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Radioactivity I INTRODUCTION Marie Curie Working with her husband, Pierre Curie, French physicist Marie Curie discovered the radioactive elements polonium and radium in 1898.

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Radioactivity I INTRODUCTION Marie Curie Working with her husband, Pierre Curie, French physicist Marie Curie discovered the radioactive elements polonium and radium in 1898.

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Aperçu du corrigé : Radioactivity I INTRODUCTION Marie Curie Working with her husband, Pierre Curie, French physicist Marie Curie discovered the radioactive elements polonium and radium in 1898.



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Radioactivity
I

INTRODUCTION

Marie Curie
Working with her husband, Pierre Curie, French physicist Marie Curie discovered the radioactive elements polonium and
radium in 1898.
Zoom

Radioactivity
I

INTRODUCTION

Marie Curie
Working with her husband, Pierre Curie, French physicist Marie Curie discovered the radioactive elements polonium and
radium in 1898. The couple won a Nobel Prize for their efforts, making Marie Curie the first woman ever to win the award.
In 1911 she won an unprecedented second Nobel Prize for further work on radioactive compounds.
Culver Pictures.

Radioactivity, spontaneous disintegration of atomic nuclei by the emission of subatomic particles called alpha particles and beta particles, or of electromagnetic rays
called X rays and gamma rays. The phenomenon was discovered in 1896 by the French physicist Antoine Henri Becquerel when he observed that the element uranium
can blacken a photographic plate, although separated from it by glass or black paper. He also observed that the rays that produce the darkening are capable of
discharging an electroscope, indicating that the rays possess an electric charge. In 1898 the French chemists Marie Curie and Pierre Curie deduced that radioactivity is a
phenomenon associated with atoms, independent of their physical or chemical state. They also deduced that because the uranium-containing ore pitchblende is more
intensely radioactive than the uranium salts that were used by Becquerel, other radioactive elements must be in the ore. They carried through a series of chemical
treatments of the pitchblende that resulted in the discovery of two new radioactive elements, polonium and radium. Marie Curie also discovered that the element
thorium is radioactive, and in 1899 the radioactive element actinium was discovered by the French chemist André Louis Debierne. In that same year the discovery of the
radioactive gas radon was made by the British physicists Ernest Rutherford and Frederick Soddy, who observed it in association with thorium, actinium, and radium.
Radioactivity was soon recognized as a more concentrated source of energy than had been known before. The Curies measured the heat associated with the decay of
radium and established that 1 g (0.035 oz) of radium gives off about 100 cal of energy every hour. This heating effect continues hour after hour and year after year,
whereas the complete combustion of a gram of coal results in the production of a total of only about 8000 cal of energy. Radioactivity attracted the attention of
scientists throughout the world following these early discoveries. In the ensuing decades many aspects of the phenomenon were thoroughly investigated.

II

TYPES OF RADIATIONS

Alpha Particles
Alpha particles consist of two protons and two neutrons that act as a single particle. An alpha particle is identical to the
nucleus of a Helium atom. When alpha particles are emitted from an unstable radioactive nucleus, the atom is transmuted
into a different element.
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Rutherford discovered that at least two components are present in the radioactive radiations: alpha particles, which penetrate into aluminum only a few thousandths of
a centimeter, and beta particles, which are nearly 100 times more penetrating. Subsequent experiments in which radioactive radiations were subjected to magnetic and
electric fields revealed the presence of a third component, gamma rays, which were found to be much more penetrating than beta particles. In an electric field the path
of the beta particles is greatly deflected toward the positive electric pole, that of the alpha particles to a lesser extent toward the negative pole, and gamma rays are
not deflected at all. Therefore, the beta particles are negatively charged, the alpha particles are positively charged and are heavier than beta particles, and the gamma
rays are uncharged.
The discovery that radium decayed to produce radon proved conclusively that radioactive decay is accompanied by a change in the chemical nature of the decaying
element. Experiments on the deflection of alpha particles in an electric field showed that the ratio of electric charge to mass of these particles is about twice that of the
hydrogen ion. Physicists supposed that the particles could be doubly charged ions of helium (helium atoms with two electrons removed). This supposition was proved by

Rutherford when he allowed an alpha-emitting substance to decay near an evacuated thin-glass vessel. The alpha particles were able to penetrate the glass and were
then trapped in the vessel, and within a few days the presence of elemental helium was demonstrated by use of a spectroscope. Beta particles were subsequently
shown to be electrons, and gamma rays to consist of electromagnetic radiation of the same nature as X rays but of considerably greater energy.

A

The Nuclear Hypothesis

Rutherford Experiment
Rutherford studied the structure of the atom by firing a beam of alpha particles at gold atoms. A few alpha particles
bounced directly back, indicating that they had struck something massive. Rutherford proposed that most of the mass of
atoms was concentrated in their centers. This concentration of mass is now known as the nucleus.
© Microsoft Corporation. All Rights Reserved.

At the time of the discovery of radioactivity physicists believed that the atom was the ultimate, indivisible building block of matter. The recognition of alpha and beta
particles as discrete units of matter and of radioactivity as a process by means of which atoms are transformed into new kinds of atoms possessing new chemical
properties because of the emission of one or the other of these particles brought with it the realization that atoms themselves must have structure and that they are
not the ultimate, fundamental particles of nature. In 1911 Rutherford proved the existence of a nucleus within the atom by experiments in which alpha particles were
scattered by thin metal foils (see Atom). The nuclear hypothesis has since grown into a refined and fully accepted theory of atomic structure, in terms of which the
entire phenomenon of radioactivity can be explained. Briefly, the atom is thought to consist of a dense central nucleus surrounded by a cloud of electrons. The nucleus,
in turn, is composed of protons equal in number to the electrons (in an electrically neutral atom), and neutrons. An alpha particle, or doubly charged helium ion,
consists of two neutrons and two protons, and hence can be emitted only from the nucleus of an atom. Loss of an alpha particle by a nucleus results in the formation of
a new nucleus, lighter than the original by four mass units (the masses of the neutron and of the proton are abou...


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