Atomic Structure Booklet

• The chemical properties of an element depend on its atomic structure and the arrangement of electrons around the nucleus
• The arrangement of electrons in orbitals is linked to the way in which elements are organised in the Periodic Table
• Chemists can measure the mass of atoms and molecules to a high degree of accuracy in a mass spectrometer
• Plum pudding model

  A model of positive charge with negative electrons
• Rutherford's α-particle scattering experiment

  1. Aimed a beam of α-radiation at a gold foil target

  2. Investigated the size and arrangement of the atoms of gold
• α-particle

  The nucleus of a helium atom
• Aston's mass spectrometer

  Showed that a sample of one pure element contained atoms of different masses
• Fundamental particles

  • Proton
  • Neutron
  • Electron
• Mass number (A)


  The total number of nucleons (protons + neutrons) in the nucleus of the atom
• Atomic number (Z)


  The total number of protons in the nucleus of the atom
• Isotopes

  Atoms of the same element with the same number of protons but different numbers of neutrons
• Ion

  An atom of an element that has gained or lost electrons
• Relative atomic mass (Ar)

  The weighted average mass of all isotopes of an element compared to 1/12th the mass of an atom of carbon-12
• Potential energy (Ep)


  Ep = qV
• Kinetic energy (Ek)


  Ek = 1/2 m * v^2
• Velocity (v)


  v = s/t
• Time of flight (t) is proportional to the square root of the mass to charge ratio (m/z)
• For a given mass, species with a higher charge arrive at the detectors before those with a lower charge
• The relative molecular mass of a covalent compound can be obtained from the mass spectrum, usually as the highest m/z value
• Electrospray ionisation can be used to avoid fragmentation, in which case the highest m/z value is Mr+1
• In the mass spectrum of krypton, a small peak with m/z of 42 corresponds to the 84Kr isotope with 2 electrons knocked out and a 2+ charge
• Atomic emission spectra are produced when electrons in an atom are excited and then return to lower energy levels, emitting photons
• Mr

  Highest m/z value, unless the compound has all fragmented during the ionisation phase
• Electrospray ionisation

  Adds a H+ ion to the molecule, so the highest m/z value is Mr+1
• Ionisation of krypton in the mass spectrometer

  Beam of electrons from an electron gun knocks out electron(s) (to form a positive ion)
• When the electron returns to the lower energy state the extra energy is emitted as light
• The pattern of emissions from a particular element is unique to that element
• The more electrons there are in an atom, the more complex the pattern produced
• Lyman series

  Emission lines when an excited electron falls back to n=1 (UV)
• Balmer series

  Emission lines when the electron falls back to n=2 (Visible)
• As the energy levels within each series increase, they converge, getting closer and closer together
• n=∞ represents an electron leaving the atom completely
• Ionisation energy

  The energy needed to remove an electron from one mole of gaseous atoms to form one mole of gaseous +1 ions
• Second ionisation energy

  The energy needed to remove an electron from one mole of gaseous +1 ions to form one mole of gaseous +2 ions
• Successive ionisation energies require progressively more energy since the closer an electron is to the nucleus, the more energy is required to remove it from the atom
• Orbital


  A volume of space in which there is a high probability of finding electron density
• The maximum number of electrons that can be fitted into a single orbital is two: this is known as the Pauli exclusion principle
• Electrons possess a property known as spin and so the two electrons of a full orbital make up a 'spin pair'
• The orbitals occur in a series of shells whose distance from the nucleus gradually increases
• The number of orbitals that can be fitted into each shell follows a fixed pattern