Question

Neither ${\mathrm{Co}}^{2+}$ nor ${\mathrm{Co}}^{3+}$ has $4\mathrm{s}$ electrons in its electron configuration. How many unpaired electrons would you expect to find in each of these ions? Explain.

Short answer

${\mathrm{Co}}^{2+}$ has 1 unpaired electron and ${\mathrm{Co}}^{3+}$ has 0 unpaired electrons.

Step-by-step solution

Determine electron configuration of neutral atom

The atomic number of Cobalt is 27, meaning it has 27 electrons in its neutral state. The electron configuration of a neutral Co atom will be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^7$.

Determine electron configuration of ${\mathrm{Co}}^{2+}$ ion

The ${\mathrm{Co}}^{2+}$ ion is formed by removing two electrons from the neutral Co atom. Electrons are removed from the highest energy level down, so they are removed from the 4s orbital first, then the 3d orbital. The electron configuration for ${\mathrm{Co}}^{2+}$ will be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^7$. Thus, ${\mathrm{Co}}^{2+}$ has 1 unpaired electron in the 3d orbital.

Determine electron configuration of ${\mathrm{Co}}^{3+}$ ion

The ${\mathrm{Co}}^{3+}$ ion is formed by removing three electrons from the neutral Co atom. Again, electrons are removed from the highest energy level down. The electron configuration for ${\mathrm{Co}}^{3+}$ will be $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^6$. Here all the electrons in the 3d orbital are paired, so there are no unpaired electrons.

Key concepts

Unpaired Electrons

When discussing electron configurations, the term "unpaired electrons" refers to electrons that occupy an atomic orbital singly without a paired electron. In the case of the electron configuration, each orbital can hold two electrons with opposite spins. Unpaired electrons are those that do not have a partner in their respective orbitals. They are important because they often determine the magnetic properties of an atom or ion.

In transition metals like cobalt, we often encounter unpaired electrons in the d orbitals. For example, a neutral cobalt atom has the configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^7$. When you ionize an atom by removing electrons, this changes.

• ${\mathrm{Co}}^{2+}$ ends up with the electron configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^7$. Here, there is 1 unpaired electron in the 3d subshell, making the ion paramagnetic.
• ${\mathrm{Co}}^{3+}$ has the electron configuration: $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^6$, with all 3d electrons paired, resulting in a diamagnetic ion.

Ionic States of Cobalt

Cobalt is a transition metal known for its ability to form various ionic states, most commonly ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$. Understanding these ionic states is essential when discussing cobalt's chemical properties. Each state differs by the oxidation number, which is the charge of the ion resulting from the loss of electrons.

Transition metals like cobalt can lose different numbers of electrons, resulting in various ionic forms:

• extbf{${\mathrm{Co}}^{2+}$}: Formed by losing two electrons, usually from the outermost $4s$ orbital. This is the more stable state and often seen in many compounds.
• extbf{${\mathrm{Co}}^{3+}$}: Formed by losing three electrons, including one from the $3d$ orbital, this state requires more energy to achieve. It is not as common but is crucial in some specific chemical reactions.

These different ionic states significantly influence the coordination chemistry and reactivity of cobalt in various environments.

Electron Removal in Transition Metals

The process of electron removal, or ionization, in transition metals like cobalt follows a unique pattern compared to other elements. Typically, for transition metals, electrons are first removed from the highest principal quantum number, which often involves the $4s$ electrons being removed before the $3d$ electrons, even though the $3d$ orbital is filled after the $4s$.

Let's break it down:

• For a neutral cobalt atom, the configuration is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^7$.
• When forming ${\mathrm{Co}}^{2+}$, two $4s$ electrons are removed, leading to $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^7$.
• For ${\mathrm{Co}}^{3+}$, an additional electron is removed from $3d$, resulting in $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^6$.

This order of electron removal affects the properties of the ions, including magnetic behavior and reactivity, and is a key feature of transition metal chemistry.