Metals, Nonmetals and Metalloids

Metals occupy most of the left and center of the periodic table. Nonmetals lie mainly toward the upper right, with hydrogen as an important nonmetal at the upper left. The staircase boundary is a guide to metalloids: B, Si, Ge, As, Sb and Te are the usual introductory set. Borderline classifications such as Po and At vary between sources, so use the convention supplied rather than treating every square touching a printed line as definitive.

Class and physical state are different descriptions. Sulfur and iodine are solid nonmetals, bromine is a liquid nonmetal, and mercury is a liquid metal near room temperature. The term metalloid refers to a set of intermediate chemical and physical behaviors, not to an exact numerical electronegativity interval.

Metals have low ionization energies and low electronegativities, so their valence electrons delocalise. That gives shiny surfaces, good electrical and thermal conductivity, malleability and ductility. Metals form cations.

Conduction requires mobile charge carriers. In a metal, valence electrons occupy states extending through the solid and can respond to an applied electric field. The positive atomic cores remain at lattice sites while electrons carry current. Metallic bonding is not confined to a single direction between a fixed pair of atoms, so layers can shift while cohesion remains; this helps explain malleability, meaning shaping into sheets, and ductility, meaning drawing into wires. These properties are tendencies, and alloy composition or crystal defects can change hardness and brittleness.

Nonmetals have high ionization energies and high electronegativities. As solids they are dull and brittle and do not conduct. They form anions with metals and share electrons with other nonmetals.

A nonmetal atom often gains electrons when combined with an electropositive metal, but two nonmetals typically share electrons. This predicts ionic KBr and covalent HCl as first approximations. It does not mean every nonmetal solid consists of molecules: diamond and silicon dioxide contain extended covalent networks, and graphite conducts despite carbon being a nonmetal. Classification of an element and classification of a solid's structure are related but distinct tasks.

Metalloids have properties between the two classes. Silicon and germanium are semiconductors: they conduct much better than nonmetals but far worse than metals, and their conductivity rises with temperature and with doping.

In silicon and germanium, a band gap separates occupied electronic states from nearby empty conducting states. Heating or light can create mobile electrons and holes; controlled doping changes their populations. That is different from the abundant mobile electrons already available in a typical metal. You do not need band calculations to classify the sample, but the carrier model explains why semiconductivity can vary strongly with conditions.

Periodic-table position is a useful first classification, but a material's behavior supplies independent evidence. Malleability and good electrical conduction support metallic bonding; brittleness and poor conduction often support nonmetallic or network structures. Metalloids can show intermediate behavior, especially semiconductivity. A single observation should not be treated as proof of an element's category or a sample's purity.

When predicting a compound, give the chain of reasoning: identify the element classes, propose the likely bonding, then test the proposal against properties. A metal/nonmetal pair often produces ions; two nonmetals often produce molecular or network covalent structures; a mixture of metals often forms an alloy. These are starting predictions. Some metal compounds have substantial covalent character, so a measured melting point or conductivity pattern can refine the classification.