Question

Although the electron affinity of bromine is a negative quantity, it is positive for \(\mathrm{Kr}\). Use the electron configurations of the two elements to explain the difference.

Short answer

In conclusion, the difference in electron affinity between bromine and krypton is due to their electron configurations. Bromine has a partially filled 4p subshell, making it favorable to accept an additional electron and release energy, resulting in a negative electron affinity. Krypton has a completely filled 4p subshell, so adding an electron requires placing it in a higher energy level (5s subshell), necessitating energy input and leading to a positive electron affinity.

Step-by-step solution

Understand Electron Affinity

Electron affinity is the amount of energy released or absorbed when an electron is added to a neutral atom, forming a negative ion. In general, nonmetals have higher (more negative) electron affinity than metals. A negative value for electron affinity means that energy is released when an electron is added to an atom, making it more favorable. A positive value means that energy needs to be added to make the process possible, which is less favorable.

Write the Electron Configurations of Bromine and Krypton

Bromine (Br) has an atomic number of 35, and Krypton (Kr) has an atomic number of 36. We can write their electron configurations as follows: Bromine (Br): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\) Krypton (Kr): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\)

Analyze Energy Released or Absorbed in the Process

For bromine, adding an electron means placing it in the 4p subshell. Since the 4p subshell in Br is not completely filled, the electron can be added to the existing 4p orbital, and there is a release in energy, thus making its electron affinity negative. Krypton, on the other hand, has a completely filled 4p subshell. Adding an electron to Kr would mean adding it to a new 5s subshell instead of the existing 4p subshell. This process requires energy, as the electron-electron repulsions in the 5s orbital are greater than the attractive forces between the nucleus and the added electron. Therefore, the electron affinity of Kr is positive.

Conclusion

In conclusion, the difference in electron affinity between bromine and krypton can be explained by their electron configurations. Bromine has a partially filled 4p subshell, and it can accept an additional electron, thereby releasing energy. Krypton, however, has a completely filled 4p subshell, and adding an electron would require placing the electron in a higher energy level, which requires energy input, giving it a positive electron affinity.

Key concepts

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom. It is crucial for understanding the chemical properties and reactivity of elements. An element is defined by its unique electron configuration, which specifies the number of electrons and their distribution in atomic orbitals.
Br> In the context of bromine and krypton:

• Bromine has an electron configuration of \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\). This means bromine's 4p subshell is not completely filled, leaving room for one more electron.
• Krypton has a configuration of \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\), indicating that all its subshells are fully occupied.

Bromine's configuration allows it to readily gain an electron, which is why its affinity is negative. Krypton’s filled subshell configuration makes it stable, requiring additional energy to add an electron to a new energy level.

Energy Release and Absorption

Energy changes occur when an electron is added to an atom. If energy is released, the process is exothermic, and the electron affinity is negative. This happens when an added electron lowers the energy of the atom, making it more stable.
On the other hand, if energy is absorbed, the electron affinity is positive, because energy is necessary for adding the electron, potentially making the atom less stable.

• For bromine, adding an electron completes its 4p orbital, releasing energy and thus having a negative electron affinity.
• Krypton absorbs energy as it requires the electron to go to a higher energy level, the 5s orbital, which is less energetically favorable, resulting in a positive electron affinity.

Nonmetals and Metals

Nonmetals and metals differ significantly in their electron affinities. Nonmetals, like bromine, usually have more negative electron affinities because they are closer to completing their outer electron shell.
This makes them more likely to attract and gain electrons.

• Bromine, a nonmetal from the halogen group, needs just one additional electron to achieve a full outer shell, making the process energetically favorable.
• Metals and noble gases, like krypton, generally have lower or even positive electron affinities. Krypton is a noble gas with a full outer shell, making it stable and less likely to gain additional electrons.

Understanding these differences in electron affinities explains why metals and nonmetals behave differently in chemical reactions.