is weaker (less able to

penetrate metals) than K radiation.

Fill in the blanks

When a metal plate is bombarded with a beam of

…....................………………..

electrons, the atoms of the target emit X-rays.

One type of X-ray was able to penetrate sheets of

…....................………………..

of a certain thickness.

It became clear that an X-ray of K type was produced when electrons knocked out of

…....................……………

are recaptured.

Verso le competenze

Nell’atomo si individuano quindi diversi livelli di energia, a ciascuno dei

quali appartengono una o (a partire dal secondo livello) più orbite (o sotto-

livelli) vicine tra loro che gli elettroni possono percorrere.

In seguito, altri problemi nell’interpretazione degli spettri imposero l’intro-

duzione, dovuta al fisico austriaco

Wolfgang Pauli

(1900-1958), di un nuovo

numero quantico, collegato alla rotazione dell’elettrone su sé stesso (

spin

),

fenomeno che influisce sull’energia dell’orbita (

Figura 16

).

Il nuovo modello atomico che si ottenne dalla quantizzazione della forma e

dell’orientazione delle orbite, conosciuto come modello di Bohr-Sommerfeld,

lasciava ancora molti problemi irrisolti. Oltre a non spiegare, come quello di

Bohr, perché un elettrone non perdesse energia nel muoversi su un orbita per-

messa, il nuovo modello dava conto solo parzialmente degli spettri di atomi

con più elettroni.

Figura 16

Gli elettroni ruotano su sé stessi, ma

possono farlo solo in due modi diversi.

energia

di ionizzazione

ionization energy

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