Successive Ionisation Energies
What Are Ionisation Energies?
Ionisation energy is the minimum amount of energy required to remove one mole of electrons from one mole of gaseous atoms or ions. This process can be represented by the following general equation for the first ionisation:
When we talk about successive ionisation energies, we mean the energy needed to remove the second electron, the third, and so on, from the resulting positive ion. Each removal requires more energy than the last because you are taking an electron from an increasingly positive ion, which holds its remaining electrons more tightly.
Think of it like a magnet holding onto a pile of paperclips. Removing the first paperclip is easy. But once you remove one, the magnet's hold on the remaining paperclips feels stronger, and pulling off the next one requires a bit more effort. In an atom, the "magnet" is the positively charged nucleus, and the "paperclips" are the negatively charged electrons.
The Factors Governing Ionisation Energy
Three main factors determine the magnitude of an ionisation energy:
1. Nuclear Charge (Atomic Number): This is the number of protons in the nucleus. A higher nuclear charge means a stronger positive pull on the electrons, resulting in a higher ionisation energy. For example, the ionisation energy of lithium (3 protons) is lower than that of neon (10 protons).
2. Shielding or Screening Effect: Inner shell electrons "shield" the outer shell electrons from the full attractive force of the nucleus. The more inner shells there are, the greater the shielding, and the easier it is to remove an outer electron (lower ionisation energy).
3. Distance from the Nucleus: Electrons in shells further from the nucleus are less strongly attracted to it and are easier to remove, leading to a lower ionisation energy.
These factors work together. The effective nuclear charge felt by an electron is approximately the actual nuclear charge minus the shielding effect of the inner electrons.
The Stepwise Pattern of Successive Ionisation Energies
The most revealing aspect of successive ionisation energies is not just that they increase, but how they increase. The increases are generally small and steady until there is a very large jump. This jump occurs when the electron being removed comes from a principal energy level (shell) closer to the nucleus.
For instance, consider the element sodium (Na), which has the electron configuration $ 1s^2 2s^2 2p^6 3s^1 $. Removing the first electron (the $ 3s^1 $ electron) is relatively easy. Removing the second through ninth electrons (from the n=2 shell) requires progressively more energy, but the increases are gradual. However, removing the tenth electron, which is from the n=1 shell, requires a massive amount of energy because it is much closer to the nucleus and experiences almost no shielding.
| Electron Removed | Ionisation Energy (kJ/mol) | Explanation |
|---|---|---|
| 1st | 496 | Removed from the 3s orbital, far from the nucleus. |
| 2nd | 4,562 | Now removing from a stable $ 2p^6 $ configuration (noble gas core). |
| 3rd | 6,912 | Removing another electron from the n=2 shell. |
| ... (up to 9th) | Gradual Increase | Progressively removing electrons from the n=2 shell. |
| 10th | 159,100 | Massive jump! Electron removed from the n=1 shell. |
A Practical Application: Identifying an Unknown Element
Successive ionisation energies are a powerful tool for deducing the identity and electronic structure of an element. Let's look at the data for an unknown element, which we'll call Element Q.
| Electron Removed | Ionisation Energy (kJ/mol) |
|---|---|
| 1st | 900 |
| 2nd | 1,757 |
| 3rd | 14,849 |
| 4th | 21,007 |
Analysis:
Notice the relatively small increase from the 1st to the 2nd ionisation energy. This suggests both electrons are being removed from the same principal energy level. However, there is a very large jump between the 2nd and 3rd ionisation energies. This indicates that the third electron is being removed from a shell much closer to the nucleus, which is held much more tightly.
Conclusion: Element Q must have two electrons in its outer shell. After these two are removed, we start removing electrons from a full inner shell, which requires significantly more energy. This pattern is characteristic of Group 2 elements[1] in the periodic table, such as magnesium (Mg). By comparing the exact values, we could confirm that Element Q is indeed magnesium.
Common Mistakes and Important Questions
Why does the second ionisation energy be higher than the first?
After the first electron is removed, the atom becomes a positive ion. The remaining electrons are now attracted to a nucleus with the same number of protons but fewer electrons. This means the effective pull per electron is stronger, and more energy is needed to remove the next one. It's like a team of people pulling a rope; if one person lets go, the remaining people have to pull harder to hold on.
Is the increase in energy always smooth between shells?
Not always. Even within the same principal energy level, there can be small jumps. For example, removing an electron from a $ p $-orbital might be slightly easier than from an $ s $-orbital in the same shell because $ s $-electrons are, on average, closer to the nucleus. A big jump, however, always signals the start of removal from a new, inner shell.
Can we predict the group of an element from its successive ionisation energies?
Absolutely. This is one of its most important applications. The number of electrons that can be removed with relatively low energy before a large jump tells you the number of valence electrons[2]. For instance, one low energy before a big jump suggests a Group 1 element; three low energies suggest a Group 13 element, and so on.
Conclusion
The study of successive ionisation energies provides a clear and experimental window into the electronic structure of atoms. The pattern of gradual increase followed by a large jump offers undeniable proof of the shell model of the atom. By analyzing these energy steps, we can not only confirm an element's position in the periodic table but also understand its reactivity and the reason it forms the ions it does. It is a fundamental concept that bridges the gap between the abstract atomic model and tangible chemical behavior.
Footnote
[1] Group 2 elements: The vertical column of elements in the periodic table including Beryllium (Be), Magnesium (Mg), Calcium (Ca), etc. They all have two electrons in their outer shell.
[2] Valence electrons: The electrons in the outermost principal energy level of an atom. These are the electrons involved in chemical bonding.