Question

Which of the following species is represented by the electron configuration \(1 s^2 2 s^2 2 p^6 3 s^2 3 p^6 4 s^1 3 d^5\) ? A. \(\mathrm{Cr}\) B. \(\mathrm{Mn}^{+}\) C. \(\mathrm{Fe}^{2+}\) D. both (A) and (B)

Short answer

(A) \( \text{Cr} \) and (B) \( \text{Mn}^{+} \)

Step-by-step solution

Identify the total number of electrons from the electron configuration

The given electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5\). Calculate the total number of electrons by adding the superscripts: \(2 + 2 + 6 + 2 + 6 + 1 + 5 = 24\).

Determine the element with 24 electrons in its neutral state

An element with 24 electrons in its neutral state is chromium (\( \text{Cr} \)), which has an atomic number of 24.

Compare the given configuration to known configurations

The electron configuration for neutral chromium is indeed \( [Ar] 3d^5 4s^1 \), which matches the given configuration. This confirms that one possibility is \( \text{Cr} \).

Check the configuration for manganese ion (\( \text{Mn}^{+} \))

Manganese in its neutral state has the electron configuration \( [Ar] 4s^2 3d^5 \) and an atomic number of 25. Upon losing one electron (\( \text{Mn}^{+} \)), it should have an electron configuration of \( [Ar] 4s^1 3d^5 \), matching the given configuration.

Confirm the electron configuration for iron ion (\( \text{Fe}^{2+} \))

Iron in its neutral state has an electron configuration of \( [Ar] 4s^2 3d^6 \) and an atomic number of 26. Losing two electrons (\( \text{Fe}^{2+} \)), it should have an electron configuration of \( [Ar] 3d^6 \), which does not match the given configuration.

Determine the correct answers

Since the given configuration matches both \( \text{Cr} \) and \( \text{Mn}^{+} \) but not \( \text{Fe}^{2+} \), the correct answers are both (A) \( \text{Cr} \) and (B) \( \text{Mn}^{+} \).

Key concepts

atomic number

The atomic number of an element represents the number of protons in its nucleus. This is a unique identifier for each element on the periodic table. For neutral atoms, the atomic number also equals the number of electrons. For example, chromium (Cr) has an atomic number of 24, meaning it has 24 protons and, in a neutral state, 24 electrons. The atomic number is crucial for understanding the element's electron configuration, which determines its chemical properties.

electron configuration

Electron configuration describes how electrons are distributed in an atom's orbitals. The arrangement follows specific rules, filling lower energy levels first. For instance, the electron configuration for chromium (Cr) can be written as \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 3d^5 \]. This shows how each electron occupies different subshells. Note that this configuration is somewhat unique due to chromium's preference for a half-filled \( 3d \) subshell, which is more stable.

ions and neutral atoms

Atoms can gain or lose electrons, becoming ions. Neutral atoms have equal numbers of protons and electrons. When a neutral atom loses or gains electrons, it becomes a positively charged ion (cation) or a negatively charged ion (anion). For example, manganese (Mn) in its neutral state has 25 electrons. However, as an ion \[ (Mn^{+}) \], it loses one electron, changing its electron configuration to \[ [Ar] 4s^1 3d^5 \], which matches the given configuration in the problem.

transition metals

Transition metals are elements found in the central block of the periodic table (Groups 3-12). They are characterized by having d orbitals that are partially filled with electrons. Transition metals, like chromium, manganese, and iron, exhibit variable oxidation states and are known for forming colorful compounds. The electron configurations of transition metals can be more complex due to the stability provided by half-filled and fully-filled d subshells. For example, neutral chromium's electron configuration is stable with a half-filled \(3d\) subshell (\(4s^1 3d^5\)).