page_title: Nonmetal
section_title: Cross-cutting relationships
hierarchical_section_title: Nonmetal / Properties of nonmetals (and metalloids) by group / Cross-cutting relationships
caption_attribution_description: English: A periodic table extract showing some relationships among the nonmetals
context_page_description: In chemistry, a nonmetal is a chemical element that mostly lacks the characteristics of a metal. Physically, a nonmetal tends to have a relatively low melting point, boiling point, and density. A nonmetal is typically brittle when solid and usually has poor thermal conductivity and electrical conductivity. Chemically, nonmetals tend to have relatively high ionization energy, electron affinity, and electronegativity. They gain or share electrons when they react with other elements and chemical compounds. Seventeen elements are generally classified as nonmetals: most are gases; one is a liquid; and a few are solids. Metalloids such as boron, silicon, and germanium are sometimes counted as nonmetals.
  The nonmetals are divided into two categories reflecting their relative propensity to form chemical compounds: reactive nonmetals and noble gases. The reactive nonmetals vary in their nonmetallic character. The less electronegative of them, such as carbon and sulfur, mostly have weak to moderately strong nonmetallic properties and tend to form covalent compounds with metals.
context_section_description: Some pairs of nonmetals show additional relationships, beyond those associated with group membership.
  H and C. Hydrogen in group 1, and carbon in group 14, show some out-of-group similarities. These include proximity in ionization energies, electron affinities and electronegativity values; half-filled valence shells; and correlations between the chemistry of H–H and C–H bonds.
  H and N. Both are relatively unreactive colourless diatomic gases, with comparably high ionization energies (1312.0 and 1402.3 kJ/mol), each having half-valence subshells, 1s and 2p respectively. Like the reactive azide N₃⁻ anion, inter-electron repulsions in the H⁻ hydride anion (with its single nuclear charge) make ionic hydrides highly reactive. Unusually for nonmetals, the two elements are known in cationic forms. In water the H⁺ "cation" exists as an H₁₃O₆⁺ ion, with a delocalised proton in a central OHO group. Nitrogen forms an N₅⁺ pentazenium cation; bulk quantities of the salt N₅⁺SbF₆⁻ can be prepared. Coincidentally, the NH₄⁺ ammonium cation behaves in many respects as an alkali metal anion.
  C and P. Carbon and phosphorus represent an example of a less-well known diagonal relationship, especially in organic chemistry. "Spectacular" evidence of this relationship was provided in 1987 with the synthesis of a ferrocene-like molecule in which six of the carbon atoms were replaced by phosphorus atoms. Further illustrating the theme is the "extraordinary" similarity between low coordinate phosphorus compounds and unsaturated carbon compounds, and related research into organophosphorus chemistry. In 2020, the first compound containing three carbon atoms and one phosphorus arranged in a tetrahedron, tri-tert-butyl phosphatetrahedrane, (PC₃)(C₄H₉)₃ was synthesised. While plain all-carbon tetrahedrane (CH)₄ has never been isolated, phosphorus was selected in light of its capacity to form tetrahedral molecules, and the similarity of some of its properties to those of carbon.
  C and N. With nitrogen, carbon forms an extensive series of nitride compounds including those with high N:C ratios, and with structures that are simple (CN₁₂); chain-like (C₆N₂ for example); graphitic (linked C₆N₇ units); fullerenic (C₄₈N₁₂) or polymeric (C₃N₃ units). Most of the compounds prepared to date also contain quantities of hydrogen.
  N and P. Like nitrogen, the chemistry of phosphorus is that of the covalent bond; the two nonmetals rarely form anions. Despite them being in the same group, and the composition of some of their compounds resembling one another, the individual chemistries of nitrogen and phosphorus are very different. That said, the two elements form an extensive series of phosphorus–nitrogen compounds having chain, ring and cage structures; the P–N repeat unit in these structures bears a strong resemblance to the S–N repeat unit found in the wide range of sulfur–nitrogen compounds, discussed next.
  N and S. Nitrogen and sulfur have a less-well known diagonal relationship, manifested in like charge densities and electronegativities (the latter are identical if only the p electrons are counted; see Hinze and Jaffe 1962) especially when sulfur is bonded to an electron-withdrawing group. They are able to form an extensive series of seemingly interchangeable sulfur nitrides, the most famous of which, polymeric sulfur nitride, is metallic, and a superconductor below 0.26 K. The aromatic nature of the S₃N₂²⁺ ion, in particular, serves as an "exemplar" of the similarity of electronic energies between the two nonmetals.
  N and O.  Nitrogen and oxygen represent the main parts of air. They both become toxic under pressure thus, nitrogen narcosis; oxygen narcosis. They react readily with one another. Nitrogen forms several oxides, including nitrous oxide, N₂O, in which nitrogen is in the +1 oxidation state; nitric oxide, NO, in which it is in the +2 state; and nitrogen dioxide, NO₂, in which it is in the +4 state.
  Many of the nitrogen oxides are extremely volatile; they are prime sources of pollution in the atmosphere. Nitrous oxide, also known as laughing gas, is sometim
caption: Periodic table extract showing some relationships among the nonmetals. The dashed line around H denotes that H is normally positioned on the far left of the periodic table, above Li in group 1. The red arrows denote that, as with the metalloids, the most stable forms of C, P, Se, and I each have a metallic appearance. The white arrow denotes that N, S, and Br are a gas, solid, and liquid, respectively. That leaves the triangle of O, F, and Cl representing the most corrosive nonmetals. Not shown here are At (a metalloid, predicted to be a post-transition metal), Rn (a noble gas, showing incipient metallic behavior), and Og (possibly a metalloid).
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