Question

(a) Compare the electronic structures of atomic chromium and atomic selenium. In what respects are they similar, and in what respects do they differ? (b) Chromium is a metal, and selenium is a nonmetal. What factors are important in determining this difference in properties?

Short answer

Chromium (Cr) and selenium (Se) share the same electron configuration for their inner shells (1s, 2s, 2p, 3s, and 3p). However, chromium has a 4s electron and five 3d electrons, while selenium has two 4s electrons, ten 3d electrons, and four 4p electrons. Chromium displays metallic properties due to its partially filled d-orbitals, metallic bonding, and its ability to lose electrons to form cations. Selenium is a nonmetal, as it has half-filled p-orbitals, and prefers to gain electrons and form covalent bonds, with higher ionization energies and poorer conductivity than metals.

Step-by-step solution

Determine Chromium's electron configuration

To find the electron configuration of chromium (Cr), we look up its atomic number on the periodic table, which is 24. Hence, chromium has 24 electrons. Using the aufbau principle and Hund's rule, the electron configuration for chromium is: \[ \text{Cr: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^1 \, 3d^5 \]

Determine Selenium's electron configuration

Now, we find the electron configuration of selenium (Se). Selenium's atomic number is 34, which means it has 34 electrons. Using the aufbau principle and Hund's rule, the electron configuration for selenium is: \[ \text{Se: } 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^{10} \, 4p^4 \]

Compare the electron configuration of chromium and selenium

Let's compare the electron configurations of chromium and selenium: Chromium (Cr): \( 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^1 \, 3d^5 \) Selenium (Se): \( 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^{10} \, 4p^4 \) Similarities: Both have the same electron configuration for the inner shells (1s, 2s, 2p, 3s, and 3p). Differences: Chromium has one 4s electron and five 3d electrons, while selenium has two 4s electrons, ten 3d electrons, and four 4p electrons.

Factors determining metallic/nonmetallic properties

The difference in metallic and nonmetallic properties is primarily determined by the element's position in the periodic table and the type of bonding it typically forms. Metals, such as chromium, tend to lose electrons to form positive ions (cations), have low ionization energies, and are good conductors of electricity and heat. The presence of partially filled d-orbitals and metallic bonding are important factors that contribute to its metallic properties. Nonmetals, like selenium, tend to gain electrons to form negative ions (anions) and have high ionization energies. They typically form covalent bonds and are poor conductors of electricity and heat. Selenium has half-filled p-orbitals, and its position in the periodic table (Group 16) makes it more likely to form covalent bonds, thus displaying nonmetallic properties. In conclusion, the factors responsible for chromium being a metal and selenium being a nonmetal are the differences in electron configuration, position in the periodic table, and bond formation tendencies for each element.

Key concepts

Understanding Electronic Structure

The electronic structure of an atom is the arrangement of electrons in its atomic orbitals and is critical for understanding chemical reactivity and bonding. When comparing the electronic structures of chromium and selenium, as seen in the exercise solution, we notice that both elements share the same electron configuration for their inner shells, but differ in the outer shells. Chromium's unique setup with a single electron in the 4s orbital and five in the 3d orbital makes it an exception to the general filling order predicted by the aufbau principle.

This deviation is due to energy considerations that favor a half-filled d-subshell because of its enhanced stability. On the other hand, selenium has a more predictable configuration, with the 4p orbital housing four electrons, two shy of being fully occupied. This difference in the outermost electrons is essential when predicting the chemical behavior of the two elements.

Metallic and Nonmetallic Properties

Exploring the properties of metals and nonmetals illuminates the nature of elements like chromium and selenium. Metals exhibit specific characteristics: they tend to lose electrons easily, possess low ionization energies, and show great electrical and thermal conductivity. Metals often form metallic bonds where electrons are shared in a 'sea' that allows them to move freely, contributing to conductivity and malleability.

In contrast, nonmetals generally gain electrons to form anions, have higher ionization energies, and are poor conductors, typically engaging in covalent bonding where electron pairs are shared between specific atoms. For instance, the half-filled p-orbitals in selenium lead to its ability to form stable covalent bonds, illustrating its nonmetallic nature. The differences in outer electron configurations between chromium and selenium directly relate to their respective metallic and nonmetallic characteristics.

Periodic Table Position and Element Properties

An element's position in the periodic table is a strong indicator of its properties. The table is organized to group elements with similar properties, and one's position provides clues about its electron configuration and typical bonding patterns. Metals are located to the left and center of the table, whereas nonmetals reside on the right.

Chromium, situated in the middle of the periodic table, falls within the transition metals, characterized by their d-orbitals. Transition metals commonly exhibit metallic properties such as malleability, ductility, and high electrical conductivity. On the other hand, selenium is found in Group 16, among the chalcogens—a group known for their strong tendency to form covalent bonds and for being nonmetals or metalloids. These groupings help predict behaviors such as chromium's tendency to form cations and selenium's affinity for anionic and covalent bonding, which explain their respective metallic and nonmetallic properties.