Transition Elements

• (Vanadium): Typically forms oxides such as , , , and , indicating it does form an type oxide.
• (Chromium): Known to form , , and hence it does form an type oxide.
• (Manganese): Can form oxides such as , , , and showing it forms an type oxide.
• (Scandium): Forms as its stable oxide and does not form a simple oxide.

In chemistry, the ability of an element to exhibit variable oxidation states and form colored compounds is characteristic of transition metals. Transition metals have electrons in d orbitals, which allow them to have multiple oxidation states and form complex ions with distinct colors.

Examining the options:

• Sodium: An alkali metal with a stable oxidation state of +1. It does not form colored compounds under normal conditions.
• Calcium: An alkaline earth metal with a stable oxidation state of +2, generally forming colorless compounds.
• Copper: A transition metal that exhibits variable oxidation states, commonly +1 and +2. Copper forms colored compounds, such as copper(II) sulfate, which is blue in color.
• Magnesium: An alkaline earth metal with a stable oxidation state of +2, typically forming colorless compounds.

Therefore, the element that exhibits variable oxidation states and forms colored compounds is Copper