Æthelmar of Valence: see Aymer of Valence.

valence, combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound. Atoms are called monovalent, divalent, trivalent, or tetravalent, according to whether they form one, two, three, or four bonds (see chemical bond).

The Relationship of Electrons and Valence

For purposes of describing chemical behavior, an atom can be considered as a positively charged nucleus surrounded by negatively charged electrons orbiting in concentric spherical shells. The number of positive charges in the nucleus determines how many electrons normally surround the nucleus; as atomic number increases, the electron shells are filled, starting with those nearest the nucleus.

The valence of an atom is determined by the number of electrons in the outermost, or valence, shell. The atom exists in its most stable configuration when its outermost shell is completely filled; in combining with other atoms, it thus tends to gain or lose valence electrons in order to attain a stable configuration. If the valence shell of the atom is nearly complete, as in chlorine and other nonmetals, the atom will tend to accept electrons to complete it; if the valence shell has few electrons, as in potassium and other metals, the atom will tend to lose these electrons, so that the next shell below the valence shell becomes a completed outermost shell.

The valence of many elements is determined from their ability to combine with hydrogen or to replace it in compounds. For example, one oxygen atom combines with two hydrogen atoms to form water and the valence of oxygen is thus determined to be 2. Similarly, chlorine accepts one electron in combining with a single atom of hydrogen to form hydrogen chloride, HCl, and chlorine's valence is 1. Zinc does not combine with hydrogen but does replace it in compounds; in a typical replacement reaction, one zinc atom replaces two hydrogen atoms, as in the equation Zn+H₂SO₄→ZnSO₄+H₂, so that zinc has a valence of 2.

Valence Number

Atoms are assigned numbers, called valence numbers, oxidation numbers, or oxidation states, which range in value from -4 through 0 to +7 and describe the combining behavior of the atoms in chemical reactions, particularly oxidation-reduction reactions (see oxidation and reduction). Metals, which commonly donate electrons and form compounds in which they exist in the positive, or cationic, state, are assigned positive oxidation numbers (see cation). For a metal such as zinc, which donates two electrons to achieve a stable electron configuration, the oxidation number is +2. Nonmetals, which commonly accept electrons and in compounds exist in the negative, or anionic, state, are assigned negative oxidation numbers (see anion). The oxidation number is -1 for chlorine and the other halogens, which accept one electron to complete their valence shell.

Some elements, like the transition metals, have electron configurations in which electrons from their inner shells can also be used as valence electrons; these elements can have several different oxidation states. For example, iron can have a valence of +2 or +3, and chromium can have a valence of +2, +3, or +6. Iron in the +3 oxidation state, Fe⁺³, acts as an oxidizing agent, accepting one electron to attain the Fe⁺² state, while ferrous iron, Fe⁺², by donating an electron in going to the +3 state, acts as a reducing agent.

Valence, city (1990 pop. 65,026), capital of Drôme dept., SE France, in Dauphiné, on the Rhône River. Its many manufactures include metallurgical products, textiles, leather goods, and jewelry. It is also a processing and trade center for a fertile farm area. An old Roman town, it was taken by the Visigoths (413) and the Arabs (c.730), then changed hands many times. Although nominally under the control of various European powers, it was actually ruled by its own bishops from 1150 until the 15th cent., when its citizens put themselves under the protection of the dauphin. Its Romanesque cathedral (11th cent.; partially restored) is a tourist attraction. Its university was founded in 1452.

Pembroke, Aymer de Valence, earl of, d. 1324, English nobleman; nephew of Aymer of Valence, bishop of Winchester. He succeeded his father, William, half brother of Henry III, as earl of Pembroke in 1296. Sent by Edward I to suppress the Scottish uprising, he defeated Robert I at Methven (1306) but was himself defeated at Loudon Hill (1307). Under Edward II he was one of the lord's ordainers, appointed to limit the king's power. Disgusted by the murder (1312) of Piers Gaveston, the royal favorite, Pembroke switched his support to the king. He fought for Edward at Bannockburn (1314). By 1318 he had organized a moderate royalist group of barons, which mediated successfully between the king and the rebellious barons until displaced (1322) by Hugh le Despenser, the elder, and his son. Pembroke died suddenly on a diplomatic mission to France.

Aymer of Valence, earl of Pembroke: see Pembroke, Aymer de Valence, earl of.

Aymer of Valence, d. 1260, bishop of Winchester; son of Isabella (widow of King John of England) and Hugh X, count of La Marche. He was thus half brother of King Henry III of England. He is sometimes called Æthelmar. Henry forced the chapter of Winchester to elect Aymer bishop in 1250, but his youth and ignorance, combined with his disinclination to assume the responsibilities along with the revenues of office, delayed his consecration. He was one of the king's 12 delegates in formulating the Provisions of Oxford. However, he refused to swear to them, and hostility toward him and his brothers was an important factor in the Barons' War. Later he and his brothers had to flee the continent. Aymer was consecrated by the pope in 1260 and was on his way back to England when he died in Paris.

Number of bonds (see bonding) an atom can form. Hydrogen (H) always has valence 1, so other elements' valences equal the number of hydrogen atoms they combine with. Thus, oxygen (O) has valence 2, as in water (H₂O); nitrogen (N) has valence 3, as in ammonia (NH₃); and chlorine (Cl) has valence 1, as in hydrochloric acid (HCl). The valence depends on the number of unpaired electrons in the outermost (and, in transition elements, the next) shell of the atom's structure. The sharing of the unpaired (valence) electrons in a bond mimics the stable configuration of the noble gases, whose outer shells are full. Elements that can achieve stable configurations by various combinations have more than one valence.

Learn more about valence with a free trial on Britannica.com.