Nuclear Physics

Atomic structure An atom consists of a small central nucleus composed of protons and neutrons surrounded by electrons.

An atom will always have the same number of electrons as protons.

A Lithium atom

protons

neutrons

electrons

Atomic and mass numberThe atomic number (Z) of an atom is equal to the number of protons in its nucleus.

The mass number (A) of an atom is equal to the number of protons plus neutrons in its nucleus.

This Lithium atom has:

atomic number = 3

mass number = 7

protons = 3

neutrons = 4

electrons = 3

Properties of protons, neutrons and electrons

Position in the atom

Relative mass

Relative electric charge

PROTON

NEUTRON

ELECTRON

nucleus

nucleus

outside nucleus

1

1

0.005

+ 1

- 1

0

IonsAn atom becomes an ion when it loses or gains one or more electrons.

Lithium atom

protons neutronselectrons

Positive Lithium ion

Negative Lithium ion

Isotopes The atoms of an element always have the same number of protons.

Isotopes are atoms of the same element with different numbers of neutrons.

The three isotopes of hydrogen neutrons

hydrogen 1 hydrogen 3 (tritium)

hydrogen 2 (deuterium)

Note: The number after ‘hydrogen’ is the mass number of the isotope.

Nuclear notation

C14

6Number of protons (Atomic number)

Chemical symbol

An isotope of carbon consists of 6 protons and 8 neutrons. This can be written as:

OR:

carbon 14 Number of protons PLUS neutrons (Mass number)

Question 1

U235

92

An isotope of uranium (chemical symbol U) consists of 92 protons and 143 neutrons. Give the two different ways of notating this isotope.

uranium 235

The mass number of the Uranium isotope:

= 92 + 143 = 235

AND

Question 2Determine the number of protons and neutrons in the isotopes notated below:

N13

7

(a) protons = 7

neutrons = 6 Co60

27

(b) p = 27

n = 33

Au197

79

(c) p = 79

n = 118 Pu239

94

(d) p = 94

n = 145

Note: Apart from the smallest atoms, most nuclei have more neutrons than protons.

Alpha decay

Alpha particles consist of two protons plus two neutrons.

They are emitted by some of the isotopes of the heaviest elements.

Example: The decay of Uranium 238

U238

92Th

234

90α

4

2+

Uranium 238 decays to Thorium 234 plus an alpha particle.

Notes:

1. The mass and atomic numbers must balance on each side of the equation: (238 = 234 + 4 AND 92 = 90 +2)

2. The alpha particle can also be notated as:He

4

2

QuestionShow the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92).

Pu239

94U

235

92α

4

2+

Beta decay

Beta particles consist of high speed electrons.

They are emitted by isotopes that have too many neutrons.

One of these neutrons decays into a proton and an electron. The proton remains in the nucleus but the electron is emitted as the beta particle.

Example: The decay of Carbon 14

C14

6N

14

7 β-

0

-1+

Carbon 14 decays to Nitrogen 14 plus a beta particle.

Notes:

1. The beta particle, being negatively charged, has an effective atomic number of minus one.

2. The beta particle can also be notated as:e

0

-1

QuestionShow the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg).

Na25

11Mg

25

12 β-

0

-1+

Gamma decayGamma decay is the emission of electromagnetic radiation from an unstable nucleus

Gamma radiation often occurs after a nucleus has emitted an alpha or beta particle.

Example: Cobalt 90

Co90

27γ

0

0+Co

90

27

Cobalt 90 with excess ENERGY decays to

Cobalt 90 with less ENERGY plus gamma radiation.

Changing elementsBoth alpha and beta decay cause the an isotope to change atomic number and therefore element. Alpha decay also causes a change in mass number.

Decay type Atomic number Mass number

alpha DOWN by 2 DOWN by 4

beta UP by 1 NO CHANGE

gamma NO CHANGE NO CHANGE

Complete the decay equations below:

Fe59

26Co

59

27 β-

0

-1+

Ra224

88Rn

220

86α

4

2+

N16

7O

16

8 β-

0

-1+

(a)

(c)

(b)

Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!)

Pb202

82Hg

198

80α

4

2+

Pt194

78Au

194

79β

-0

-1+

Element Sym Z

Platinum Pt 78

Gold Au 79

Mercury Hg 80

Thallium Tl 81

Lead Pb 82

Bismuth Bi 83

Hg198

80Pt

194

78α

4

2+

There are other correct solutions

Background radiationBackground radiation is ionising radiation from space (cosmic rays), devices such as X-ray tubes, and from radioactive isotopes in the environment (for example radon gas from rocks in the ground).

Most of this radiation occurs naturally but a small amount is due to nuclear weapon testing and nuclear power stations.

Background radiation pie-chart

Nuclear reactions ANSWERS

In text questions:

(a) 92p, 143n

(b) 23890Th = 90p + 138n;

22488Ra = 88p + 136n

(c) 4019K = 19p + 21n; 40

20Ca = 20p +20n

Summary questions:

1. (a) 6p + 6n

(b) 27p + 33n

(c) 92p + 143n

2. (a) 92p + 146n

(b) 90p + 144n

(c) 91p + 143n

The Plum Pudding Atomic Model

Before about 1910 many scientists believed that an atom consisted of:Positively charged matter spread out like a pudding embedded by negatively charged electrons (like plums in a pudding).

The ‘Plum Pudding’ Model

Rutherford’s Atomic ModelIn 1909 Ernest Rutherford suggested that an atom consists of a a tiny positively charged nucleus surrounded by negatively charged electrons.

Lord Rutherford 1871 - 1937

Geiger & Marsden’s alpha particle scattering experiment

In 1909 Hans Geiger and Ernest Marsden performed an experiment using alpha particles to determine which of the two models was the better in describing the structure of an atom.

Geiger and Marsden

The apparatus 2

1

5

3

4

What was observedthin metal foil

1. Virtually all of the alpha particles went straight through the metal foil.

2. A few alpha particles were deflected through a small angle.

3. About 1 in 10 000 were deflected backwards.

alpha source

nucleus (highly enlarged)

atom

How the results can be explained

1. Deflections occur because there is a force between the charged nucleus and the positively charged alpha particles.

2. Most of the alpha particles do not go near enough to the nucleus to be deflected.

3. Backwards deflections occur when the alpha particles make near head on collisions with the positively charged nucleus.

How their results supported Rutherford’s atomic model

1. The relatively small number of deflections indicates that most of the atom is empty space with only a very small nucleus.

2. The backward deflections can only occur if the nucleus is positively charged and contains most of the atom’s mass.

3. The ‘plum pudding’ model would not produce backward deflections.

Choose appropriate words to fill in the gaps below:

According to __________ an atom consists of a tiny, ___________ charged __________ surrounded by a cloud of ________ electrons. The nucleus also contains most of the ______ of an atom.

This model was supported by the ______ particle scattering experiment in 1909. In this experiment most alpha particles passed ________ through a thin metal foil with only about 1 in 10000 being deflected _________.

positively nucleus

mass

alpha

Rutherford

straight

backwards

WORD SELECTION:

negative

positively nucleus

mass

alpha

Rutherford

straight

backwards

negative

Nuclear fission

Nuclear fission is the splitting of an atomic nucleus.

Nuclear fission can be used as an energy source in a nuclear reactor.

There are two fissionable substances in common use in nuclear reactors, uranium 235 and plutonium 239.

Chain reactionThe fission of a nucleus of Uranium 235 can be initiated by a neutron.

When this nucleus splits further neutrons are produced.

These neutrons in turn can cause more nuclei to split.

An avalanche effect, called a ‘chain reaction’ can then occur.

A chain reaction

neutron

Nuclear fission reactor

1

2

3 & 4

5

6

7

Nuclear reactor parts1. Fuel rodsThese contain U235 or Pu239. They become very hot due to nuclear fission.

2. Control rodsMade of boron, when placed in-between the fuel rods these absorb neutrons and so reduce the rate of fission. Their depth is adjusted to maintain a constant rate of fission.

3. ModeratorThis surrounds the fuel rods and slows neutrons down to make further fission more likely. The moderator can be water or graphite.

4. CoolantThis transfers the heat energy of the fuel rods to the heat exchanger. Coolant be water, carbon dioxide gas or liquid sodium.

5. Heat exchangerHere water is converted into high pressure steam using the heat energy of the coolant. 6. Reactor coreThis is a thick steal vessel designed to withstand the very high pressure and temperature in the core.

7. Concrete shieldThis absorbs the radiation coming from the nuclear reactor.

Choose appropriate words to fill in the gaps below:

Nuclear fission is the _________ up of the nucleus of an atom into two smaller nuclei. ________ and neutrons are also usually emitted.

Nuclear ________ use Uranium _____ or Plutonium _____to produce energy by nuclear ________. A controlled chain reaction is maintained by the use of _______ rods which absorb some of the _________ produced.

An _______ bomb is the consequence of an uncontrolled chain reaction.

239

neutrons

reactors

control

splitting

fission

energyWORD SELECTION:

235

atomic

239

neutrons

reactors

control

splitting

fission

energy

235

atomic

Nuclear fusion

Nuclear fusion is the joining of two atomic nuclei to form a larger one.

Hydrogen nuclei undergo fusion in stars to make helium nuclei

Energy from fusionNuclear fusion is the process by which energy is released in the Sun and other stars.

It is also the energy source of the hydrogen bomb.

Nuclear fusion reactors

Scientists are currently working to make nuclear fusion reactors.

The fuel for fusion reactors is the isotope hydrogen 2 (deuterium) which is found in sea water.

An experimental fusion reactor in Seatle USA

Virtual Physics Laboratory SimulationsNOTE: Links work only in school

Alpha Scattering.exe

Geiger Muller Tube

Nuclear Stability With stability curve

Nuclear Reactions Fission & Fusion with binding energy

Online SimulationsAtoms, ions & isotopes (GCSE) - Powerpoint presentation by KT Build an atom - eChalk Atomic Structure Quiz - by KT - Microsoft WORD Hidden Pairs Game on Atomic Structure - by KT - Microsoft WORD Fifty-Fifty Game on What particles are positive - by KT - Microsoft WORD Rutherford Scattering - PhET - How did Rutherford figure out the structure of the atomic nucleus without looking at it? Simulate the famous experiment in which he disproved the Plum Pudding model of the atom by observing alpha particles bouncing off atoms and determining that they must have a small core. Rutherford Scattering Experiment Thomson Model of evenly distributed charge and Nuclear Model - Michael Fowler Types of Radiation - S-Cool section on types of radiations including an animation of absorption and a couple of decay equations to fill in on screen. Alpha Decay - PhET - Watch alpha particles escape from a Polonium nucleus, causing radioactive alpha decay. See how random decay times relate to the half life. Beta Decay - PhET - Watch beta decay occur for a collection of nuclei or for an individual nucleus Decay series - Fendt Andy Darvill's Radioactivity Pages Understanding Radiation - National Radiological Protection Board - Useful starting point to get at useful areas of the site.

Nuclear Fission - PhET - Start a chain reaction, or introduce non-radioactive isotopes to prevent one. Control energy production in a nuclear reactor! Nuclear Fission - Powerpoint presentation by Richard Miller of 5SJW (2005) Nuclear Fission - Powerpoint presentation that includes a link to the 'mousetrap' demonstration Power Station Animation - eChalk Managing a Nuclear Power Plant Simulation - by Henrik Eriksson BBC AQA GCSE Bitesize Revision: Atoms, isotopes & radioactivity - Core Science Structure of an atom Isotopes Evidence for atomic structure Alpha, beta & gamma radiation Detecting radiation Radioactive decay equations Natural sources of background radiation Artificial radiation Nuclear fission Nuclear fusion