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.