Question

How many d-electrons are present in \(\mathrm{Cr}^{2+}\) ion? (a) 5 (b) 6 (c) 3 (d) 4

Short answer

The \\(Cr^{2+}\) ion has 4 d-electrons.

Step-by-step solution

Determine the Atomic Number

Chromium (Cr) is an element with the atomic number 24. This atomic number tells us that a neutral Cr atom has 24 electrons.

Identify the Electron Configuration of Chromium

The electron configuration for a neutral chromium atom (Cr) is: \\[ 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1 \] \Chromium is an exception to the typical filling order because having half-filled 3d orbitals provides extra stability.

Account for the Ionization of Electrons in \\(\\)Cr^{2+} Ion

When Cr becomes \(Cr^{2+}\), it loses 2 electrons. Electrons are removed first from the outermost shell, which is the 4s orbital. So: \\[ Cr^{2+}: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^4 \] \The \(Cr^{2+}\) ion configuration indicates that 2 electrons are removed from the 4s orbital, which was initially 1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1.

Count the Number of d-electrons

In the electron configuration of \(Cr^{2+}\), there are four electrons in the 3d subshell. Therefore, \(Cr^{2+}\) has 4 d-electrons.

Key concepts

Electron Configuration

In chemistry, "electron configuration" refers to the arrangement of electrons in an atom or ion. This distribution is particularly significant as it determines how an atom will interact with others during chemical reactions. Electrons fill orbitals in a specific order, usually following the Aufbau principle, Hund’s rule, and the Pauli exclusion principle. These principles guide electrons to occupy the lowest energy levels available before filling higher levels, share orbitals singly before pairing, and ensure no two electrons in an atom have the same set of quantum numbers, respectively.

For chromium, a neutral atom with atomic number 24, the usual order might be interrupted to achieve greater stability. Chromium’s electron configuration is interesting because it's an exception among transition metals. Instead of \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^4 4s^2\), the configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1\). This is due to the extra stability provided by a half-filled d subshell.

Chromium Ionization

Ionization refers to the process by which an atom gains or loses electrons, forming ions. For chromium, especially, ionization involves shedding specific electrons to form positively charged ions. When chromium becomes a doubly charged cation, \(Cr^{2+}\), it must lose two electrons to achieve this state.

Electrons are removed from the outermost shell or energy level. In the chromium atom, after the first ionization, it liberates the single electron in the 4s orbital due to energy consideration. Despite the 4s orbital filling before 3d under normal circumstances, electrons in 4s are typically more accessible for removal than those in 3d. After losing these electrons, the new configuration of \(Cr^{2+}\) becomes: \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^4\). This shows that two electrons have been removed from the 4s orbital.

3d Subshell

The "3d subshell" is a set of orbitals part of the d-block in the periodic table, which is typically filled after the 3p subshell and during the filling of the 4s. These d orbitals can contain a maximum of 10 electrons, as each orbital can hold 2 electrons, and there are 5 orbitals in total in a d subshell.

Chromium's unique electron configuration results in a 3d subshell filled up to just 5 electrons in its neutral state, due to its quest for greater stability. In the \(Cr^{2+}\) ion, while two electrons are removed from the outer 4s shell, the 3d configuration changes to harboring 4 electrons instead of 5. Understanding the behavior of 3d subshells is crucial for grasping the chemistry of transition metals because it heavily influences their chemical and physical properties.

Transition Metals

Transition metals are found in the middle section of the periodic table. These elements are characterized by having partially filled d or f subshells in any of their oxidation states. They're known for certain distinct properties, such as the ability to form colorful compounds, being good conductors of electricity and heat, and having high melting and boiling points.

The chemistry of transition metals is dominated by the involvement of d electrons. These electrons can engage in bonding, influence electron configurations, and impact physical properties. Chromium, as a transition metal, exhibits typical behavior due to its d electrons. The variability in the 3d subshell influences traits such as oxidation states, magnetic properties, and complex formation, making transition metals quite versatile in chemistry.