CIE AS Physics (9702) exams from 2022

Revision Notes

11.1.1 Atomic Structure

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

Alpha particle experiment diagram, downloadable AS & A Level Physics revision notes

α-particle scattering experiment set up

  • α-particles are the nucleus of a helium atom and are positively charged

Alpha particle scattering, downloadable AS & A Level Physics revision notes

When α-particles are fired at thin gold foil, most of them go straight through but a small number bounce straight back

  • From this experiment, Rutherford results were:
  • The majority of α-particles went straight through (A)
    • This suggested the atom is mainly empty space
  • 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)
  • 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, downloadable AS & A Level Physics revision notes

An atom: a small positive nucleus, surrounded by negative electrons

  • (Note: The atom is around 100,000 times larger than the nucleus!)

Worked example

Worked example - Rutherford scattering, downloadable AS & A Level Physics revision notes

  • The Rutherford scattering experience directed parallel beams of α-particles at gold foil
  • The observations were:
    • Most of the α-particles went straight through the foil
    • The largest value of n will therefore be at small angles
    • Some of the α-particles were deflected through small angles
    • n drops quickly with increasing angle of deflection θ
  • These observations fit with graph A

Atomic Structure

  • The atoms of all elements are made up of three types of particles: protons, neutrons and electrons.

 

Nucleus and electrons, downloadable AS & A Level Physics revision notes

Protons and neutrons are found in the nucleus of an atom while electrons orbit the nucleus

  • The properties of each particle are shown in the table below:

Table of nuclear properties, downloadable AS & A Level Physics revision notes

  •  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

Exam Tip

Remember not to mix up the ‘atom’ and the ‘nucleus’. The ‘atom’ consists of the nucleus and electrons. The ‘nucleus’ just consists of the protons and neutrons in the middle of the atom, not the electrons.

Antimatter

  • We live in a universe made up of matter particles (protons, neutrons, electrons etc.)
  • All matter particles have antimatter counterparts
    • Antimatter particles are identical to their matter counterpart but with the opposite charge
  • This means if a particle is positive, its antimatter particle is negative and vice versa
  • Common matter-antimatter pairs are shown in the diagram below:

Matter antimatter pairs, downloadable AS & A Level Physics revision notes

  • Apart from electrons, the corresponding antiparticle pair has the same name with the prefix ‘anti-’ and a line above the corresponding matter particle symbol
  • A neutral particle, such as a neutron or neutrino, is its own antiparticle

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
  • This value will be given on your data sheet in the exam
  • The a.m.u is commonly used in nuclear physics to express the mass of subatomic particles. It is equal to 1/12 of the mass of the carbon-12 atom

Table of common particles with mass in a.m.u

Table of common particles with mass in a.m.u, downloadable AS & A Level Physics revision notes

  • The mass of an atom in a.m.u is roughly equal to the sum of its protons and neutrons (nucleon number)
    • For example, the mass of Uranium-235 is roughly 235u

Worked example

Worked example - amu, downloadable AS & A Level Physics revision notes

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