AQA GCSE Physics contents

Unit 4 · Atomic Structure

Atoms, Isotopes & The Development of the Model

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The structure of an atom

  • Atoms are very small: radius about 1×10−101 \times 10^{-10} m
  • A positively charged nucleus of protons and neutrons, surrounded by negatively charged electrons
  • The nucleus radius is less than 110 000\frac{1}{10\,000} of the atom's radius (about 1×10−141 \times 10^{-14} m), but most of the mass is concentrated in it
  • Electrons are arranged at different distances from the nucleus: different energy levels
    • Absorbing electromagnetic radiation moves an electron to a higher energy level, further from the nucleus
    • Emitting electromagnetic radiation moves an electron to a lower level, closer to the nucleus
Particle Relative charge Relative mass Where
Proton +1+1 1 nucleus
Neutron 0 1 nucleus
Electron −1-1 very small (about 12000\frac{1}{2000}) energy levels around the nucleus

Mass number, atomic number and isotopes

  • The number of electrons equals the number of protons, so an atom has no overall charge
  • The atomic number is the number of protons; all atoms of an element have the same atomic number
  • The mass number is the total number of protons and neutrons
  • Written as atomic numbermass numberX^{\text{mass number}}_{\text{atomic number}}\text{X}: 1123Na^{23}_{11}\text{Na} has 11 protons, 12 neutrons, 11 electrons
  • Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons: same atomic number, different mass number. 12C^{12}\text{C} and 14C^{14}\text{C}
  • Atoms become positive ions if they lose one or more outer electrons

The development of the model of the atom

  • New experimental evidence can lead to a model being changed or replaced
  • Before the electron was discovered, atoms were thought to be tiny solid spheres that could not be divided
  • The discovery of the electron led to the plum pudding model: a ball of positive charge with negative electrons embedded in it
  • The alpha particle scattering experiment (Rutherford, Geiger and Marsden): alpha particles were fired at thin gold foil
    • Most passed straight through: the atom is mostly empty space
    • Some were deflected: the centre carries a positive charge, which repels the positive alpha particles
    • A few bounced back: the mass is concentrated in a tiny, dense nucleus
  • The plum pudding model could not explain the deflections, so it was replaced by the nuclear model
  • Niels Bohr adapted the nuclear model: electrons orbit the nucleus at specific distances (energy levels). His calculations agreed with experiment
  • Later experiments showed the nuclear charge is made of whole numbers of smaller particles with the same positive charge: protons
  • James Chadwick provided the evidence for neutrons in the nucleus, about 20 years after the nucleus was accepted
  • Details of Bohr's and Chadwick's experiments are not required

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alpha scattering experimentring of detectorsthin gold foilsourcemost pass straight through, hardlydeflected at alla few are deflected throughlarge anglesa very few bounce almoststraight backthe model it impliedmost of the atom is EMPTY SPACE — so mostparticles pass throughthe mass and the positive charge are concentratedin a tiny NUCLEUS — so a few are deflected, andthe very few that hit it head-on bounce backthe electrons orbit at a distanceThe plum pudding model predicted that every particle would pass through with only a slightdeflection. It could not explain the large deflections — so the model had to be replaced. This ishow scientific models work: evidence that does not fit forces a new theory.
The alpha scattering experiment: a source firing alpha particles at gold foil inside a ring detector, with most particles passing through, a few deflected at angles and one bouncing back, next to the nuclear model it implied