S1.3.7 Successive ionization energies (Higher Level Only) Notes

Using Ionization Energy Data to Determine Group Numbers and Analyze Trends

Successive Ionization Energy and Group Number

What Is Ionization Energy?

These processes can be represented as follows:

1. For the first ionization: $$\text{X(g)} + \text{energy} \rightarrow \text{X}^+ \text{(g)} + e^-$$
2. For the second ionization: $$\text{X}^+ \text{(g)} + \text{energy} \rightarrow \text{X}^{2+} \text{(g)} + e^-$$

Using Ionization Energy Data to Deduce Group Number

1. The key to identifying an element’s group lies in identifying large jumps in successive ionization energy values.
2. These jumps occur when an electron is removed from a stable, filled energy level (or noble gas configuration).
3. Let’s break this process down:
4. Identify the Large Jump: Examine the ionization energy data for significant increases. For example:
   1. $IE_1 = 500 \, \text{kJ mol}^{-1}$
   2. $IE_2 = 4560 \, \text{kJ mol}^{-1}$
   3. $IE_3 = 6910 \, \text{kJ mol}^{-1}$
5. A massive jump between $IE_1$ and $IE_2$ suggests that the first electron is being removed from a new energy level, while the second electron is removed from a stable noble gas configuration.
6. This indicates the element has one valence electron, placing it in Group 1.
7. Relate to the Periodic Table:
   1. For main group elements (Groups 1, 2, and 13–18), the number of valence electrons corresponds to the group number.
   2. For transition metals, interpreting ionization energy trends requires understanding $d$-sublevel electron configurations.

Graphical Trends in Ionization Energy

Trends Across a Period

1. As you move across a period in the periodic table (from left to right), the first ionization energy generally increases.
2. This happens because:
   1. The nuclear charge (number of protons) increases, pulling the electrons closer to the nucleus.
   2. Shielding remains relatively constant as electrons are added to the same energy level.

Trends Down a Group

As you move down a group in the periodic table, the first ionization energy decreases:

1. The outermost electrons are farther from the nucleus due to additional energy levels.
2. Increased shielding by inner electrons reduces the effective nuclear charge felt by the valence electrons.

Graphical Representation of Trends

1. When ionization energy is graphed against atomic number, the periodic nature of the elements becomes clear.
2. Peaks correspond to noble gases (stable configurations with high $IE_1$), while troughs correspond to alkali metals (low $IE_1$).

Discontinuities in Trends

Two notable exceptions occur in the general trend across a period:

1. Group 2 to Group 13 (e.g., Be to B):
   1. Beryllium $Be$: $1s^2 2s^2$
   2. Boron $B$: $1s^2 2s^2 2p^1$
   3. The 2p electron in boron is higher in energy and shielded by the 2s electrons, making it easier to remove than the 2s electron in beryllium.
2. Group 15 to Group 16 (e.g., N to O):
   1. Nitrogen $N$: $1s^2 2s^2 2p^3$ (half-filled p sublevel, stable)
   2. Oxygen $O$: $1s^2 2s^2 2p^4$ (paired electrons in the 2p sublevel experience repulsion)
   3. The increased repulsion in oxygen’s 2p sublevel makes it slightly easier to remove an electron compared to nitrogen.