electronegativity, electron affinity, ionisation energy

• first ionisation energy

  the energy required to remove the outermost electron from a gaseous atom
• first ionisation energy decreases down a group
• why does first ionisation energy decrease down a group

  because the size of the atoms increases so that the outer electron is further from the nucleus and therefore less strongly attracted by the nucleus
• variation in the first ionisation energy across a period


  the general trend is that the first ionisation energy increases from left to right across a period because there is an increase in nuclear charge with no significant change in shielding
• going across a period, the ionisation energy increases so elements lose electrons less easily
• Going down a group in the periodic table, ionisation energy decreases; therefore, elements are much more likely to exhibit metallic behaviour lower down a group
• first electron affinity

  • the energy change when one electron is added to a gaseous atom (X+e-=X-)
  • electron affinity is the enthalpy change when one electron is added to each atom in one mole of gaseous atoms under conditions
• electron affinity is difficult to measure experimentally, and data are incomplete
• What factors could affect the magnitude of an ionisation energy?

  Ionisation energies can be affected by nuclear charge, atomic radius and electron shielding.
• the first electron affinity is exothermic for virtually all elements
• an exothermic process responds to a negative voltage.
• the magnitude of the electron affinity decreases down a group - the values becomes less exothermic
• Usually the first member of the group will have a less exothermic reaction affinity than the second member and then the electron affinity becomes less exothermic down the group after that
• Electron affinities become more exothermic across a period

  As effective nuclear charge increases the attraction

  between the nucleus and the valence electrons will

  increase. This results in a stronger attraction and a more

  negative energy change.
• Electron affinities become less exothermic down a group:


  As the number of energy levels and electron shielding

  increases. Weaker attraction between the nucleus and the

  valence electrons. Hence a less negative energy change.
• Electronegativity


  Electronegativity is the ability of an atom to attract a shared pair of electrons in a covalent bond.
• Electronegativity increases from left to right across a period

  Increase in nuclear charge and smaller atomic radii, results in an increased attraction between the nucleus and shared

  electrons.
• Electronegativity decreases down a group:


  Atomic radii increase down a group. Therefore, there is a

  reduced attraction between the nucleus and the shared

  electrons.
• Variations in ionisation energy:

  • Ionisation energy decreases from Group 2 to Group 13 as the
  • outermost electron is added to a p sub-level which is at a higher energy level than a s sub-level.
  • Ionisation energy decreases from Group 15 to 16 as the outermost
  • electron is added to an already occupied p orbital, increasing electron repulsion.
• Variations in electron affinity:

  • Group 15 elements (e.g. phosphorous) will have the added electron placed into a singly occupied p orbital. This will lead to increased
  • electron repulsions and a weaker attraction. Therefore, the energy change is less negative.
• an exothermic process corresponds to a negative value for an enthalpy change
• the general trend is that the magnitude of the electron affinity decreases down a group - the values become less endothermic
• the first member of the group will have a less exothermic electron affinity than the second then the electron affinity becomes less exothermic down the group after that
• the electron affinity becomes less exothermic from cl to i as the size of the atom increases
• why does the electron affinity become less exothermic from cl to I as the size of the atom increases 


  The electron is brought into the outer shell of the atom and as the atom gets bigger there is a weaker attraction between the added electron and the nucleus as it is brought to a positive that is further from the nucleus if there is a weaker attraction less energy is released
• why does the first member of a group often have a less exothermic electron affinity than the second member 


  electron

  atoms
• atoms

  as atoms get larger, the electron is brought into a shell further from the nucleus, so is less strongly attracted. This should make the electron affinity less exothermic as we descend a group
• electron

  Electron repulsion between the incoming electron and the electrons already in the atom also affects the electron affinity. As the atoms gets larger , the electrons are further apart, and there is less electron - electron repulsion. This means that the electron affinity should become more exothermic as a group is descended
• electron affinity across a period

  becomes more exothermic
• why does electron affinity become more exothermic across a period?

  There is an increase in nuclear charge with no significant change in shielding during the period. The increase in nuclear charge also results in a decrease in atomic radius from left to right during the period
• Electronegativity is a measure of the attraction of an atom in a molecule to the electron in the covalent bond of which it is a part
• In a covalent bond between two different atoms, the atoms do not attract the electrons in the bond equally. How strongly the electrons are attracted depends on the size of the individual atoms and their nuclear charge
• electronegativity decreases down a group because the size of the atoms increase down a group
• Electronegativity increases from left to right across a period because there is an increase in nuclear charge with no significant change in shielding. shielding remains constant because atoms in the same period have the same number of inner shells
• trends down a group


  due to the increase in size of the atoms down a group
• trends across a period 


  due to an increase in nuclear charge with no significant change in shielding because all the atoms have the same number of inner shells
• atomic radius down a group


  increases
• ionic radius down a group

  increases
• ionisation energy down a group

  decreases
• electron affinity down a group

  becomes less exothermic