Transition Elements: Definition and Key Properties
What Defines a Transition Element?
- The term "transition element" refers to elements in the d-block of the periodic table.
- These elements are defined by having an incomplete d-sublevel either in their neutral atom or in at least one of their ions.
- This distinction sets them apart from other d-block elements, such as zinc, which does not qualify as a transition element because its $d$-sublevel is completely filled in both its neutral and ionic forms.
Electron Configuration and the d-Sublevel
- Transition elements occupy groups 3–12 of the periodic table.
- Their electron configurations typically follow the general pattern:
$$(n-1)d^{1-10}ns^{0-2}$$ where $n$ represents the principal quantum number of the outermost shell.
- Iron (Fe): $[Ar] 3d^6 4s^2$
- Copper (Cu): $[Ar] 3d^{10} 4s^1$ (an exception due to enhanced stability)
- For an element to qualify as a transition element, it must have a partially filled $d$-sublevel.
- For instance, zinc ($[Ar] 3d^{10} 4s^2$) is excluded because its $d$-sublevel is completely filled.
Key Properties of Transition Elements
Transition elements exhibit a range of unique properties stemming from their partially filled $d$-sublevels.
Variable Oxidation States
- One of the defining features of transition elements is their ability to exhibit variable oxidation states.
- This is due to the relatively small energy difference between the $s$- and $d$-sublevel electrons, allowing both to participate in bonding.
- Manganese (Mn): Can exhibit oxidation states from $+2$ to $+7$, as seen in ions like Mn$^{2+}$, Mn$^{4+}$, and Mn$^{7+}$.
- Iron (Fe): Commonly forms $+2$ and $+3$ oxidation states, as in Fe$^{2+}$ and Fe$^{3+}$.
To determine the possible oxidation states of a transition element, examine its electron configuration and consider how many electrons can be removed from the $s$- and $d$-sublevels.
Formation of Colored Compounds
- Transition elements are renowned for the vibrant colors of their compounds.
- This property arises from the splitting of the $d$-orbitals in the presence of ligands (molecules or ions that coordinate to the metal ion).
- The energy gap between the split $d$-orbitals corresponds to the wavelength of visible light.
