Electromagnetic spectrum Mini-book – Due 17th September
• One type of EM wave for each page• Each page should contain the wavelength• Each page should contain uses and other
interesting information (including dangers)• Attractive to look at (and funny?)-
Last year June 2012
• describe the structure of an atom in terms of protons, neutrons and electrons and use symbols to describe particular nuclei
• Rutherford’s alpha scattering experiment • understand the terms atomic (proton) number,
mass (nucleon) number and isotope
Nuclide notation
Li3
7
Atomic number (proton number) = number of protons
Atomic mass (mass number) = number of protons and neutrons
Isotopes
Li3
7
It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons)
Li3
6
Isotopes
Li3
7
For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7
Li3
6
4 neutrons3 neutrons
Relative atomic mass
On average, lithium atoms have a mass of 6.941 (relative to Carbon 12)
Li3
6.941
Let’s try some questions.
Do Now!1.Date &
Title – Properties of Radiation
2. Do exam question 11 and stick in your book
Unstable nuclei
Some nuclei are unstable, for example Uranium 235
Hi! I’m uranium-235 and I’m unstable. I really need to
lose some particles from my nucleus to become more
stable.
Unstable nuclei
The decay of an unstable nucleus is random. We know it’s going to happen, but we can’t say when! It cannot be affected by temperature/pressure etc.
Weeeeeeeeeeeeee!
Today’s lesson
• 7.4 understand that alpha and beta particles and gamma rays are ionising radiations emitted from unstable nuclei in a random process
• 7.5 describe the nature of alpha and beta particles and gamma rays and recall that they may be distinguished in terms of penetrating power
• 7.6 Begin to describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the three main types of radiation
3 types of particle
There are 3 (at least in IGCSE!) types of particles that can be ejected from an unstable nuclei.
Please fill in the table with the information in the Powerpoint
You’ll learn about some really weird
ones in year 12!
Ionisation – Please copy
Ionisation occurs when an atom loses or gains one or more electrons.
When an atom loses electrons it becomes a positive ion.
When an atom gains electrons it becomes a negative ion.
Lithium atom (uncharged)
Lithium ion (positively charged)
Alpha particles
• 2 protons and 2 neutrons joined together
• The same as the nucleus of a helium atom
• Stopped by paper or a few cm of air
• Highly ionising
• Deflected by electric and strong magnetic fields
He2
4 2+
Beta particles
• Fast moving electrons from the nucleus
• Stopped by about 3 mm of aluminium
• Weakly ionising
• Deflected by electric and magnetic fields
e-1
0
Beta decay
• In the nucleus a neutron changes into an electron (the beta particle which is ejected) and a proton (which stays in the nucleus)
• During beta decay the mass number stays the same but the proton number goes up by 1.
e-1
0Th90
231
Pa91
231
+
Gamma rays
• High frequency electromagnetic radiation
• several cm of lead or thick concrete will reduce the intensity.
• Very weakly ionising
• NOT affected by electric or magnetic fields
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
+ + +
- - -
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
Stick your table in your books
• please complete information about speed and mass from the text book
The penetrating power of alpha, beta and gamma radiation – Please copy
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
reduces the intensity of gamma rays
Alpha, beta and gamma radiationAn alpha particle consists of two protons and two neutrons.It is strongly ionising.
A beta particle is a high speed electron.It is produced when a neutron has decays into an electron and proton.It is moderately ionising.
Gamma rays are very high frequency electromagnetic waves.They are produced when an unstable nucleus loses energy..They are weakly ionising.