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Structure of the atomA hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…
I did an experiment (with my colleagues Geiger and Marsden)
that proved this idea was wrong. I called it the “Scattering
Experiment”
Ernest Rutherford, British scientist:
The Rutherford Scattering ExperimentAlpha
particles (positive
charge, part of helium
atom)
Thin gold foil
Most particles passed through, 1/8000 were
deflected by more than 900
Conclusion – atom is made up of a small, positively charged nucleus surrounded by
electrons orbiting in a “cloud”.
The structure of the atomELECTRON –
negative, mass nearly
nothing
PROTON – positive,
same mass as neutron
(“1”)
NEUTRON – neutral,
same mass as proton
(“1”)
Atoms are roughly 10-10m in diameter, while the nucleus is 10-15 – 10-14m
The structure of the atomParticle Relative Mass Relative Charge
Proton 1u (1.7x10-27kg) +1.6x10-19C
Neutron 1u (1.7x10-27kg) 0
Electron 0 -1.6x10-19C
MASS NUMBER (A) = number of protons + number of neutrons
SYMBOL
No. of neutrons N = A - Z
PROTON NUMBER (Z) = number of protons (obviously)
IsotopesAn isotope is an atom with a different number of neutrons:
Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.
Notice that the mass number is different. How many neutrons does each isotope have?
A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.
Quarks
Pe-
Low energy scattering
We can investigate the structure of protons by bombarding them with electrons:
High energy scattering
Elastic collision. Electrons and protons behave as expected.
Pe-
Inelastic collision. Energy is “absorbed” by the proton and increases its internal energy. This is Deep Inelastic Scattering and suggests that the proton is made of smaller particles called quarks.
Introduction to RadioactivitySome substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation:
Radiation
The nucleus is more stable after emitting some radiation – this is called “radioactive decay”.
IonisationRadiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by giving them enough energy to “knock off” electrons:
Alpha radiation is the most ionising (although short range). Ionisation causes cells in living tissue to mutate, usually causing cancer.
The Geiger-Muller Tube
Metallic case (cathode)
Mixture of argon and halogen
Central anode
Mica end window
Types of radiation1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)
2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle.
3) Gamma – after or decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.
Unstable nucleus
Unstable nucleus
Unstable nucleus
New nucleus
New nucleus
New nucleus
Alpha particle
Beta particle
Gamma radiation
Words – frequency, proton, energy, neutrons, helium, beta
Changes in Mass and Proton NumberAlpha decay:
Am241
95Np
237
93α
4
2+
11
5
0
+1C
11
6B β+
90
39Sr
90
38Y β
0
-1+
Beta - decay:
Beta + decay:
“positron”
Blocking RadiationEach type of radiation can be blocked by different materials:
Sheet of paper (or 6cm of air
will do)
Few mm of aluminium
Few cm of lead
SummaryProperty Alpha Beta - Beta + Gamma
Charge
Rest mass
Penetration
What is it?
Ionising ability
Deflection by Magnetic FieldsAlpha and beta particles have a charge:
++
-
2 protons, 2 neutrons, therefore charge =
+21 electron, therefore
charge = -1
Because of this charge, they will be deflected by electric and magnetic fields:
+
-
+
Background Radiation
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power
13% are man-made
Nuclear fission
Uranium nucleus
Unstable
nucleus New nuclei (e.g. barium
and krypton)
More neutron
s
Neutron
Chain reactions
Each fission reaction releases neutrons that are used in further reactions.
Radioactive DecayRadioactivity is a random process. The number of radioisotopes that will decay clearly depends on the number of radioisotopes present at that point in time:
Activity (in Bq) = λN
λ = “The decay constant” and has units of s-1. It is constant for a particular radioisotope.
Half LifeThe decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…
At start there are 16 radioisotope
s
After 1 half life half have
decayed (that’s 8)
After 3 half lives another
2 have decayed (14 altogether)
After 2 half lives another
half have decayed (12 altogether)
= radioisotope = new atom formed
A radioactive decay graph
Time
Count
1 half life
1 half life
1 half life
Half LifeTo calculate half life there are a few methods:
1) Read from a graph
2) Calculate using an equation
t½ = ln2
λ
Half Life questions
100s
1) The graph shows the activity of a radioisotope. Determine the half life and decay constant.
2) If there are 106 atoms present right now calculate how many will decay over the next second.3) What percentage of a sample of radioactive material will exist after 200 years if the half life is 50 years?
4) Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.
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