Although a distinction had existed between metals and other mineral substances since ancient times, it was only towards the end of the 18th century that a basic classification of chemical elements as either metallic or nonmetallic substances began to emerge. It would take another nine decades before the term "nonmetal" was widely adopted. The two tables in this section list some physical and chemical properties of metals [n 38] and those of the three to four types of nonmetals, based on the most stable forms of the elements in ambient conditions. H 2O, CO, NO, N 2O are neutral oxides; CO and N 2O are "formally the anhydrides of formic and hyponitrous acid, respectively viz. CO + H 2O → H 2CO 2 (HCOOH, formic acid); N 2O + H 2O → H 2N 2O 2 (hyponitrous acid)" [318] Labinger JA 2019, "The history (and pre-history) of the discovery and chemistry of the noble gases", in Giunta CJ, Mainz VV & Girolami GS (eds.), 150 Years of the Periodic Table: A Commemorative Symposium, Springer Nature, Cham, Switzerland, ISBN 978-3-030-67910-1

Chemically, the halogen nonmetals exhibit high ionization energies, electron affinities, and electronegativity values, and are mostly relatively strong oxidizing agents. [148] These characteristics contribute to their corrosive nature. [149] All four elements tend to form primarily ionic compounds with metals, [150] in contrast to the remaining nonmetals (except for oxygen) which tend to form primarily covalent compounds with metals. [n 27] The highly reactive and strongly electronegative nature of the halogen nonmetals epitomizes nonmetallic character. [154] Cahn RW & Haasen P, Physical Metallurgy: Vol. 1 4th ed., Elsevier Science, Amsterdam, ISBN 978-0-444-89875-3 Cotton A et al. 1999, Advanced Inorganic Chemistry, 6th ed., Wiley, New York, ISBN 978-0-471-19957-1 King RB 1994, Encyclopedia of Inorganic Chemistry, vol. 3, John Wiley & Sons, New York, ISBN 978-0-471-93620-6The Goldhammer-Herzfeld ratio is roughly equal to the cube of the atomic radius divided by the molar volume. [247] More specifically, it is the ratio of the force holding an individual atom's outer electrons in place with the forces on the same electrons from interactions between the atoms in the solid or liquid element. When the interatomic forces are greater than, or equal to, the atomic force, outer electron itinerancy is indicated and metallic behaviour is predicted. Otherwise nonmetallic behaviour is anticipated.

Hare [268] observed that the line of demarcation between metals and nonmetals had been "annihilated" by the discovery of alkaline metals having a density less than that of water:Most nonmetallic elements were not identified until the 18th and 19th centuries. While a distinction between metals and other minerals had existed since antiquity, a basic classification of chemical elements as metallic or nonmetallic emerged only in the late 18th century. Since then nigh on two dozen properties have been suggested as single criteria for distinguishing nonmetals from metals. Powell P & Timms P 1974, The Chemistry of the Non-Metals, Chapman and Hall, London, ISBN 978-0-412-12200-2 Goodrich 1844, p. 264; The Chemical News 1897, p. 189; Hampel & Hawley 1976, pp. 174, 191; Lewis 1993, p. 835; Hérold 2006, pp. 149–50

Emsley J 1971, The Inorganic Chemistry of the Non-metals, Methuen Educational, London, ISBN 978-0-423-86120-4 Knight J 2002, Science of Everyday Things: Real-life chemistry, Gale Group, Detroit, ISBN 9780787656324Traversing the periodic table from right to left, three or four types of nonmetals can be discerned: The pivotal moment in the systematic classification of chemical elements, distinguishing between metallic and nonmetallic substances, came in 1789 with the groundbreaking work of Antoine Lavoisier. Lavoisier, a French chemist, published the first modern list of chemical elements in his revolutionary [231] work Traité élémentaire de chimie. In this work he categorized elements into distinct groups, including gases, metallic substances, nonmetallic substances, and earths (heat-resistant oxides). [232] Lavoisier's work gained widespread recognition and was republished in twenty-three editions across six languages within its first seventeen years, significantly advancing the understanding of chemistry in Europe and America. [233] When Davy isolated sodium and potassium their low densities challenged the conventional wisdom that metals were ponderous substances. It was thus proposed to refer to them as metalloids, meaning "resembling metals in form or appearance". [267] This suggestion was ignored; the two new elements were admitted to the metal club in cognizance of their physical properties (opacity, luster, malleability, conductivity) and "their qualities of chemical combination".

For example, Wulfsberg divides the nonmetals, including B, Si, Ge, As, Sb, Te, Xe, into very electronegative nonmetals (Pauling electronegativity over 2.8) and electronegative nonmetals (1.9 to 2.8). This results in N and O being very electronegative nonmetals, along with the halogens; and H, C, P, S and Se being electronegative nonmetals. Se is further recognized as a semiconducting metalloid. [168] Unclassified nonmetals have electrical conductivity values of from ca. 1 ×10 −18S•cm −1 for the elemental gases to 3 ×10 4 in graphite [99] Fortescue JAC 2012, Environmental Geochemistry: A Holistic Approach, Springer-Verlag, New York, ISBN 978-1-4612-6047-9 Elliot A 1929, "The absorption band spectrum of chlorine", Proceedings of the Royal Society A, vol. 123, no. 792, pp.629–644, doi: 10.1098/rspa.1929.0088Chemically, the noble gases exhibit relatively high ionization energies, negligible or negative electron affinities, and high to very high electronegativities. The number of compounds formed by noble gases is in the hundreds and continues to expand, [139] with most of these compounds involving the combination of oxygen or fluorine with either krypton, xenon, or radon. [140] Main article: Discovery of the nonmetals The Alchemist Discovering Phosphorus (1771) by Joseph Wright. The alchemist is Hennig Brand; the glow emanates from the combustion of phosphorus inside the flask. The absorbed light may be converted to heat or re-emitted in all directions so that the emission spectrum is thousands of times weaker than the incident light radiation [46] Six nonmetals are classified as noble gases: helium, neon, argon, krypton, xenon, and the radioactive radon. In conventional periodic tables they occupy the rightmost column. They are called noble gases due to their exceptionally low chemical reactivity. [115] While CO and NO are commonly referred to as being neutral, CO is a slightly acidic oxide, reacting with bases to produce formates (CO + OH − → HCOO −); [71] and in water, NO reacts with oxygen to form nitrous acid HNO 2 (4NO + O 2+ 2H 2O → 4HNO 2). [72]