Problem 26 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

The electronic structures of chromium (Cr: \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1\)) and selenium (Se: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^4\)) have some similarities in the lower energy levels and in both elements filling the 3d orbitals. However, they differ in the number of electrons in the 3d, 4s, and 4p orbitals, as well as the location of their highest energy electrons. Chromium's metallic properties stem from its few valence electrons and partially filled 3d orbital, while selenium's nonmetallic properties are due to its greater number of valence electrons and preference for gaining electrons to achieve a stable octet.

Step 1: Determine the electronic structures of atomic chromium and atomic selenium

To determine the electronic structure of an atom, we need to know its atomic number, which represents the number of protons (and also electrons in a neutral atom). Chromium has an atomic number of 24 and selenium has an atomic number of 34. To find their electron configuration, follow the aufbau principle, which states to fill the atomic orbitals in the order of increasing energy. Chromium (Cr, Z=24): \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1\) Selenium (Se, Z=34): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^4\)

Step 2: Compare the electronic structures

Now that we have the electronic configurations for both chromium and selenium, we will compare them: Similarities: - Both atoms have the same electron configuration for the first three energy levels (1s, 2s, 2p, 3s, and 3p orbitals). - Both elements fill the 3d orbitals. Differences: - Chromium has 5 electrons in the 3d orbital and 1 electron in the 4s orbital, while selenium has 10 electrons in the 3d orbital, 2 electrons in the 4s orbital, and 4 electrons in the 4p orbital. - The highest energy electrons for chromium are in the 3d and 4s orbitals, whereas for selenium, they are in the 4p orbital.

Step 3: Identify factors determining metal and nonmetal properties

The properties of metals and nonmetals are determined by their electron configurations, particularly the number of valence electrons (electrons in the outermost energy level) and their ability to gain or lose electrons to form chemical bonds. Factors determining chromium's metallic properties: - Chromium has few valence electrons (3d^5 4s^1), which makes it easy to lose them and form positive ions (cations). - The 3d orbital is not fully filled, which allows chromium to participate in metallic bonding, where electrons are delocalized and freely shared among multiple atoms. Factors determining selenium's nonmetallic properties: - Selenium has more valence electrons (4s^2 4p^4), making it more likely to gain electrons and form negative ions (anions). - The 4p orbital is only two electrons short of being fully filled, indicating that selenium prefers to gain electrons to achieve a stable octet rather than lose electrons like a metal. In conclusion, the electronic structures of chromium and selenium are mostly different, with their main similarities being in the lower energy levels and in both elements filling the 3d orbitals. The differences in the number of valence electrons and their tendency to gain or lose electrons are the main factors determining their properties as a metal (chromium) and a nonmetal (selenium).