Transition metal

In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings:

Variable oxidation states

Compared to Group II elements such as calcium, transition elements form ions with a wide variety of oxidation states. The transition metals show such a range of oxidation states because their partially filled d orbitals can accept or donate electrons in chemical reactions. Calcium ions typically do not lose more than two electrons, whereas transition metals can lose up to nine.

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The reason for this can be obtained by studying the ionisation enthalpies of both groups. The energies required to remove electrons from calcium are low until you try to remove electrons from below its outer two s orbitals.

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In fact Ca3+ has an ionisation enthalpy so high that it rarely occurs naturally.

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However a transition element like vanadium has roughly linear increasing ionisation enthalpies throughout its s and d orbitals, due to the close energy difference between the 3d and 4s orbitals.

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Transition metal ions are therefore commonly found in very high states.

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Certain patterns can be seen to emerge across the period of transition elements:

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• The number of oxidation states of each ion increases up to Mn, after which they start to drop. This drop is due to the stronger pull from the protons in the nucleus towards the electrons, making them harder to remove.
• When the elements are in lower oxidation states, they can be found as simple ions. However elements in higher oxidation states are usually bonded covalently to electronegative compounds such as O or F, often as a polyatomic ion such as chromate, vanadate, and permanganate ions.

Properties with respect to the stability of oxidation states:

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• Higher oxidation state ions become less stable across the period.
• Ions in higher oxidation states tend to make good oxidising agents, whereas elements in low oxidation states become reducing agents.
• The 2+ ions across the period start as strong reducing agents, and become more stable.
• The 3+ ions start stable and become more oxidizing across the period.