Ionisation Energies and Radii

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Ionisation Energies

  • When an atom loses one or more electrons to form a positive ion, it is said to become ionised. This process requires energy to overcome the attraction between the electron and the nucleus
  • The amount of energy needed to excite an electron to the point at which it can escape is called the ionisation energy
  • Ionisation energies can be found for all elements, not just those that commonly form positive ions

'The first ionisation energy of an element is the energy needed to remove one electron from each atom in a mole of gaseous atoms of an element'

'The first ionisation energy for any element X is therefore the energy change, in jKmol-1, for the process;

X(g) -> X+(g) + e-

  • More than one electron can be removed from an atom but we always consider them one at a time
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Ionisation Energies

'The second ionisation energy of an element is the energy needed to remove a second electron from each ion in a mole of gaseous 1+ ions of an element'

'The second ionisation energy of X is therefore the energy change, in kJmol-1 for the process;

X+(g) -> X 2+(g) + e-'

Factors affecting ionisation energies

1. The shell that the outermos electron will be removed from

This affects the strength of attraction between electron and the nucleus. The higher the shell, the weaker the attraction because;

  • Distance from the nucleus - the attraction between a positive and a negative charge gets weaker the furthur they are from each other. The higher the shell the greater the average distance of the electron from the nucleus. Hence, the weaker the attraction of the nucleus for the electron and the lower the ionisation energy, other factors being equal
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Ionisation Energies

  • Shielding by electrons in inner shells - An electron being removes is repelled by other electrons in the inner shell and this is called shielding and it makes an electron easier to remove than if there were no inner electrons. Other electrons in the same shell have much less shielding effect, only electrons in inner shells are important

2. The Nuclear Charge

  • (The positive charge on the nucleus, which is given by the atomic number of the element)
  • The higher the nuclear charge, the more strongly the electrons will be attracted and hence the higher the ionisation energy, all other factore being equal

Variation of first ionisation energies

  • There is a lot of variation from one element to another but the variation is not random - it shows periodicity

Across a period

  • Overall first ionisation energy increases
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Ionisation Energies

This is because;

  • The nuclear charge is increasing across the period, so the outer electrons are pulled in closer to the nucleus
  • Extra electrons are being added across the period but because they go into the same shell, the shielding does not change much
  • Overall, the attraction to the nucleus increases across the period

Down the group

  • First ionisation energy decreases

This is because;

  • An extra shell has been added furthur from the nucleus
  • The outer electron therefore experiences more shielding
  • The two above effects outweigh the increasing nuclear charge, so the outer electron is less attracted to the nucleus
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Ionisation Energies

Trends in Successive Ionisation Energies

  • There is always an increase in ionisation energy as each successive electron is removed. This is because as the number of electrons decreases, there is less repulsion between them, so they can move in closer where attraction to the nucleus is greater
  • There is always a large increase in ionisation energy when the electron being removed comes from a different shell. This is because lower shells are closer to the nucleus and are less shielded, so the electrons in them are much more strongly attracted to the nucleus
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Atomic and Ionic Radii

  • The more shells of electrons an atom has, the bigger it will be 
  • The stronger the attraction to the nucleus, the more closely the outermost electrons will be pulled in towards it, so the smaller the atom will be

Variation of Atomic Radius across a Period

  • On going across a period, there is a decrease in atomi radius

This is because;

  • The nuclear charge increase
  • As extra electrons are being added to the same shell, shielding does not increase much
  • Consequently, the attraction of the nuclues for the outer electrons increases, so the electrons are pulled in closer to the nuclues and the atomic radii decrease

Variation of Atomic Radius across a Period

  • On going down the group, there is an increase in atomic radius
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Atomic and Ionic Radii

This is because;

  • There are more shells of electrons
  • There is more shielding by inner shells of electrons
  • The two above factors outweigh the increasing nuclear charge so the atomic radii increase

Ionic Radii

  • When atoms gain or lose electrons, the extent of repulsion between the electrons changes and as a result the radius changes too

Positive ions

  • These are always smaller than the atoms they are formed from because, as electrons are removed there is less repulsion between those that are left so they are pulled in closer to the nucleus
  • Also, in the case of group 1 and 2 ions, there is one less shell in the positive ion than in the atom because all outer shell electrons are lost when the ion is formed. This furthur reduces the radius
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Ionic Radii

Negative Ions

  • These are always larger than the atoms they are formed from because there is more repulsion between electrons as the number of them increases

Isoelectronic Species

  • If two ions, or an atom and an ion contain the same number of electrons, then the one with more protons is always smaller as the nucleus attracts the electrons more strongly
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