EDEXCEL IGCSE PHYSICS 7-1

Atoms and RadioactivityEdexcel IGCSE Physics pages 199 to 208

December 4th 2010

All content applies for Triple & Double Science

THIS POWERPOINT IS NOT DUE FOR COMPLETION UNTIL

JULY 2012

Edexcel IGCSE Specification

Section 7: Radioactivity and particlesb) Radioactivitydescribe the structure of an atom in terms of protons, neutrons and electrons and use symbols such as 14

6C to describe particular nucleiunderstand the terms atomic (proton) number, mass (nucleon) number and isotopeunderstand that alpha and beta particles and gamma rays are ionising radiations emitted from unstable nuclei in a random processdescribe the nature of alpha and beta particles and gamma rays and recall that they may be distinguished in terms of penetrating powerdescribe the effects on the atomic and mass numbers of a nucleus of the emission of each of the three main types of radiationunderstand how to complete balanced nuclear equations

Atomic structureAn 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 of an atom is equal to the number of protons in its nucleus.

The mass number 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

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)

IsotopesThe 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.

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.

RadioactivityThe atoms of some substances are unstable and they give out radiation from their nuclei all the time, whatever is done to them. These substances are said to be radioactive.

The first three types of radiation discovered were alpha particles, beta particles and gamma rays. Henri Becquerel discovered

radioactivity in 1896

Alpha, beta and gamma radiationAn alpha particle is the same as a helium nucleus.It consists of two protons and two neutrons.

A beta particle is a high speed electron.It has come from the nucleus where a neutron has decayed into an electron and proton.

Gamma rays are very high frequency electromagnetic waves.They are produced when an unstable nucleus loses energy.

The penetrating power of alpha, beta and gamma radiation

Paper or a few cm of air stops alpha particles

1cm or 1m of air of aluminium

stops beta particles

Several cm of lead or 1m of concrete is needed to stop

gamma rays

Choose appropriate words to fill in the gaps below:

Atoms consist of a very small _______, containing protons and neutrons, surrounded by _______. Atoms of the same element will always have the same number of _______ but different ________ of the same element will have different numbers of _________.

The atoms of some substances are unstable and _________. They may give off alpha or ______ particles or gamma rays.

Gamma rays are the most penetrating type of radiation, _____ is the least.

isotopes

radioactivenucleus

protons electronsalpha

neutrons

WORD SELECTION:

beta

isotopes

radioactive

nucleus

protonselectrons

alpha

neutrons

beta

SimulationsBuild an atom - eChalk Atomic Structure Quiz - by KT - Microsoft WORD Hidden Pairs Game on Atomic Structure - by KT - Microsoft WORD 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. Andy Darvill's Radioactivity Pages Understanding Radiation - National Radiological Protection Board - Useful starting point to get at useful areas of the site. BBC Bitesize Revision: Introduction Page to AQA Radioactive SubstancesAtoms & Isotopes Alpha, beta & gamma radiation - what they are Penetrating power of radiations - includes applet - also see page on detecting radiations (two after)

Ionisation

Ionisation occurs when an atom loses one or more of its electrons. The atom becomes a positive ion.

Alpha particles cause intense ionisation due to their large mass double positive charge.Beta particles cause moderate ionisation.Gamma rays only cause weak ionisation because they are uncharged.

Lithium atom (uncharged)

Lithium ion (positively charged)

S

Deflection by magnetic fields

Alpha and beta particles are deflected in opposite directions due to their opposite charges.

Due to their much larger mass alpha particles are deflected far less than beta.

Gamma rays are not deflected because they are not charged.

Magnetic south pole placed behind the rays

Deflection by electric fields

Alpha and beta particles are deflected in opposite directions due to their opposite charges.

Due to their much larger mass alpha particles are deflected far less than beta.

Gamma rays are not deflected because they are not charged.Electric field produced by

positively and negatively charged plates

+ + +

- - -

Choose appropriate words to fill in the gaps below:

Magnetic and ________ fields deflect alpha and beta particles in ________ directions due to their opposite ________. Beta particles deflect more because their ______ is about 8000 times ______ than alpha particles. Gamma rays, being _________, are not deflected by either type of field.

Radioactivity causes __________ which can cause living cells to undergo genetic _________ leading on to possibly cancerous growth. It is therefore important to minimalise exposure especially to ______ particles which cause the most intense ionisation.

mass uncharged electricopposite chargesmutationionisation

WORD SELECTION:

less alpha

mass

uncharged

electric

opposite

charges

mutation

ionisation

less

alpha

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

Choose appropriate words to fill in the gaps below:

When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______.

Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged.

Background radiation is mainly due to natural sources of _________ radiation such as from ________ gas that seeps out from rocks in the ground.

ionising radonmasstwofour one

WORD SELECTION:

neutrons

ionising radon

mass

two four

one

neutrons

SimulationsTypes 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. Andy Darvill's Radioactivity Pages Understanding Radiation - National Radiological Protection Board - Useful starting point to get at useful areas of the site. BBC Bitesize Revision: Alpha, beta & gamma radiation - what they are Penetrating power of radiations - includes applet - also see page on detecting radiations (two after) Deflecting radiations using electric and magnetic fields - includes applets showing deflections Detecting radiation using photographic film (badges) & GM tube - includes applet testing penetrating power with GM tube detector Hazards of radiation

SimulationsVarious Radioactive Materials in the Home - 'Whys Guy' Video Clip (4:30mins)

Andy Darvill's Radioactivity Pages

Understanding Radiation - National Radiological Protection Board - Useful starting point to get at useful areas of the site.

Radon Gas - National Radiological Protection Board

BBC Bitesize Revision:

Using radiation - tracers & thickness measurement - includes applet showing sheet rolling application

Test bite on Radioactive Sources

Atoms and RadioactivityNotes questions from pages 199 to 208

1. Answer the questions on pages 207 and 208.

2. Verify that you can do all of the items listed in the end of chapter checklist on page 207.

Online Simulations