The nucleus consists of protons and neutrons, collectively called nucleons. The number of protons is the atomic number. Protons plus neutrons is the atomic mass number or the nucleon number. - PowerPoint PPT Presentation

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The nucleus consists of protons and neutrons, collectively called nucleons. The number of protons is the atomic number. Protons plus neutrons is the atomic mass number or the nucleon number.

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Title: The nucleus consists of protons and neutrons, collectively called nucleons. The number of protons is the atomic number. Protons plus neutrons is the atomic mass number or the nucleon number.


1
The nucleus consists of protons and neutrons,
collectively called nucleons. The number of
protons is the atomic number. Protons plus
neutrons is the atomic mass number or the nucleon
number.
2
Shorthand notation42He, 2713Al, 11H, 23592UIso
topes are atoms that have the same number of
protons, but a different number of
neutrons. 126C 136C 146C
3
The radius r of the nucleus depends on the atomic
number A. r ª (1.2 x 10-15 m)A1/3 Nuclear
density has nearly the same value for all atoms.
4
Why doesnt the nucleus fly apart due to the
positive charges? The strong nuclear force holds
the nucleus together. This force is independent
of charge. It has a limited range of action.
5
To balance the electrostatic forces as the number
of protons increases, more neutrons must be
present (more strong nuclear force without more
electrostatic force).
6
Eventually a point is reached where the number of
protons cannot be balanced by adding more
neutrons. 20983Bi is the stable nucleus with the
most protons.
7
All atoms with more than 83 protons are unstable
and spontaneously break down or rearrange their
internal structure.
8
This spontaneous break down or rearrangement is
called radioactivity.
9
The more stable the nucleus is, the greater the
amount of energy needed to break it apart. This
is the binding energy of the nucleus.
10
The mass of a stable nucleus is less than the
total mass of the individual particles that make
up the nucleus. This difference in mass ?m is
the mass deficit of the nucleus.
11
The mass deficit is directly related to the
binding energy by E mc2. Binding energy
(Mass deficit)c2 (?m)c2
12
Ex 2 - The mass of a 42He nucleus is 6.6447 x
10-27 kg. Find (a) the mass deficit and (b) the
binding energy.
13
The atomic mass unit (u) is often used instead of
the kilogram. 1 u 1.6605 x 10-27 kg
14
In energy, one atomic mass unit is equal to
1.4924 x 10 -10 J 9.315 x 108 eV 931.5 MeV.
15
The binding energy per nucleon is too low for
nuclei with more than 83 protons. That is why
these elements are radioactive.
16
Three kinds of rays are produced by
radioactivity alpha rays, beta rays, and gamma
rays.
17
The production of alpha rays is called alpha
decay. An alpha particle is a helium nucleus 42He.
18
23892U ---gt 23490Th 42HeAlpha decay
converts one element into another. This is called
transmutation. Energy is released in this change
and can be calculated from the loss of mass.
19
Ex 4 - Determine the energy released when a
decay converts 23892U into 23490Th.
20
The energy released appears as kinetic energy of
the two particles except for a small amount
released as a gamma ray.
21
Beta particles are electrons. Beta decay is also
an example of transmutation.23490Th ---gt 23491Pa
0-1e
22
This released electron is created when a neutron
decays to a proton and an electron.
23
Ex 6 - Find the energy released when b-decay
changes 23490Th into 23491Pa .
24
In a second kind of b decay a positron is emitted
when a nuclear proton is converted to a neutron.
The mass number is unchanged, but the atomic
number decreases by one.
25
A third type of b decay is electron capture where
the nucleus captures an electron from outside the
nucleus.
26
An excited nucleus can change to a lower energy
state releasing a photon, in this case a gamma
ray photon. This does not cause transmutation of
the element.
27
Ex 7 - What is the wavelength of the 0.186 MeV
gamma ray photon emitted by radium 22688Ra?
28
All of the energy emitted during b decay does not
end up in the beta particle. Some of the energy
goes to another particle called a neutrino.
29
Neutrinos are mass-less or almost mass-less. They
have no charge, and can pass through a light-year
of lead without interacting with it.
30
The emission of neutrinos and b particles
involves a force called the weak nuclear force.
Actually the weak nuclear force and the
electrostatic force are one fundamental force,
the electroweak force.
31
The electroweak force, the gravitational force,
and the strong nuclear force are the three
fundamental forces in nature.
32
Half-life T1/2 is the time required for one-half
of the nuclei present in a sample to disintegrate.
33
Half-lives range from a fraction of a second to
billions of years.
34
Ex 8 - Suppose 3.0 x 107 radon atoms are trapped
in an enclosure. The half-life of radon is 3.83
days. How many radon atoms remain after 31 days?
35
If the product of a radioactive decay is an
unstable isotope, this isotope will go through
radioactive decay. This will continue until a
stable isotope is reached. This is a radioactive
decay series.
36
In a Geiger counter particles and photons cause a
gas to ionize and conduct a current. The degree
to which it conducts the charge flow is
determined by the level of radiation.
37
In a cloud chamber, a gas is cooled to the point
of condensation. When particles pass through the
gas droplets form along the path of the particle.
38
A bubble chamber contains a liquid just at the
point of boiling. The particles leave a trail of
bubbles.
39
A photographic emulsion (film) directly produces
a record of the particles path.
40
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