Atomic Strucutre

• Plum Pudding Model


  Atoms consisted of a sphere of positive charge, with small negative charges distributed evenly within it
• Electron Shell Model


  Atom consists of a small, dense central nucleus surrounded by orbiting electrons in electron shells

  
• The nucleus consists of protons and neutrons giving it an overall positive charge. It contains almost the entire mass of the atom
• In a neutral atom, the number of electrons is equal to the number of protons due to the relative charges
• Particle


  • Proton: Relative Charge +1, Relative Mass 1
  • Neutron: Relative Charge 0, Relative Mass 1
  • Electron: Relative Charge -1, Relative Mass 1/1840

  
• Maximum number of orbiting electrons in a shell

  2n^2 where n is the number of the shell
• Each electron shell must fill before the next one can hold any electrons
• Mass number (A)


  Sum of protons and neutrons in an atom
• Atomic number (Z)


  Equal to the number of protons in an atom
• Relative atomic mass (Ar)


  The mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope

  
• Isotopes


  Atoms of the same element with the same atomic number, but with a different number of neutrons, resulting in a different mass number
• Neutral atoms of isotopes will react chemically in the same way as their proton number and electron configuration is the same. The sharing and transfer of electrons is unaffected. However, the different mass number means they have different physical properties
• Ions


  Formed when an atom loses or gains electrons meaning it is no longer neutral and will have an overall charge
• Mass Spectrometry


  1. Ionisation

  2. Acceleration

  3. Ion Drift

  4. Detection

  5. Analysis
• During the ionisation process, a 2+ charged ion may be produced. This means it will be affected more by the magnetic field producing a curved path of smaller radius. As a result, its mass to charge ratio (m/z) is halved and this can be seen on spectra as a trace at half the expected m/z value

  
• Spectra produced by the mass spectrometry of chlorine display a characteristic pattern in a 3:1 ratio for Cl+ ions and a 3:6:9 ratio for Cl2+ ions. This is because one isotope is more common than the other and the chlorine molecule can form in different combinations

  
• Electron Orbitals


  s, p, d and f orbitals have different shapes

  
• Each orbital can hold a different number of electrons before the next one is filled: s-orbital = 2 electrons, p-orbital = 6 electrons, d-orbital = 10 electrons
• The energy of the orbitals increases from s to d meaning the orbitals are filled in this order. Each orbital is filled before the next one is used to hold electrons
• Spin


  Within an orbital, electrons pair up with opposite spin so that the atom is as stable as possible. Electrons in the same orbital must have opposite spin
• There are three rules for writing out electron configurations: 1. The lowest energy orbital is filled first. 2. Electrons with the same spin fill up an orbital first before pairing begins. 3. No single orbital holds more than 2 electrons

  
• If electron spins are unpaired and therefore unbalanced, it produces a natural repulsion between the electrons making the atom very unstable. If this is the case, the electrons may take on a different arrangement to improve stability
• Ionisation Energy


  The minimum energy required to remove one mole of electrons from one mole of atoms in a gaseous state. It is measured in kJmol^-1
• Successive ionisation energies


  Usually require more energy because as electrons are removed the electrostatic force of attraction between the positive nucleus and the negative outer electron increases. More energy is therefore needed to overcome this attraction so ionisation energy increases
• Along a Period


  First ionisation energy increases due to a decreasing atomic radius and greater electrostatic forces of attraction
• Down a Group


  First ionisation energy decreases due to an increasing atomic radius and shielding which reduces the effect of the electrostatic forces of attraction
• When successive ionisation energies are plotted on a graph, a sudden large increase indicates a change in energy level. This is because the electron is being removed from an orbital closer to the nucleus so more energy is required
• The first ionisation energy of Aluminium is lower than expected due to a single pair of electrons with opposite spin. As a result there is a natural repulsion which reduces the amount of energy needed to be put in to remove the outer electron