The IE of H is 1.31 MJ/mole. If all three electrons in Li were in the 1st shell at a distance equal to that of H, which of the following values would be the better estimate of the first ionization energy of Lithium: 3.6 MJ/mole or 0.6 MJ/mole? Why?

1 Answer
Aug 11, 2016


The most suitable estimate given the stated criteria would be #3.6# #"MJ/mol"#.


As is often the case with questions such as these, I often find that the best approach begins with a definition. Here, let's consider a typical definition of first ionisation energy:

The first ionisation energy of an element X is the energy required to remove one mole of electrons from one mole of atoms of X in the gaseous state, to form one mole of #"X"^+#anions in the gaseous state.

With that in mind, we may now consider any trends in changing first ionisation energy between adjacent elements - this is an important aspect of periodicity because it allows us to better understand the nature of the atom.

As you go from left to right across a period in the periodic table, say from hydrogen to helium, you will find that the first ionisation energy of an element will increase. This is because there is:

  1. An increased number of protons in the nucleus and so an increase in nuclear charge.

  2. No change in the amount of shielding, since there has been no change in the number of electron shells.

In short, the attraction between the nucleus and the outermost electrons is increased; because of this, you would also observe a decrease in atomic radius if the question was non-constraining. The important thing of note here is that since the question supposes that we remain in the first shell, the trend for first IE change by period is applicable here.

All that remains is to realise that the more strongly attracted the outermost electron(s) is to the nucleus, the harder it is to remove and the more energy is required to do so. It is therefore completely irrational to conclude that #0.6# #"MJ/mol"# would be sufficient in the first ionisation of lithium given that #""_1^1"H"##""_I""_E = 1.31# #"MJ/mol"#. The most appropriate estimate would therfore be that #""_7^3"Li"##""_I""_E = 3.6# #"MJ/mol"#.