Cambridge International AS Level Chemistry
QUESTION
The 1st ionisation energy of an element is the energy needed to remove one electron from each atom in one mole of atoms of the element in the gaseous state to form one mole of gaseous 1+ ions.
1 Write the simple electronic configuration of the following atoms, showing the principal quantum shells only: a sulfur; the atomic number of sulfur, Z = 16 b magnesium, Z = 12 c
fluorine, Z = 9
d potassium, Z = 19 e carbon, Z = 6
Evidence for electronic structure Ionisation energy, ∆Hi
By firing high-speed electrons at atoms, scientists can work out how much energy has to be supplied to form an ion by knocking out one electron from each atom. The energy change that accompanies this process is called the ionisation energy.
Ionisation energies are measured under standard conditions. The general symbol for ionisation energy is ΔHi. Its units are kJ mol–1. The symbol for the 1st ionisation energy is ΔHi1. Using calcium as an example: 1st ionisation energy: Ca(g)
Ca+(g) + e–
ΔHi1 = 590 kJ mol–1 If a second electron is removed from each ion in a mole of gaseous 1+ ions, we call it the 2nd ionisation energy, ΔHi2. Again, using calcium as an example: 2nd ionisation energy: Ca+(g)
Ca2+(g) + e–
ΔHi2 = 1150 kJ mol–1 Removal of a third electron from each ion in a mole of gaseous 2+ ions is called the 3rd ionisation energy. Again, using calcium as an example: 3rd ionisation energy: Ca2+(g)
34
Ca3+(g) + e–
ΔHi3 = 4940 kJ mol–1 We can continue to remove electrons from an atom until only the nucleus is left. We call this sequence of ionisation energies, successive ionisation energies. The successive ionisation energies for the first 11 elements in the Periodic Table are shown in Table 3.2. The data in Table 3.2 shows us that: ■
■
Figure 3.3 The frequencies of the lines in an atomic emission spectrum can be used to calculate a value for the ionisation energy.
For each element, the successive ionisation energies increase. This is because the charge on the ion gets greater as each electron is removed. As each electron is removed there is a greater attractive force between the positively charged protons in the nucleus and the remaining negatively charged electrons. Therefore more energy is needed to overcome these attractive forces. There is a big difference between some successive ionisation energies. For nitrogen this occurs between the 5th and 6th ionisation energies. For sodium the first big difference occurs between the 1st and 2nd ionisation energies. These large changes indicate that for the second of these two ionisation energies the electron being removed is from a principal quantum shell closer to the nucleus.
For example, for the 5th ionisation energy of nitrogen, the electron being removed is from the 2nd principal quantum shell. For the 6th ionisation energy of nitrogen, the electron being removed is from the 1st principal quantum shell.