14.1 Structure of the Atom In order to understand atoms, we
need to understand the idea of electric charge. We know of two
different kinds of electric charge and we call them positive and
negative.
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14.1 Electric charge in matter We say an object is electrically
neutral when its total electric charge is zero.
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14.1 An early model In 1897 English physicist J. J. Thomson
discovered that electricity passing through a gas caused the gas to
give off particles that were too small to be atoms. These negative
particles were eventually called electrons.
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14.1 The nuclear model In 1911, Ernest Rutherford, Hans Geiger,
and Ernest Marsden did a clever experiment to test Thomsons model.
We now know that every atom has a tiny nucleus, which contains more
than 99% of the atoms mass.
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14.1 Inside an atom The mass of the nucleus determines the mass
of an atom because protons and neutrons are much larger and more
massive than electrons. In fact, a proton is 1,836 times heavier
than an electron.
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14.1 Force inside atoms Electrons are bound to the nucleus by
the attractive force between electrons (-) and protons (+).
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14.1 Force inside atoms What holds the nucleus together? There
is another force that is even stronger than the electric force. We
call it the strong nuclear force.
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14.1 How atoms of various elements are different The atoms of
different elements contain different numbers of protons in the
nucleus. Because the number of protons is so important, it is
called the atomic number.
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14.1 How atoms of various elements are different Isotopes are
atoms of the same element that have different numbers of neutrons.
The mass number of an isotope tells you the number of protons plus
the number of neutrons. How are these carbon isotopes
different?
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14.1 Radioactivity Almost all elements have one or more
isotopes that are stable. Stable means the nucleus stays together.
Carbon-14 is radioactive because it has an unstable nucleus.
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Solving Problems How many neutrons are present in an aluminum
atom that has an atomic number of 13 and a mass number of 27?
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1.Looking for: number of neutrons in aluminum-27 2.Given atomic
no. = 13; mass no. = 27 3.Relationships: Periodic table says atomic
no. = proton no. protons + neutrons = mass no. 4.Solution neutrons
= mass no. protons neutrons = 27 13 = 14 Solving Problems
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14.2 Electrons in the atom Each different element has its own
characteristic pattern of colors called a spectrum. The colors of
clothes, paint, and everything else around you come from this
property of elements to emit or absorb light of only certain
colors.
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14.2 Electrons in atoms Each individual color in a spectrum is
called a spectral line because each color appears as a line in a
spectroscope. A spectroscope is a device that spreads light into
its different colors.
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14.2 Bohr model of the atom Danish physicist Neils Bohr
proposed the concept of energy levels to explain the spectrum of
hydrogen. When an electron moves from a higher energy level to a
lower one, the atom gives up the energy difference between the two
levels. The energy comes out as different colors of light.
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14.2 The quantum theory Quantum theory says that when things
get very small, like the size of an atom, matter and energy do not
obey Newtons laws or other laws of classical physics.
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14.2 The quantum theory According to quantum theory, particles
the size of electrons are fundamentally different An electron
appears in a wave-like cloud and has no definite position.
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14.2 The quantum theory The work of German physicist Werner
Heisenberg (19011976) led to Heisenbergs uncertainty principle. The
uncertainty principle explains why a particles position, momentum
or energy can never be precisely determined. The uncertainty
principle exists because measuring any variable disturbs the others
in an unpredictable way.
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14.2 The uncertainty principle
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14.2 Electrons and energy levels In the current model of the
atom, we think of the electrons as moving around the nucleus in an
area called an electron cloud. The energy levels occur because
electrons in the cloud are at different average distances from the
nucleus.
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14.2 Rules for energy levels Inside an atom, electrons always
obey these rules: 1.The energy of an electron must match one of the
energy levels in the atom. 2.Each energy level can hold only a
certain number of electrons, and no more. 3.As electrons are added
to an atom, they settle into the lowest unfilled energy level.
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14.2 Models of energy levels While Bohrs model of electron
energy levels explained atomic spectra and the periodic behavior of
the elements, it was incomplete. Energy levels are predicted by
quantum mechanics, the branch of physics that deals with the
microscopic world of atoms.
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14.2 Energy levels In the Bohr model of the atom, the first
energy level can accept up to two electrons. The second and third
energy levels hold up to eight electrons each. The fourth and fifth
energy levels hold 18 electrons.