Flashcards in Models Deck (28)
What did Dalton propose?
that an element is made up of atoms and that every element has its own kind of atom.
-An element is a basic chemical which cannot be broken down into a smaller substance.
-Atoms combine in whole number ratios to form compounds
What is an atom?
The smallest particle of an element which can have the element's properties.
What did Thomson discover?
The electron - an extremely small subatomic particle carrying a negative charge, present in all matter.
What is the Thomson model?
Plum Pudding Model: a uniform sphere/cloud of positive charge embedded with electrons. The positive and negative charges balance to give a neutral atom.
What is the Rutherford model?
A tiny, positive, dense nucleus with electrons orbiting around it. Atom still neutral overall.
-Mainly empty space, as the nucleus has 99.95% of the mass of the atom, but is very small compared to the size of the atom.
Describe the set up of Rutherford's experiment
Radium source emitting a beam of alpha particles (which are positive charged) from a narrow tube or in lead block/box (to fire straight at Au foil). The beam was sent through the gold foil, surrounded by a fluorescent screen. Any alpha particles coming away from the foil will strike the screen, producing tiny flashes of light.
-vacuum because alpha particles are highly ionising so would ionise air particles, meaning it would interfere with the interaction with Au foil.
The experiment was carried out in a totally dark room
The observations of the Rutherford Experiment
Most alpha particles passed straight through the foil without any deflection.
Some alpha particles were deflected at large angles
A few bounced straight back
What did the observations conclude
Most alpha particles passed straight through the foil without any deflection. = Au atoms mainly empty space.
- the alpha particles were far away from the nucleus to not be repelled by the positive charge of the nucleus
Some alpha particles were deflected at large angles = the electric repulsive force between the positive alpha particle and the nucleus was repulsive so alpha particles were deflected.
A few bounced straight back = the very unlikely event (due to small size) of a head-on collision of an alpha particle with a very small gold nucleus would cause a large deflection of the alpha particle. The nucleus of Au is very dense because it was the alpha particle that was deflected, not the Au nucleus.
Symbol for Atomic number and mass number
Atomic = Z
Mass = A
What is the centripetal force holding the electrons in circular orbits around the nucleus?
electric force attracting the negatively charged electrons to the positive charge of the protons in the nucleus
What binds the protons and neutrons together in the nucleus?
Strong nuclear force. This prevents the protons from pushing each other apart, by the electric repulsive forces between positive charges.
What is the penetration ability of a radiation?
How far it travels before being absorbed
How far can the decay particles travel in air?
What are the things stopping the rays?
Alpha: sheet of paper
Beta: 3-5mm Al
Gamma: thick lead/concrete
What is the ionising ability of a radiation?
How much ionisation (production of charged particles) is produced by the radiation.
i.e. how easily it removes or adds electrons onto the particles for them to gain a positive or negative charge.
Relationship between speed of radiation and ionisation ability
In general, the quicker the radiation is absorbed, the more ionisation is caused.
What is the absorption ability of a material?
A measure of how much material is needed to protect against the harmful nature of the radiation
What are the two principles used for decay
Conservation of charge: number of protons same/total charge remains constant
Conservation of mass: total number of nucleons remains constant
Relationship between activity of radioactive sample to the number of undecided nuclear in the sample
Proportional. As the sample decays, it becomes less active i.e. it gives off fewer particles per second.
What is half life
The time taken for half the radioactive substance to decay to just half the original amount
Time taken for the decay counts to half its value
What is nuclear power
The use of sustained nuclear fission to generate heat and electricity.
Nuclear power is generated using Uranium, a metal that is mined as an ore in large quantities.
What is nuclear fission
Splitting of large nucleus into two smaller parts. Exothermic reaction.
How does U decay?
When a neutron strikes a U-235 nucleus, it is absorbed and the resulting nucleus becomes unstable and splits.
EM energy (in the form of gamma radiation) is released as several different atoms and neutrons are produced. This E is proportional to the MASS LOSS.
Explain E = mc^2
m is the mass LOSS in kilograms.
c = 3x10^8 ms^-1
E = energy released in Joules.
What is U fission chain reaction?
Each time U-235 splits, it releases 3 neutrons. If at least one of those neutrons encounters another U-235 nucleus, then further fission occurs. This process may continue as a chain reaction, producing an uncontrolled nuclear explosion.
Describe the nuclear reactor in nuclear power station
In a nuclear reactor, the chain reaction is controlled so it takes place at a chosen steady rate. The reactor consists of a core which contains natural U.
The fuel rods contain 99% U-238, which does not undergo fission, and 1% U-235, which does undergo chain reaction fission.
Cadmium control rods have the property of absorbing neutrons and so slow the chain reaction down or even stop it. Both the fuel and cadmium rods are mounted in graphite. The graphite is a moderator which slows the neutrons down without absorbing them.
U-235 gives off neutrons, and the cadmium rods absorb them. The cadmium rods are gradually removed to start the reactor. The number of cadmium rods is very carefully controlled so that exactly the right number of neutrons is free to react. All the cadmium rods can be dropped into the reactor, to shut it off quickly in case of an emergency.
Describe the 3 steps of workings of a nuclear power station
1. Fission reaction produces heat energy
2. water is used to carry heat energy away from the reactor
3. steam produced drives a steam turbine which drives an electrical generator.