What is the experiment to show the existence of a small charged nucleus in an atom?
- positively charged alpha particles are incident on a thin gold foil in a vacuum chamber
- majority went straight through or deviated by small angles, indicating that the nucleus is very small compared to the size of an atom and the atom is mainly made of empty spaces
- some deflected at more than 10°, indicating that the nucleus is positively charged
- a very small proportion of alpha particles are deviated at large angles and came straight back, indicating that the mass of nucleus is massive and highly concentrated in a very small charged volume
Why are beta particles not used in Rutherford’s scattering experiment?
- beta-particles have a range of energies and are easily deviated by the orbital electrons of a gold nucleus
- hence unable to interact with nucleus and deduce that nucleus is positively charged
- beta particles are too light so unable to interact with nucleus and deduce that nucleus is massive
How to ensure the accuracy of Rutherford’s experiment?
- ensure that gold foil is as thin as possible to limit the number of collisions between alpha particles and gold nuclei
- experiment to be done in vacuum to avoid collisions between alpha particles and air molecules
- use target element of high atomic number to ensure large scattering of alpha particles
What affects the degree of scattering of alpha particles?
The greater the charge of the target nuclei, the stronger the repulsive electrostatic force exerted on the approaching alpha particle and the greater the degree of scattering
Wht is very thin gold foil used in Rutherford’s experiment?
- prevent too many alpha particles from being absorbed
- allow alpha particles to be scattered only once
Is the interaction between alpha particle and gold nucleus considered an elastic collision?
- as alpha particle apporaches the nucleus, kinetic energy of the alpha particle is converted to electrical potential energy
- when they separate, potential energy is converted back to kinetic energy of the alpha particle and the nucleus
- kinetic energy of the system remains constant before and after the experiment
- thus the interaction is considered elastic
What is an atomic mass unit?
1/12 of the mass of a Carbon-12 atom
What is nuclear force?
Strong attractive force that binds protons and neutrons together in nucleus
What are the characteristics of nuclear force?
- identical between proton-proton, proton-neutron, neutron-neutron
- extremely short range, 1 to 3fm
Why is the density of the nucleus independent of atomic number?
Nuclear attractive force is much larger than the electrostatic forces of repulsion between protons in the nucleus
What is mass defect?
The difference between the total mass of separate nucleons and the mass of nucleus
What is binding energy?
The energy to separate nucleons in a nucleus completely to infinity
Why is nuclear reactions being able to produce large amounts of energy not a violation of conservation of energy?
- some mass from reactant is converted into energy and released
- total mass-equivalent energy in the system is conserved
What is binding energy per nucleon?
total energy needed to completely separate all the nucleons in a nucleus divided by the number of nucleons in the nucleus
What is the relationship between binding energy per nucleon and nucleus stability?
The higher the binding energy per nucleon, the greater the nucleus stability
What is the trend of BE per nucleon for light elements?
- increasing binding energy per nucleon
- the increase in attractive nuclear forces per nucleon outweights the increase in electrostatic forces of repulsion between protons
What is the trend of BE per nucleon for large elements?
- decreasing BE per nucleon
- electrostatic forces of repulsion between protons are beginning to overcome attractive nuclear forces between nucleons
What is conserved in nuclear reactions?
- proton number
- mass number
- mass-energy
- momentum
What is nuclear fission?
Splitting of heavy nucleus into two or more light nuclei of approximately the same mass, with emission of a few neutrons and/or other radiations
What is a nuclear chain reaction?
a series of similar reactions in which one of the products of a nuclear reaction causes further reactions to occur
How can fission be controlled?
absorbing excess neutrons using water/boron/cadmium sheets
What is nuclear fussion?
combination of two light nuclei to form a more massive nucleus, with possible emission of other radiations
What are the conditions required for fusion and fission to occur?
- fusion: high temperature
- fission: bombardment of small electrons (minimal KE)
What are the advantage/disadvantage of fusion/fission?
- fusion produces radioactive waste that has very short half life, less hazardous
- fission produces radioactive waste, hence hazardous
- fusion release more energy than fission
- fusion energy released is not controllable while fission energy can be controlled by absorbing neutrons away using control rod
- fusion is more efficient than fission
