Atomic Structure (CIE A Level Physics)

• 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 the nucleus of a helium atom and are positively charged

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 < 10^o
  • 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 > 90^o (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

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

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

     ANSWER:   A

• 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

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

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:

•  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

• 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:

• 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

• 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

• 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