Download Structure of the nucleus • It is now known that the nucleus consists of

Structure of the nucleus

It is now known that the nucleus consists of:
o
protons: positive charge: charge +1, mass number 1
o
neutrons: no charge: charge 0,mass number 1
o
protons and neutrons are collectively known as nucleons.

The total number of protons and neutrons in the nucleus is called the mass number A.

The number of protons in the nucleus is called the atomic number Z.

In a neutral atom the number of protons equals the number of electrons.

The mass numbers, charges and symbols for protons, neutrons and electrons are given below.

The type of nucleus, and the corresponding atomic number and mass number is normally
written using the standard representation
A
Z
X
for example,
14
6
C
means that this is a carbon nucleus of atomic number 6 and mass number 14.

This means that this carbon nucleus has 6 protons and 8 neutrons (14 – 6 = 8).

The table below provides information on protons, neutrons and electrons with the last column
showing the standard representation described above.

Particle
Mass number
Charge
Symbol
Proton
1
+1
1
1
p
Neutron
1
0
1
0
n
Electron
0
-1
0
1
The mass number of an electron is taken to be zero since mass of electron =
proton.
e
1
mass of
1840
Isotopes

Each element in the periodic table has a different atomic number and is identified by that
number.

It is possible to have different versions of the same element, called isotopes.

An isotope of an atom has the same number of protons but a different number of neutrons, i.e.
the same atomic number but a different mass number.
An isotope is identified by specifying its chemical symbol along with its atomic and mass numbers.
For example:
Carbon-12
12
6
C
contains 6 protons and 6 neutrons (12 – 6)
Carbon-14
14
6
C
contains 6 protons and 8 neutrons (14 – 6)
Radioactive decay

Many nuclei are unstable.

In order to achieve stability, they can emit nuclear radiation: alpha, beta or gamma.
o

Such unstable nuclei are called radioisotopes or radionuclides.
The process of emitting radiation is called decay.
Alpha, beta and gamma
 or 24 He

An alpha particle is the nucleus of a helium atom
4
2

A beta particle is a fast-moving electron
0
1
o
 or 10 e
In beta decay, a neutron within the unstable nucleus changes into a proton, with the
emission of a fast-moving electron from with the nucleus (discussed in more detail in Key
Area - Standard Model)
n  11 p  01n
1
0

Gamma rays are photons of high-energy electromagnetic radiation

0
0
Representation of radioactive decay by symbols and equations

In the following equations, both mass number and atomic number are conserved,
o

i.e. the totals are the same before and after the decay.
The original radionuclide is called the parent and the new radionuclide produced after decay is
called the daughter product (sometimes this may go on to decay further).
Alpha decay

Uranium 238 decays by alpha emission to give Thorium 234.
238
92

U
234
90
Th + 42 He
Mass number decreases by 4, atomic number decreases by 2 (due to loss of 2 protons and 2
neutrons).
Beta decay

Lead 210 decays by beta emission to give Bismuth 210.
210
82

Pb 
210
83
Bi + 01e
Mass number is unchanged, atomic number increases by 1
Gamma decay

Only energy is emitted from within the nucleus and the daughter product is the same as the
parent.
Example
Thorium-230 decays by alpha emission.
Write the equation for this decay and state the name of the daughter nucleus produced.
You need a Periodic Table to identify the daughter nucleus
Th 
230
90
Ra  24
226
88
The daughter nucleus produced is Radon-226
(the equation is balanced, giving unknown daughter nucleus atomic number 88 and mass number
226. The periodic table is used to identify the element with atomic number 88 as Radon).
Nuclear Fission


Nuclear fission is when a heavy nucleus breaks up into two nuclei of smaller mass number,
usually along with several neutrons.
o
Fission can occur spontaneously, i.e. a heavy nucleus can break up randomly and of its
own accord
o
In nuclear power stations, fission is induced by bombarding the heavy nucleus with
neutrons.
The following equation represents the fission of Uranium-235
235
92
141
1
U + 01n  92
36 Kr + 56 Ba + 3 0 n + energy
Nuclear Fission and E=mc2

Mass number and atomic number are both conserved during the fission reaction shown above.

Even though the mass number is conserved, when the masses before and after the fission are
compared accurately, there is a mass difference.
o
The total mass before fission is greater than the total mass of the products.

Einstein suggested that mass and energy are equivalent, and that when there was a decrease in
mass, an equivalent amount of energy was produced.

Einstein’s famous equation shows how mass and energy are related:

In fission reactions, the energy released is carried away as kinetic energy of the fission products.
Example
The equation shown below represents a fission reaction
235
92
U + 01 n 
137
56
Ba +
97
36
Kr + 2 01 n + energy
The masses of the nuclei are:
235
92
U
Ba
2273  10-27 kg
Kr
1609  10-27 kg
137
56
97
36
1
0
3902  10-27 kg
n
1675  10-27 kg
Mass before fission (kg)
235
-27
92U 3902 × 10
1
0
n
Mass after fission (kg)
Ba
2273 × 10-27
Kr
1609 × 10-27
137
56
1675 × 10-27
97
36
___________________
2 01n 2  1.67510-27 = 3350 × 10-27
391875 × 10-27
______________________________________________
391550 × 10-27
Decrease in mass = (391875 – 391550) × 10-27 = 0325 × 10-27 = 325  10-28 kg
Energy released during this fission reaction, using E = mc2
E  mc 2
E  3  25 1028  (3 108 ) 2
E  2  8 1011 J
This is the energy released by fission of a single nucleus.
Nuclear fission reactor

Fuel Rods:
Generate heat by nuclear fission

Moderator:
fission
Slows neutrons down since slow neutrons have more chance of inducing

Control Rods:
Absorb excess neutrons. Control the rate of heat production

Coolant:
Removes heat from the reactor and passes the heat to the boiler

Containment
Vessel:
Absorbs radiation. Shields the outside from radiation produced in the
reactor
Nuclear Fusion

Fusion is when two light nuclei combine to form a nucleus of larger mass number,
o
e.g. the fusion of two deuterium nuclei (an isotope of hydrogen) to form helium – as
shown below

Like fission, there is a decrease in mass in the fusion process and the energy released is
produced as kinetic energy of the fusion products.

Unlike fission however, there are no radioactive fission fragments left over from the reaction.

The energy released by the sun and other stars is produced by nuclear fusion.

Fusion of deuterium requires incredibly high temperatures found at the core of stars
o
Producing a working fusion reactor would give society an amazingly efficient energy
source which uses only hydrogen fuel and no radioactive waste
o
To recreate the conditions that occur at the core of stars on Earth presents a huge
scientific and engineering challenge
Example
The equation shown below represents a fusion reaction
2
1
H + 31 H 
4
2
He + 01 n + energy
The masses of the nuclei are:
2
1
H
3345  10-27 kg
5008  10-27 kg
3
1
4
2
He 6647  10-27 kg
1
0
n
H
1675  10-27 kg
Mass before fusion (kg)
2
3345  10-27 kg
1H

 10-27 kg
3
5 008
1
___________________
H
8353 × 10
Mass after fusion (kg)
4
6647  10-27 kg
2 He
-27
1.67510-27
______________________________________________
8322 × 10-27
Decrease in mass = (8353 – 8322) × 10-27 = 0031 × 10-27 = 31  10-29 kg
Energy released during this fission reaction, using E = mc2
E  mc 2
E  3 11029  (3 108 ) 2
E  2  8 1012 J
This is the energy released by one fusion reaction.
Fusion reactor – the goal of plasma physicists

The fourth state of matter which is a high-temperature, ionised gas called plasma
o
Physicists who study fusion are called plasma physicists

Fusion experiments involve heating a very small container of hydrogen fuel to phenomenally
high temperatures usually only found at the core of stars.

A self-sustaining fusion reactor would generate its own incredibly high temperatures which
would then continue to generate more and more fusion reactions.
o
The point where the fusion reactions generate the conditions to sustain further
fusion reactions is called ignition.

The challenge is in heating the fuel to very high temperatures – and in confining the plasma
within itself to prevent any plasma touching the walls of its container – any contact with the
container results in cooling.

For ignition to occur, three conditions must be met
o
Extremely high plasma temperature (over 100 million Kelvin)
o
A stable period of heat energy production via fusion lasting over 2 seconds

o
Stable plasma density (mass per unit volume) of 1020 nuclei/m3


This is where the confinement by huge magnetic fields is required
250,000 times less dense than the earth's atmosphere
Teams of physicists have come close to achieving ignition and a self-sustaining fusion reactor.
o
The leading centre for fusion research is the Joiunt European Torus (JET) facility in
Oxfordshire. JET is the largest fusion fusion exeperiment in the world involving a
huge team of plasma physicists and engineers.
o
The product of temperature, density, and thermal insulation attained is only a factor
of 5 short of the conditions required for ignition.
Diagram of a magnetic confinement fusion
reactor. The plasma is confined and
stopped from touching the walls of the
reactor by the huge magnetic fields
generated by the complex magnet system.