Post on 27-Dec-2015
transcript
Traditional atomic model Not correct but it is a good enough
approximation. Nucleus contains 2 types of nucleons:
Protons and Neutrons. Orbiting electrons. Atomic number is number of protons. Mass number is number of nucleons. Elements have the same atomic
number but can have different mass numbers = isotopes.
Properties of subatomic particles
Property
Proton Neutron Electron
Mass 1.673 x 10-27 kg
1.675 x 10-27 kg
9.109 x 10-31 kg
Charge +1 0 -1
Location Nucleus Nucleus Orbiting
Radioactive isotopes
Every element has some unstable isotopes called radioisotopes.
These are rare. The most common isotope is usually stable (eg. C-12 not C-13 or C-14).
Radioisotopes become stable by decaying into a stable isotope.
They decay by emitting radiation.
Alpha decay The nucleus ejects an α particle (2
protons and 2 neutrons) to become smaller and more stable.
Uranium-238: 92 protons, 146 neutrons.
Emits an α particle: 2 protons, 2 neutrons.
Thorium-234: 90 protons, 144 neutrons.
Th-234 is the daughter nucleus of the alpha decay
Beta decay
A neutron can decay into a proton and an electron (after about 10 minutes).
The electron (β particle) is ejected from the nucleus at close to the speed of light.
The atomic number increases by 1. The mass number stays the same.
Thorium-234: 90 protons, 144 neutrons
One neutron decays into a proton and an electron
Protactinium-234: 91 protons, 143 neutrons
Gamma decay
After an alpha or beta decay the nucleus can be momentarily “excited”.
The excited nucleus emits a high energy gamma ray (which is a photon).
If a nucleus is in an excited state we put an asterisk next to the symbol.
Protactinium-234*: 91 protons, 143 neutrons
Emits a γ particle Protactinium-234: 91 protons, 143
neutrons
Decay of Uranium-238
http://en.wikipedia.org/wiki/Uranium-238
http://en.wikipedia.org/wiki/Decay_chain
Penetration power
Smoke detectors Americium-241 emits α particles. These charged particles allow a
current to flow between two electrodes. If smoke blocks the α particles then the current is reduced and an alarm sounds.
Ionising power Radiation can ionise atoms by removing one or
more electron from it’s orbit. α particles interact strongly with atoms because
of their slow speed and +2 charge. This slows them down quickly (low penetration power).
β particles also with atoms but are moving faster and only interact with the outside electrons. They take longer to slow down and have higher penetration power than α particles.
γ particles interact with atoms very weakly because they have no charge and most of an atom is empty space. This means they pass through most materials.
Properties of α,β,γ particlesProperty α
particleβ particle
γ particle
Mass 6.644 x 10-27 kg
9.109 x 10-31 kg
0 kg
Charge +2 -1 0
Effect of electric or magnetic field
Small deflection Large deflection No deflection
Speed (c%) 5-7% 30-99% 100%
Penetration power
A few cm in airA Piece of paper
1-2m of airA few cm of Al
Goes through airMany cm of lead absorbs most
Ionising power Strong Weak Very weak