4.5: Transition Metal Ion Formation


How do transition metals form ions?

The transition metals are an interesting and challenging group of elements. They have perplexing patterns of electron distribution that do not always follow the electron filling rules. Predicting how they will form ions is also not always obvious.

Transition Metal Ions

Transition metals belong to the $$d$$ block, meaning that the $$d$$ sublevel of electrons is in the process of being filled with up to ten electrons. Many transition metals cannot lose enough electrons to attain a noble-gas electron configuration. In addition, the majority of transition metals are capable of adopting ions with different charges. Iron, which forms either the $$\ce{Fe^{2+}}$$ or $$\ce{Fe^{3+}}$$ ions, loses electrons as shown below.

$\begin{array}{lcl} \ce{Fe} & \rightarrow & \ce{Fe^{2+}} + 2 \ce{e^-} \\ \left[ \ce{Ar} \right] \: 3d^6 \: 4s^2 & & \left[ \ce{Ar} \right] \: 3d^6 \end{array}\nonumber$

$\begin{array}{lcl} \ce{Fe} & \rightarrow & \ce{Fe^{3+}} + 3 \ce{e^-} \\ \left[ \ce{Ar} \right] \: 3d^6 \: 4s^2 & & \left[ \ce{Ar} \right] \: 3d^5 \end{array}\nonumber$

According to the Aufbau process, the electrons fill the $$4s$$ sublevel before beginning to fill the $$3d$$ sublevel. However, the outermost $$s$$ electrons are always the first to be removed in the process of forming transition metal cations. Because most transition metals have two valence electrons, the charge of $$2+$$ is a very common one for their ions. This is the case for iron above. A half-filled $$d$$ sublevel $$\left( d^5 \right)$$ is particularly stable, which is the result of an iron atom losing a third electron.

Some transition metals that have relatively few $$d$$ electrons may attain a noble-gas electron configuration. Scandium is an example.

$\begin{array}{lcl} \ce{Sc} & \rightarrow & \ce{Sc^{3+}} + 3 \ce{e^-} \\ \left[ \ce{Ar} \right] \: 3d^1 \: 4s^2 & & \left[ \ce{Ar} \right] \end{array}\nonumber$

Others may attain configurations with a full $$d$$ sublevel, such as zinc and copper.

$\begin{array}{lcl} \ce{Zn} & \rightarrow & \ce{Zn^{2+}} + 2 \ce{e^-} \\ \left[ \ce{Ar} \right] \: 3d^{10} \: 4s^2 & & \left[ \ce{Ar} \right] \: 3d^{10} \end{array}\nonumber$

$\begin{array}{lcl} \ce{Cu} & \rightarrow & \ce{Cu^+} + \ce{e^-} \\ \left[ \ce{Ar} \right] \: 3d^{10} \: 4s^1 & & \left[ \ce{Ar} \right] \: 3d^{10} \end{array}\nonumber$

The resulting configuration above, with 18 electrons in the outermost principal energy level, is referred to as a pseudo noble-gas electron configuration. It gives particular stability to the $$\ce{Zn^{2+}}$$ and $$\ce{Cu^+}$$ ions.

Summary

• Transition metal ion formation is more complex than simple cation formation.
• Transition metal ions often involve rearrangements of both $$d$$ and $$s$$ electrons.

Review

1. What block do the transition metals fall in?
2. When writing the electron configuration for a transition metal, which sublevel ($$s,p,d,f$$) is filled first?
3. When forming transition metal ions, which sublevel ($$s,p,d,f$$) loses electrons first? What is the pseudo noble-gas electron configuration?

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