Question

Although it is possible to determine the second, third, and higher ionization energies of an element, the same cannot usually be done with the electron affinities of an element. Explain.

Short answer

Higher electron affinities are usually not determined because adding extra electrons to negative ions is unstable and uncommon.

Step-by-step solution

Understanding Ionization Energy

Ionization energy is the energy needed to remove an electron from an atom or ion. The first ionization energy refers to removing the first outermost electron, the second ionization energy refers to removing the second outermost electron, and so on. Since removing additional electrons becomes progressively harder as they are held more tightly by the nucleus, it is possible to measure second, third, and higher ionization energies.

Understanding Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom, forming a negative ion. The first electron affinity refers to the energy change involved in adding the first electron to the system. However, adding more electrons to an already negatively charged ion is uncommon because it typically does not produce a stable configuration.

Comparing Stability and Feasibility

When adding additional electrons beyond the first, the element already has a negative charge, repelling incoming electrons due to electrostatic repulsion. As such, higher electron affinities (second, third, etc.) typically do not exist because the added electron would not be stable as part of the ion and does not form under normal conditions.

Concluding Why Higher Electron Affinities Cannot Be Determined

Given the factors of repulsion and lack of stability in adding more electrons, the process lacks a feasible and measurable energy change. Therefore, unlike with ionization energies, higher levels of electron affinity beyond the first are impractical to determine for most elements.

Key concepts

Electron Affinity

Atom's electron affinity is the energy change when it gains an extra electron. This process turns a neutral atom into a negatively charged ion. When an atom gains an electron, energy is released, which is measured as electron affinity. Some key points about electron affinity include:

• It is generally an exothermic process (releasing energy).
• The more negative the electron affinity value, the more energetically favorable it is for the atom to gain an electron.
• First electron affinities are commonly measured because they involve adding the first extra electron to a neutral atom, which often leads to a stable electron configuration.

However, measuring electron affinities beyond the first is rare. This is because adding a second electron to an already negatively charged ion increases repulsion and energy is required for that to happen.

Stability

Stability in atoms refers to their tendency to remain in a low-energy, balanced state. Atoms achieve stability by having complete electron shells, also known as achieving noble gas configuration. For many atoms, gaining an electron can help complete their outer shell.
Once an atom becomes a negatively charged ion, adding further electrons leads to instability. This is because the natural configuration of the ion does not comfortably allow for more electrons due to

• Increased electron-electron repulsion, disrupting harmony.
• Greater energy expenditure than energy gain, which opposes stability.

In general, the added electrons beyond the first do not contribute to stability but rather increase the internal repulsion forces within the ion, leading to a less stable state.

Electron Repulsion

Electron repulsion occurs when negatively charged electrons push against each other due to their like charges. This phenomenon is significant when assessing why further electron affinities after the first are difficult to measure.
When an atom initially gains an electron and becomes negatively charged, additional electrons are repelled by the existing negative charge. This occurs because of:

• Electrostatic forces, where like charges repel each other.
• The valence electrons naturally spreading apart to minimize this repulsion.

Due to heightened repulsion, it becomes energetically unfavorable to add a second or third electron, making it impractical to measure these higher electron affinities.

Energy Change

Energy change is a vital concept when discussing electron affinity and ionization. It describes how much energy either is absorbed or released during reactions.
For a process to occur naturally, it usually needs to result in a lower energy state. In the case of electron affinity, the energy change is most commonly negative, meaning energy is released when an electron is added to a neutral atom.

• First electron affinities are typically negative, as they release energy and form stable ions.
• Higher electron affinities can result in a positive energy change, where energy needs to be supplied to add an extra electron.

This means energy input that surpasses the gain in stability is generally impractical, making additional electron affinities beyond the first rarely observed or measured. The focus is mainly on the energy balance between electron gain and the corresponding stability of the ion formed.