Question

In the second row of the periodic table, \(\mathrm{Be}, \mathrm{N}\), and \(\mathrm{Ne}\) all have endothermic (unfavorable) electron affinities, whereas the other second-row elements have exothermic (favorable) electron affinities. Rationalize why Be, \(\mathrm{N}\), and Ne have unfavorable electron affinities.

Short answer

Be, N, and Ne have endothermic electron affinities because adding an electron would disrupt their stable electronic configurations: Be has a completely filled 2s subshell, N has a half-filled 2p subshell, and Ne has a completely filled 2p subshell. Energy is needed to accommodate the extra electron, making their electron affinities unfavorable. In contrast, other second-row elements have exothermic electron affinities, as their electronic configurations become more stable when gaining an electron.

Step-by-step solution

Understanding Electron Affinity

Electron affinity is the energy change associated with adding an electron to a gaseous atom. The more negative the value, the more exothermic the process, which indicates that the atom has a favorable tendency to gain an electron. Conversely, elements with endothermic electron affinities have a positive value, meaning that energy must be provided for the atom to accept an electron, making the process unfavorable.

Analyzing Electronic Configurations

To understand the reason behind the endothermic electron affinity of Be, N, and Ne, we should analyze their electronic configurations. Be: \[1s^2 \ 2s^2\] N: \[1s^2 \ 2s^2 \ 2p^3\] Ne: \[1s^2 \ 2s^2 \ 2p^6\]

Relating Configurations to Electron Affinities

Let's now relate the electronic configurations to the electron affinity of these elements: 1. Be: Beryllium has a completely filled 2s subshell. When an electron is added to the beryllium atom, it will go to the 2p subshell. However, adding an electron to the 2p subshell would break the stability of the completely filled 2s subshell, so the electron affinity for Be is endothermic. 2. N: Nitrogen has exactly half-filled 2p subshell, which is considered to be a stable configuration. Adding an electron to nitrogen would disrupt this stable half-filled configuration, resulting in an endothermic electron affinity. 3. Ne: Neon has a completely filled 2p subshell, and the next available subshell for an additional electron would be the 3s subshell. However, this requires more energy than is released in the process, so the electron affinity for Ne is endothermic. In summary, Be, N, and Ne have endothermic electron affinities because the addition of an electron would disrupt their stable electronic configurations, requiring energy to accommodate the extra electron. Other elements in the second row have exothermic electron affinities because their electronic configurations become more stable when gaining an electron.

Key concepts

Periodic Table

The periodic table is an organized chart of chemical elements categorized by increasing atomic number. It helps in understanding the chemical behavior of elements. Each element's position reveals vital information:

• Rows are called periods, while columns are known as groups.
• Elements in the same group share similar properties.
• The arrangement highlights trends in electron affinity and other properties.

In the second row, elements like Beryllium (Be), Nitrogen (N), and Neon (Ne) exhibit unique characteristics that affect their electron affinity.

Electronic Configuration

Electronic configuration is the distribution of electrons in an atom’s orbitals. It is crucial for predicting element behavior:

• Electrons fill orbitals from lower to higher energy levels, following the Aufbau principle.
• Understanding configurations helps in identifying stability and reactivity.

For example:

• Be: \[1s^2 \ 2s^2\] shows a stable filled 2s subshell.
• N: \[1s^2 \ 2s^2 \ 2p^3\] has a half-filled stable 2p subshell.
• Ne: \[1s^2 \ 2s^2 \ 2p^6\] features a fully filled 2p subshell.

These configurations explain why adding an electron can be energetically unfavorable.

Endothermic Process

An endothermic process absorbs energy from its surroundings. When discussing electron affinity, it refers to the need for additional energy to add an electron:

• If the electron affinity is endothermic, the process is unfavorable as energy input is needed.
• Be, N, and Ne exhibit endothermic electron affinities due to the disruption of stable configurations.

In contrast, most other elements in the second period have exothermic affinities, releasing energy when they gain electrons, making this process favorable.

Chemical Stability

Chemical stability refers to an atom’s resistance to changing its current electron configuration. It's a key factor in determining electron affinity:

• Be's filled 2s subshell, N’s half-filled 2p subshell, and Ne's filled 2p subshell represent stable configurations.
• Gaining an electron disturbs this stability and requires extra energy, leading to endothermic affinities.

In summary, stability influences how easily an atom gains or loses electrons, explaining why certain elements resist changes in electronic configuration.