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Atomic Structure (CIE A Level Physics)

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Alpha-particle scattering experiment

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

Structure of an atom 

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

Table of properties of subatomic particles

Particle Relative charge Relative mass
Proton +1 1
Neutron 0 1
Electron -1 1/2000 (negligible)

  •  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

Examiner 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 table below:

Charges of matter and antimatter particles

Matter Charge Antimatter Charge
Electron, e to the power of minus -1 Positron, e to the power of plus +1
Proton, straight p +1 Anti-proton, straight p with bar on top -1
Neutron, straight n 0 Anti-neutron,  0

  • Apart from electrons, the corresponding antiparticle pair has:
    • the same name with the prefix ‘anti-’
    • a line above the corresponding matter particle symbol

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

  • The atomic mass unit 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

Particle Mass / u
Proton 1
Neutron 1
Electron 0.0005
Alpha (α) 4

  • 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

Estimate the mass of the nucleus of element Copernicium-285 in kg.

Give your answer to 2 decimal places.

Answer:

Step 1: Determine the mass in a.m.u.

  • The mass of an atom in a.m.u. is roughly equal to its nucleon number
  • Therefore, the mass of copernicum-285 in a.m.u. is

285 space straight u

Step 2: Convert to kg

1 space straight u space equals space 1.66 cross times 10 to the power of negative 27 end exponent space kg

285 space straight u space equals space 285 space cross times space open parentheses 1.66 cross times 10 to the power of negative 27 end exponent close parentheses space kg

285 space straight u space equals space 4.73 cross times 10 to the power of negative 25 end exponent space kg space open parentheses 3 space straight s. straight f. close parentheses

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Leander

Author: Leander

Expertise: Physics

Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.