Problem 49 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write an equation for the first electron affinity of helium. Would you predict a positive or a negative energy value for this process? Is it possible to directly measure the first electron affinity of helium?

Step-by-Step Solution

Answer

The first electron affinity of helium can be represented by the equation: He(g) + e⁻ ⟶ He⁻(g) Since helium is a noble gas with a stable electron configuration (1s^2), its energy value for the first electron affinity would be positive, indicating the process is not favorable. It is not possible to directly measure the first electron affinity of helium due to its unwillingness to gain electrons and the instability of the resulting helium anion.

Step 1: Understanding Electron Affinity

Electron affinity is the energy change associated with the addition of an electron to a gaseous atom. A more negative electron affinity value indicates a more stable state after capturing the electron. Most elements have a negative electron affinity; however, some elements, like noble gases, have a positive electron affinity, meaning they resist capturing an electron. The first electron affinity is the energy change associated with gaining one electron to form a negative ion.

Step 2: Electron Configuration of Helium

Helium is a noble gas with an electron configuration of 1s^2. It has a completely filled s-orbital, which gives it maximum stability. Thus, helium has a very low tendency to gain or lose electrons.

Step 3: Writing an Equation for the First Electron Affinity of Helium

The equation for the first electron affinity of helium is: He(g) + e⁻ ⟶ He⁻(g) This equation represents the addition of an electron to a gaseous helium atom, forming a helium anion (He⁻).

Step 4: Predicting the Energy Value for the First Electron Affinity of Helium

As helium is a noble gas with a completely filled energy level, it has no affinity for extra electrons. In the case of helium, the electron-electron repulsion in a helium anion is stronger than the attraction between the new electron and the helium nucleus. Therefore, we would predict a positive energy value for the first electron affinity of helium, indicating that the process is not favorable.

Step 5: Measuring the First Electron Affinity of Helium

It is not possible to directly measure the first electron affinity of helium because helium, being a noble gas, has a stable electron configuration and does not readily gain electrons. Any attempt to add an electron would lead to an unstable helium anion, which quickly loses the electron due to the strong electron-electron repulsion. The experimental difficulties in forming and stabilizing a helium anion make it impossible to directly measure the first electron affinity of helium.

Key Concepts

Noble Gases

Noble gases are a group of chemical elements that are known for their remarkable stability and lack of reactivity. They are found in Group 18 of the periodic table and include elements such as helium, neon, argon, krypton, xenon, and radon. One of the key features that make noble gases unique is their filled valence electron shells. This full shell configuration renders them highly resistant to gaining or losing electrons, hence the minimal reactivity.

Noble gases have the following distinctive characteristics:

• They are colorless, odorless, and tasteless in their natural state.
• Under standard conditions, they are all gases.
• They have very low melting and boiling points due to weak intermolecular forces.
• They exhibit little to no reaction with other elements under normal conditions, a trait often attributed to their electron configurations being very stable.

Helium, for instance, has a complete outer shell, making it highly stable and unreactive, similar to its noble gas counterparts.

Electron Configuration

The electron configuration of an element describes the distribution of electrons in the different orbitals around the nucleus. It is a vital concept for understanding the chemical behavior and reactivity of an element. For noble gases, the electron configuration ends in a filled or complete outer shell.

Noble gases typically have a stable electron configuration, making them unlikely to react with other elements. For example, helium, which has the simplest electron configuration of this group, is represented as 1s². This means it has two electrons occupying its first and only energy level, providing maximum stability and no inclination to accept additional electrons.

The stability of noble gases is due to the filled state of their valence shells:

• Helium: 1s²
• Neon: 1s² 2s² 2p⁶
• Argon: 1s² 2s² 2p⁶ 3s² 3p⁶
• Krypton: [Ar] 3d¹⁰ 4s² 4p⁶

This complete electron configuration means that noble gases have little tendency to engage in chemical reactions, explaining their negative or low electron affinities, with some even having positive affinities, like helium.

Helium

Helium is the second lightest element and is part of the noble gas family. It remains abundant in the universe and is often utilized in various applications due to its unique properties.

Helium displays distinct features:

• It has an atomic number of 2.
• Its symbol is He.
• Heavier than only hydrogen, helium is renowned for its low density.
• It is completely non-reactive under normal conditions, which can be attributed to its full electron shell.
• Helium is crucial for applications where the avoidance of chemical reactions is desired, such as in inert gas shields for arc welding.

Given helium's electron configuration of 1s², its valence shell is complete, comprising entirely filled orbitals. This contributes to the unusual characteristic of having a positive electron affinity. This means adding an electron to helium would result in an unstable helium anion, which is unfavored energetically, highlighting why its direct electron affinity cannot be measured experimentally.

Helium remains an interesting subject for further exploration due to these distinctive characteristics that distinguish it within the periodic table.