Chapter 11 Particle Physics Flashcards
Rutherford Scattering
- Evidence for the structure of the atom was discovered by Ernest Rutherford in the beginning of the 20th century from the study of α-particle scattering
- The experimental setup consists of alpha particles fired at thin gold foil and a detector on the other side to detect how many particles deflected at different angles
α-particle scattering experiment set up
α-particles are what?
the nucleus of a helium atom and are positively charged
From this experiment, Rutherford results were: (alpha scattering)
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The majority of α-particles went straight through (A)
- This suggested the atom is mainly empty space
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Some α-particles deflected through small angles of < 10o
- This suggested there is a positive nucleus at the centre (since two positive charges would repel)
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Only a small number of α-particles deflected straight back at angles of > 90o (C)
- This suggested the nucleus is extremely small and this is where the mass and charge of the atom is concentrated
- It was therefore concluded that atoms consist of small dense positively charged nuclei, surrounded by negatively charged electrons
Atomic Structure
- The atoms of all elements are made up of three types of particles: protons, neutrons and electrons.
- A stable atom is neutral (it has no charge)
- Since protons and electrons have the same charge, but opposite signs, a stable atom has an equal number of both for the overall charge to remain neutral
Antimatter
- All matter particles have antimatter counterparts
- Antimatter particles are identical to their matter counterpart but with the opposite charge
Atomic Mass Unit (u)
- The unified atomic mass unit (u) is roughly equal to the mass of one proton or neutron:
- 1 u = 1.66 × 10−27 kg
- It is sometimes abbreviated to a.m.u
The term nuclide
is used to refer to a nucleus with a specific combination of protons and neutrons
The term nucleon
is the used to mean a particle in the nucleus – i.e. a proton or neutron
Isotopes
- An isotope is an atom (of the same element) that has an equal number of protons but different number of neutrons
- Remember, the neutron number of an atom is found by subtracting the proton number from the nucleon number
- Since nucleon number includes the number of neutrons, an isotope of an element will also have a different nucleon/mass number
- Since isotopes have an imbalance of neutrons and protons, they are unstable. This means they constantly decay and emit radiation to achieve a more stable form
AZX Notation
- The top number A represents the nucleon number or the mass number
- Nucleon number (A) = total number of protons and neutrons in the nucleus
- The lower number Z represents the proton or atomic number
- Proton number (Z) = total number of protons in the nucleus
Conservation of Nucleon Number & Charge
- Nuclear processes such as fission and fusion are represented using nuclear equations (similar to chemical reactions in chemistry)
- The number of protons and neutrons in atom is known as its constituents
what does this equation represent?
- The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and Xenon nucleus, releasing two neutrons in the process
- In nuclear equations, the nucleon number and charge are always conserved
- This means that the sum of the nucleons and charge on the left hand side must equal the sum of the number of nucleons and charge on the right hand side
- In the above equation, the sum of the nucleon (top) numbers on both sides are equal
235 + 1 = 236 = 90 + 144 + 2 × 1
- The same is true for the proton (bottom) numbers
92 + 0 = 92 = 38 + 54 + 2 × 0
- By balancing equations in this way, you can determine the nucleon, proton number or the number of missing elements
Balancing the number of nucleons shows that 3 neutrons must be released in the reaction
Alpha, Beta & Gamma Particles
- Some elements have nuclei that are unstable
- This tends to be when the number of nucleons does not balance
- In order to become more stable, they emit particles and/or electromagnetic radiation
- These nuclei are said to be radioactive
Alpha (α) particles
- are high energy particles made up of 2 protons and 2 neutrons (the same as a helium nucleus)
- They are usually emitted from nuclei that are too large