Atomic structure

Atomic models, Ionisation energies, ionisation trends, electronic structure

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  • Atomic structure
    • Atomic Models
      • Thompson: Plum pudding model - atoms where still a solid sphere but contain -vely charged particles = electrons.
      • Dalton: Atoms are solid spheres that made up an element.
      • Rutherford: gold foil experiment - firing alpha particles at thin gold foil sheet so alpha particles would be slightly deflected by +ve 'pudding' making up most of the atom.
        • Findings: most alpha particles passed through the gold atoms, very few were deflected back.
      • Bohr's model: a cloud of electrons would spiral into nucleus and atom would collapse.
        • Electrons must exist in fixed shells. Each shell has a fixed energy.  Electrons move between shells and EM radiation is emitted or absorbed. energy in shells is fixed so the radiation has a fixed frequency.
    • Electronic structure
      • Each electron shell has a principal quantum number.
      • The further away a shell is from the nucleus the higher its energy level and PQN.
      • Not all electrons in the same shell have the same energy.
        • Shells are divided into subshells. Different electron shells have different numbers of sub shells.
          • Sub shells: S,P,D,F
            • S = 1 orbital  P = 3 orbitals D = 5 orbitals F= 7 orbitals
              • Each orbital holds 2 electrons.
      • 4s subshell has lower energy than 3d subshell.
      • Elements in periodic table are classified depending on what letter orbital their outermost electron is in.
    • Ionisation energy
      • measured for gaseous atoms so use (g)
      • Factors affecting IE: Nuclear charge, Distance from nucleus and shielding.
        • Nuclear charge: more protons in nucleus = stronger +ve charge and attraction for electrons.
        • Distance from nucleus: attraction falls with distance.
        • Shielding: as number of electrons between nucleus and outer electrons increases the outer electron is less attracted to the nucleus.
      • High IE = high attraction between nucleus and electrons so more energy is needed to remove the electron.
    • Ionisation trends
      • Al has lower IE than Mg. Al outer electron is in a 3p orbital not 3s.
        • 3p is found further away from the nucleus. 3p experiences additional shielding from 3s electrons.
          • These two factors override the effect of increased nuclear charge.
            • Evidence for theory of electron shells
      • S has a lower IE than P.
        • Shielding is identical as being removed from the same orbital - 3p.
          • S outer electron experiences repulsion as its in a paired orbital.
            • Repulsion between 2 electrons in an orbital makes electrons easier to remove from shared orbitals.
              • Evidence for electronic structure model.


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