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The Structure of the Atom
Today, we define an atom as the smallest particle of an element that retains the chemical properties of
that element
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The nucleus is a very small regions located at the center
of an atom
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Surrounding the nucleus is a region
occupied by negatively charged particles called electrons
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Protons, neutrons, and electrons are often referred as subatomic particles
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In the late 1800s, many experiments were performed in which electric current was
passed through various gases at low pressures
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Such tubes are known as cathode-ray tubes
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They hypothesized that the glow was caused by a stream of
particles, which they called a cathode ray
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Experiments devised to test this hypothesis revealed the following
observations
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Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electric
current, which was known to have a negative charge
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The rays were deflected to have a negative charged object
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This hypothesis was strongly supported by a series of experiments carried out in 1897 by the
English Joseph John Thomson
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He found that this ratio was always the same, regardless of the metal
used to make the cathode or the nature of the gas inside the cathode-ray tube
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Charge and Mass of the Electron
Thomson’s experiment revealed that the electron has a very large charge-to-mass ratio
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Based on what was learned about electrons, two other inferences were made about atomic structure,
1
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2
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Thomson proposed a model for the atom that is called the plum pudding model (after the English dessert)
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However, shortly thereafter, new experiments disproved this model
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The scientists bombarded a thin piece of gold foil with fastmoving alpha particles, which are positively charged particles with about four times the mass of a
hydrogen atom
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They expected the alpha particles to pass through with only a slight
deflection, and for the vast majority of the particles, this was the case
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After thinking about the startling result for a few months, Rutherford finally came up with an explanation
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He figured that the source of this force must occupy a very small amount of space because so few
of the total number of alpha particles had been affected by it
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Rutherford called this positive
bundle of matter the nucleus
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In fact, if the nucleus were the size of a marble, then the size of the atom would be about the
size of a football field
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Composition of the Atomic Nucleus
Except for the nucleus of the simplest type of hydrogen atom (discussed in the next section), all atomic
nuclei are made of two kinds of particles, protons and neutrons
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Atoms are electrically neutral because they contain
equal numbers of protons and electrons
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The simplest hydrogen atom consists of a single-proton nucleus with a single electron moving about it
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673 x 10-27kg, which is 1836 times larger than the mass of an electron and
1836/1837, or virtually all, of the mass of the simplest ydrogen atom
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The mass of a neutron is 1
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The nuclei of atoms of different elements differ in their number of protons and therefore in the amount of
positive charge they possess
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Physicists

have identified other subatomic particles, but particles other than electrons, protons, and neutrons have
little effect on the chemical properties of matter
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Therefore, we would expect a
nucleus with more than one proton to be unstable
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A similar attraction exists when neutrons are very
close together
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The Sizes of Atoms
It is convenient to think of the region occupied by the electrons as an electron cloud—a cloud of negative
charge The radius of an atom is the distance from the center of the nucleus ot the outer portion of this
electron cloud
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This unit is the picometer
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