(a) Explain the major difference between the number of oxidation states of most transition elements and that of most maingroup elements. (b) Why is the +2 oxidation state so common among transition elements? (c) What is valence- state electronegativity? Is the electronegativity of Cr different in \(\mathrm{CrO}, \mathrm{Cr}_{2} \mathrm{O}_{3},\) and \(\mathrm{CrO}_{3} ?\) Explain.

Transition elements can be found in the d-block of the periodic table. They include elements such as iron (Fe), copper (Cu), and chromium (Cr). What makes them special is their ability to exhibit multiple oxidation states. This is because both their 3d and 4s electrons can be used in bonding.
Most main group elements, on the other hand, mainly use their s and p electrons for bonding. This limits the number of oxidation states they can show.
For example, sodium (Na) typically shows a +1 oxidation state, while chlorine (Cl) shows a -1 oxidation state. But manganese (Mn), a transition element, can have oxidation states of +2, +3, +4, +6, and +7!
This variability is what makes transition metals so versatile in forming different chemical compounds. They play crucial roles in catalysis, biological systems, and materials science.

An oxidation state, also known as oxidation number, is a measure of the degree of oxidation of an atom in a substance. It indicates the number of electrons that an atom gains, loses, or shares when it forms chemical bonds.
For most transition elements, the oxidation state can vary significantly. This is in stark contrast to main group elements, which typically exhibit fewer oxidation states. The variability in the oxidation states of transition elements arises because both the 3d and 4s sub-levels of electrons can participate in bonding.
Consider iron (Fe) as an example. It can exist in oxidation states of +2 (as in FeO) and +3 (as in Fe₂O₃). This flexibility to exist in multiple oxidation states is essential in many industrial and biochemical processes.
In brief, oxidation states are essential in understanding the reactivity and bonding of elements, especially those in the transition series.

The +2 oxidation state is especially common among transition elements. But why is this the case?
When transition elements lose electrons, they often start by losing their two 4s electrons first. This is because these 4s electrons are higher in energy and less tightly held compared to the 3d electrons.
Take manganese (Mn) as an example. When it loses two 4s electrons, it forms Mn²⁺. This +2 oxidation state is stable and frequently encountered in various compounds.
Since the energy gap between the 4s and 3d orbitals is relatively small, the removal of two electrons from the 4s orbital occurs with little energy input, making the +2 state energetically favorable.
Overall, the prevalence of the +2 oxidation state in transition elements can be attributed to the ease with which they lose their 4s electrons.

Valence-state electronegativity refers to how strongly an atom in a specific oxidation state attracts electrons towards itself. This property can change depending on the oxidation state of the element.
For instance, chromium (Cr) exhibits different electronegativity values in its compounds CrO, Cr₂O₃, and CrO₃. In CrO, chromium is in the +2 oxidation state; in Cr₂O₃, it is in the +3 state; and in CrO₃, it is in the +6 state.
As the oxidation state of chromium increases, so does its valence-state electronegativity. This happens because a higher positive charge on the atom pulls the bonding electrons closer.
Understanding valence-state electronegativity is crucial as it impacts the chemical behavior and reactivity of the element in various compounds.
So, the higher the oxidation state, the more electronegative the element becomes in that particular state.