Fortunately,
we can acquire a qualitative understanding of atomic structure without having
to retrace all the discoveries that preceded atomic physics. We do, however,
need a few key ideas about the interrelated phenomena of electricity and
magnetism, which we will briefly discuss here. Electricity and magnetism were
used in the experiments that led to the current theory of atomic structure.
Certain
objects display a property called electric charge, which can be either positive
(+) or negative (-). Positive and negative charges attract each other, while
two positive or two negative charges repel each other. As we learn in this
section, all objects of matter are made up of charged particles. An object having
equal numbers of positively and negatively charged particles carries no net
charge and is electrically neutral. If the number of positive charges exceeds
the number of negative charges, the object has a net positive charge. If
negative charges exceed positive charges, the object has a net negative charge.
Sometimes
when one substance is rubbed against another, as in combing hair, net electric
charges build up on the objects, implying that rubbing separates some positive
and negative charges (as shown in the figure below). Moreover, when a
stationary (static) positive charge builds up in one place, a negative charge
of equal size appears somewhere else; charge is balanced.
The
second figure below shows how charged particles behave when they move through
the field of a magnet. They are deflected from their straight-line path into a
curved path in a plane perpendicular to the field. Think of the field or region
of influence of the magnet as represented by a series of invisible "lines
of force" running from the North Pole to the south pole of the magnet.
The Discovery of Electrons
CRT,
the abbreviation for cathode-ray tube, was once a familiar acronym before
liquid crystal display (LCD) was available, the CRT was the heart of computer
monitors and TV sets. The first cathode-ray tube was made by Michael Faraday (1791- 1867) about 150
years ago. When he passed electricity through glass tubes from which most of
the air had been evacuated, Faraday discovered cathode
rays, a type of radiation emitted by the negative terminal, or cathode. The
radiation crossed the evacuated tube to the positive terminal, or anode. Later
scientists found that cathode rays travel in straight lines and have properties
that are independent of the cathode material (that is, whether it is iron,
platinum, and so on). The construction of a CRT is shown in figure below.
The
cathode rays produced in the CRT are invisible, and they can be detected only
by the light emitted by materials that they strike. These materials, called phosphors,
are painted on the end of the CRT so that the path of the cathode rays can
be revealed. (Fluorescence is the term used to describe the emission of
light by a phosphor when it is struck by energetic radiation.) Another significant
observation about cathode rays is that they are deflected by electric and
magnetic fields in the manner expected for negatively charged particles (see
figure below).
In
1897, by the method outlined in Figure (c), J. J. Thomson (1856 1940)
established the ratio of mass (m) to electric charge (e) for
cathode rays, that is, m/e. Also,
Thomson concluded that cathode rays are negatively charged fundamental
particles of matter found in all atoms. (The properties of cathode rays are independent
of the composition of the cathode.) Cathode rays subsequently became known
as electrons, a term first proposed by George Stoney in 1874.
Robert Millikan (1868 1953) determined the electronic charge e
through a series of oil-drop experiments (1906 1914), described in Figure below.
The currently accepted value of the electronic charge e, expressed in coulombs to five
significant figures, is (-1.6022 x 10 - 19 C). By combining this value with an accurate value
of the mass-to-charge ratio for an electron, we find that the mass of an
electron is 9.1094 x 10 -28 g.
Once the electron was seen to be a fundamental particle of
matter found in all atoms, atomic physicists began to speculate on how these
particles were incorporated into atoms. The commonly accepted model was that
proposed by J. J. Thomson. Thomson
thought that the positive charge necessary to counterbalance the negative
charges of electrons in a neutral atom was in the form of a nebulous cloud.
Electrons, he suggested, floated in a diffuse cloud of positive charge (rather
like a lump of gelatin with electron fruit embedded in it).
This model became known as the plum-pudding model because of its
similarity to a popular English dessert. The plum-pudding model is illustrated
in figure below for a neutral atom and for atomic species, called ions,
which carry a net charge.
X-Rays and Radioactivity
Cathode-ray research had many important spin-offs. In
particular, two natural phenomena of immense theoretical and practical significance
were discovered in the course of other investigations.
In 1895, Wilhelm Roentgen (1845 1923) noticed that when
cathode-ray tubes were operating, certain materials outside the tubes
glowed or fluoresced. He showed that this fluorescence was caused by radiation
emitted by the cathode-ray tubes. Because of the unknown nature of this
radiation, Roentgen coined the term X-ray. We now recognize the X-ray as
a form of high-energy electromagnetic radiation, which is discussed in other
blog posts (use the search bar on the top).
Antoine Henri Becquerel (1852 1908) associated X-rays with
fluorescence and wondered if naturally fluorescent materials produce X-rays. To
test this idea, he wrapped a photographic plate with black paper, placed a coin
on the paper, covered the coin with a uranium-containing fluorescent material,
and exposed the entire assembly to sunlight. When he developed the film, a
clear image of the coin could be seen. The fluorescent material had emitted
radiation (presumably X-rays) that penetrated the paper and exposed the film.
On one occasion, because the sky was overcast, Becquerel placed the
experimental assembly inside a desk drawer for a few days while waiting for the
weather to clear. On resuming the experiment, Becquerel decided to replace the
original photographic film, expecting that it may have become slightly exposed.
He developed the original film and found that instead of the expected feeble
image, there was a very sharp one. The film had become strongly exposed because
the uranium-containing material had emitted radiation continuously, even when
it was not fluorescing. Becquerel had discovered radioactivity.
Ernest Rutherford (1871 1937) identified two types of radiation
from radioactive materials, alpha and beta Alpha particles (α) carry two
fundamental units of positive charge and have essentially the same mass as
helium atoms. In fact, alpha particles are identical to He2+ ions. Beta
particles (β) are negatively charged particles produced by changes
occurring within the nuclei of radioactive atoms and have the same properties
as electrons. A third form of radiation, which is not affected by electric or
magnetic fields, was discovered in 1900 by Paul Villard. This radiation, called
gamma rays (γ), is not made up of particles; it
is electromagnetic radiation of extremely high penetrating power. These three
forms of radioactivity are illustrated in figure below.
By the early 1900s, additional radioactive elements were
discovered, principally by Marie and Pierre Curie. Rutherford and Frederick
Soddy made another profound finding: The chemical properties of a radioactive
element change as it undergoes radioactive decay. This observation
suggests that radioactivity involves fundamental changes at the subatomic level
in radioactive decay, one element is changed into another, a process known as transmutation.
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