Question

The first ionization energies of As and Se are \(0.947\) and \(0.941\) \(\mathrm{MJ} / \mathrm{mol}\), respectively. Rationalize these values in terms of electron configurations.

Short answer

The first ionization energies of As and Se can be rationalized based on their electron configurations. Both elements have the same number of electron shells, but As has one less electron in the 4p orbital, leading to slightly less electron-electron repulsion. Additionally, Se has a more stable 4p^4 orbital, making it harder to remove an electron compared to As's 4p^3 orbital. Thus, the first ionization energy of As is slightly lower than that of Se (0.947 MJ/mol for As and 0.941 MJ/mol for Se).

Step-by-step solution

1. Electronic Configuration of As and Se

First, we need to write the electronic configuration of As and Se. The atomic number of As (Arsenic) is 33, which means it has 33 electrons. Its electron configuration is: \[1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2, 3d^{10}, 4p^3\] The atomic number of Se (Selenium) is 34, which means it has 34 electrons. Its electron configuration is: \[1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 4s^2, 3d^{10}, 4p^4\]

2. Analyzing Electron Configurations

Next, we will analyze the electron configurations of As and Se and identify differences that might contribute to their ionization energies. - The final electron in As is in a 4p^3 orbital, while the final electron in Se is in a 4p^4 orbital. - Both elements are in the same period (4), which means they have the same number of electron shells. - As has one less electron in the 4p orbital compared to Se.

3. Rationalizing Ionization Energies based on Electron Configuration

The difference in ionization energies of As and Se can be explained in terms of their electron configurations: 1. Both As and Se have the same number of electron shells, so the difference in ionization energies is not due to electron shielding or distance from the nucleus. 2. In As, the last electron is in a 4p^3 orbital, while in Se it is in a 4p^4 orbital. Since 4p^4 is more stable than 4p^3, it is harder to remove an electron from the 4p^4 orbital. 3. As has one less electron in the 4p orbital, which means that As experiences slightly less electron-electron repulsion than Se. This makes it slightly easier to remove an electron from As compared to Se. Based on the above analysis, we can rationalize that the first ionization energy of As is slightly lower than that of Se (0.947 MJ/mol for As and 0.941 MJ/mol for Se) due to the differences in their electron configurations.

Key concepts

Electron Configuration

Understanding electron configuration is crucial when studying ionization energy. The electron configuration of an element is a description of the distribution of its electrons in atomic orbitals. The configuration follows a specific order based on the increasing energy levels of the orbitals: starting from the 1s orbital and moving to higher energy orbitals like 2s, 2p, and so forth. Each orbital can hold a set number of electrons, for example, an s orbital can hold up to 2 electrons, while a p orbital can hold up to 6.

For arsenic (As) and selenium (Se), the outermost electrons are in the 4p orbital, which can influence their ionization energies. As has a 4p^3 configuration, with three electrons occupying that orbital. Conversely, Se has a 4p^4 configuration, with an additional electron occupying that orbital. As has its electrons less tightly held due to electron-electron repulsion with fewer electrons in the same subshell, which in part explains its slightly lower ionization energy compared to Se.

Atomic Number

The atomic number is a fundamental property of an element, indicating the number of protons in an element's nucleus, and by extension, the number of electrons in a neutral atom. As the atomic number increases, the number of protons and electrons in an element also increases, generally leading to a greater attraction between the positively charged nucleus and negatively charged electrons. This means that elements with higher atomic numbers can have higher ionization energies because their electrons are more strongly attracted to the nucleus.

In our specific example, the atomic number of As is 33, while Se has an atomic number of 34. The additional proton corresponds to an additional electron in Se, which results in a stronger nuclear charge attracting electrons more strongly. This difference provides another layer of explanation for the slightly higher ionization energy of Se compared to As.

Electron-Electron Repulsion

Electron-electron repulsion plays a significant role in the determination of ionization energy. When electrons are added to the same subshell, they start to repel each other due to their like-charges, this repulsion force can make it easier to remove an electron as it counteracts the attraction to the nucleus. The magnitude of repulsion increases with the number of electrons present in a subshell.

With arsenic's 4p^3 configuration, there is less electron-electron repulsion compared with selenium's 4p^4 configuration where four electrons are present. As a result, the outermost electron in As experiences less repulsion, and is easier to remove, than the outermost electron in Se, contributing to the lower ionization energy in As.

Periodic Trends

Periodic trends allow scientists to predict properties of elements, including ionization energy, which is the energy needed to remove the most loosely held electron from an atom. As you move across a period (a row in the periodic table), ionization energies generally increase. This is because electrons are being added to the same principal energy level while the nucleus gains more protons, thus strengthening the nuclear attraction on the electrons.

However, within the same period, small variations can occur due to subshell configurations, as we see between As and Se. Both are in the same period, so the general trend would predict a slightly higher ionization energy for Se as it is to the right of As. This trend stands true in the context of our discussion, where Se, having more electrons and hence a more stable electron configuration in the 4p subshell, has a slightly higher ionization energy.