metalloids

Meaning: Metalloids sit along the diagonal staircase between the metals on the left and the nonmetals on the right. The usual list is boron, silicon, germanium, arsenic, antimony and tellurium; some texts also include polonium and astatine. They often look shiny like metals but are brittle like nonmetals, and their electronegativity and ionization energy fall between those of metals and nonmetals. Several, notably silicon and germanium, are semiconductors. Their band gap is small enough that heating moves electrons into a conducting state, so conductivity increases as temperature rises. A metal's conductivity decreases as temperature rises.

On the MCAT: Expect questions that place an element on the table and ask you to predict its behavior, such as an electronegativity between its metal neighbor and its nonmetal neighbor, or a bond to a nonmetal that is largely covalent. A passage may show a graph of conductivity against temperature and ask you to identify a conductor, an insulator or a semiconductor. Silicon in electronic devices is the standard example.

Common confusion: Aluminum borders the staircase but is generally classed as a metal. It is malleable, conducts well and forms Al³⁺. Lists of metalloids vary slightly between texts, so rely on staircase position and intermediate behavior.