Nonmetals

FIELDS OF STUDY: Organic Chemistry; Inorganic Chemistry; Geochemistry; Metallurgy

ABSTRACT

The basic properties of the nonmetals and their chemical behavior are described. The nonmetal elements make up all biochemicals and can form a near-infinite variety of compounds due to their properties of electronic structure and atomic size.

The Nature of the Nonmetals

Seventeen elements in the periodic table are generally considered to be nonmetals. These include four of the five halogens (astatine is more often classified as a metalloid) as well as the six noble gases. The remaining seven elements—hydrogen (H), carbon (C), nitrogen (N), oxygen (O), phosphorus (P), sulfur (S), and selenium (Se)—make up what is known as the nonmetals group. The term "nonmetals" can refer either to these seven elements specifically or to all seventeen nonmetallic and non-metalloid elements.

Nonmetals are elements that do not have metallic character and do not exhibit the behavior of metals. They are not malleable or ductile, are poor conductors of heat and electricity, and are characterized by their high electronegativity. The noble gases (group 18)—helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn)—are noted for their almost complete lack of chemical activity and exist as monatomic gases. Xenon and radon are known to form compounds with the extremely electronegative element fluorine. The halogens (group 17)—fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At)—are noted for their very high reactivity. The halogens exist in elemental form as diatomic molecules, either as a gas (fluorine and chlorine), a liquid (bromine), or a solid (iodine and possibly astatine). The halogen group is the only group on the periodic table to contain elements that exist in all three states of matter at standard temperature and pressure.

Seven nonmetals exist as diatomic molecules in their standard state: hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. Sulfur, in the same group as oxygen (group 16), exists in elemental form as a bright yellow crystalline solid consisting of a polyatomic molecule with the formula S₈. Elemental sulfur can be found in a number of different allotropes, depending on the manner in which the S₈ molecules form the crystal lattice of the solid. Most nonmetal elements, except the noble gases, normally form anions by accepting electrons from other atoms that are electropositive in character. Other nonmetal elements—particularly carbon, nitrogen, and phosphorus—typically do not form anions but are most likely to participate in covalent bonds due to the prohibitive energy levels associated with either gaining or losing enough electrons to form a complete octet of electrons in the valence shell.

CHNOPS

Life on earth depends on the characteristic chemical behavior of the nonmetals, particularly carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS). Carbon has a total of four electrons in its outermost electron shell, or valence shell—that is, half of a complete octet. Because of this, carbon molecules typically share their four valence electrons through covalent bonds, thereby forming an octet electron configuration, rather than losing or gaining molecules to form ionic bonds. Carbon’s valence electrons are distributed as 2s²2p², meaning that there are two electrons in the 2s orbital and one each in two of the three 2p orbitals. The s and p orbitals, belonging to the same electron shell, are sufficiently close in energy that they are able to combine to form four equivalent hybrid sp³ orbitals. Furthermore, carbon’s intermediate electronegativity makes a carbon atom more likely to share electrons than to gain or lose them. This unique combination of characteristics enables carbon atoms to form strong, stable covalent bonds with other carbon atoms, facilitating the formation of molecules that contain long chains or rings of carbon. Carbon is also capable of forming strong bonds with other nonmetals, such as hydrogen, oxygen, sulfur, and the halogens. These large, complex carbon-based molecules are the structural basis of the many compounds that make up living cells.

Nitrogen and oxygen have similar electronic properties to carbon, though with one and two more electrons, respectively. These extra valence electrons increase the electronegativity of nitrogen and oxygen relative to carbon, but they do not significantly affect the size of the atoms. Both nitrogen and oxygen, like carbon, have their valence electrons in the 2s orbital and the three 2p orbitals, and they can easily form covalent bonds with carbon atoms. The elements phosphorus and sulfur are also necessary to the molecules of biochemistry, though they have their valence electrons in the 3s and 3p orbitals. These five elements, along with hydrogen, form the basic chemical components of life itself and are the constituents of essentially all biochemical compounds. Minor quantities of metals (such as iron, sodium, and magnesium) and other nonmetal elements (such as iodine and chlorine) are significant cofactors in biochemical processes, but the molecular structures with which they interact consist primarily of the CHNOPS elements.

Nonmetal Compounds

The nonmetals, especially carbon, can form a nearly infinite variety of chemical compounds. Carbon atoms can catenate without limit, often bonding with hydrogen to form a series of linear saturated hydrocarbons, or alkanes. Ring structures, double bonds, and triple bonds between carbon atoms produce an even greater variety of corresponding organic compounds, including alkenes (which contain at least two carbon atoms connected by a double bond) and alkynes (two carbon atoms connected by a triple bond). The presence of oxygen, nitrogen, and sulfur significantly increases the number of possible organic compounds. Carbon’s unusual versatility in forming bonds enables the formation of isomeric compounds, which are compounds that share the same molecular formula but have different molecular structures due to the various possible combinations of the molecule’s substituent groups. Though several million organic compounds of carbon, hydrogen, oxygen, nitrogen, and sulfur are known, many millions more have yet to be identified and studied. Oxygen and sulfur also commonly form oxides and sulfides with most other elements.

The other nonmetal elements have markedly different chemical behaviors. The noble gases are also called inert gases because they typically do not form compounds under standard conditions. The exceptions are xenon and radon, which form compounds when combined with elemental fluorine (F₂). The extremely high electronegativity and charge density of the fluorine atom is able to induce sufficient polarity within the xenon atom to create a bond. The halogens, as mentioned, are highly electronegative and readily form halide anions with a charge of 1−. Halogen elements are also able to form covalent bonds with carbon atoms to create halocarbon compounds that are commonly found in nature. Organic compounds isolated from marine organisms often include a chlorine or other halogen atom in the place where a hydrogen atom would normally be found in a similar compound from a terrestrial organism.

Occurrence of Nonmetals

Although there are over five times more metal than nonmetal elements in the periodic table, nonmetal compounds are more readily found in nature, making up nearly 99 percent of the earth’s atmosphere (particularly N₂, O₂, and CO₂) and more than 97 percent of a human body (especially oxygen, carbon, hydrogen, nitrogen, and phosphorus) by mass. The noble gases are found in the gaseous state in nature. A large quantity of each noble-gas element is also believed to exist in clathrate compounds, which are compounds that trap atoms or molecules within a crystal structure. All halides, with the exception of many fluoride compounds, exist as either ionic solids or ions in solution. Seawater, for example, contains sodium and chloride ions that form sodium chloride (NaCl).

Oxygen is found in huge quantities as a component of various minerals, especially silicates. Oxygen also forms carbon dioxide (CO₂) in the atmosphere. Carbon dioxide can react with water (H₂O) to form carbonic acid (H₂CO₃), which can then be trapped as the carbonate salts of various metallic elements, such as calcium carbonate (CaCO₃). Plants also trap oxygen by converting carbon dioxide into glucose (C₆H₁₂O₆) in photosynthesis. In respiration, atmospheric oxygen (O₂) is used to convert the glucose back into carbon dioxide. Oxygen is the most abundant element by mass in the earth’s crust and oceans and, after nitrogen, the second-most abundant element in its atmosphere.

PRINCIPAL TERMS

• allotrope: one of two or more principal physical forms in which a single pure element occurs, due to differences in chemical bonding or the structural arrangement of the atoms; for example, diamond and graphite are two allotropes of carbon.
• anion: any chemical species bearing a net negative electrical charge, which causes it to be drawn toward the positive pole, or anode, of an electrochemical cell.
• diatomic: describes a molecule or ion consisting of two atoms that are chemically bonded to each other.
• monatomic: describes a molecule or ion consisting of only one atom.
• polyatomic: describes a molecule or ion consisting of multiple atoms that are chemically bonded to each other.

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