1. What are elements with variable valency called?

The elements with variable valency are called transition elements or transition metals.

These elements are found in the middle of the periodic table and have properties that make them unique. They have variable valency because their valence electrons can be easily lost or gained, resulting in different oxidation states or valences. This ability to show different valencies is what sets them apart from other elements.

Transition elements have partially filled d-subshells, which allows them to form multiple stable oxidation states. Their valency depends on the number of valence electrons in the d-subshell that can participate in chemical reactions. This variability in valency gives transition metals the ability to form complex compounds and exhibit various physical and chemical properties.

Some well-known examples of transition elements with variable valency include iron (Fe), copper (Cu), chromium (Cr), and manganese (Mn). These elements play crucial roles in many biological, industrial, and technological processes due to their unique properties resulting from their variable valency.