Final answer:
A single unpaired electron on a central atom is called a radical, which forms highly reactive species known as free radicals. These unpaired electrons make the radicals chemically reactive until they can pair up and form stable molecules.
Step-by-step explanation:
The term used to describe a single unpaired electron, also known as a 'lone-half pair', on a central atom is called a radical. When atoms have unpaired electrons, these electrons can lead to the formation of highly reactive species known as free radicals. Unlike paired electrons in covalent bonds, which are stable, the unpaired electrons in radicals are seeking to pair up, making them very chemically reactive. A free radical can be represented by a dot next to the element's symbol, such as Cl· for chlorine. It's important to understand that a single bond shares one pair of electrons between two atoms, and a double bond shares two pairs of electrons, but neither of these bond types refer to unpaired electrons. Free radicals are often involved in chemical reactions that lead to the formation of more stable molecules.