Almost all of the hundred or so chemical elements form chemical compounds. In these compounds, atoms of different elements are held together in fixed proportions by some form of chemical bonding. Moreover, many of the elements exist in bonded forms. Metals, for example, are held together by their own form of bonding, in which a pool of electrons are shared by all the atoms in the sample. Most nonmetallic elements exist as molecules; chlorine (Cl2), phosphorus (P4), and sulfur (S8) are examples of this phenomenon.
Valency is the number of bonds that an atom can form with other atoms. Hydrogen has a valency of one, since an atom of hydrogen forms a single bond when it participates in molecules. Nitrogen has a valency of three, so an atom of nitrogen combines with three atoms of hydrogen in ammonia molecules (NH3). The valency of an element is not necessarily equal to the number of atoms with which each atom of that element will bond. In a molecule of nitrogen (N2), for example, each atom bonds with only one other, yet the valency of nitrogen is still three because a triple bond joins the two atoms in the molecule, so each atom participates in three bonds.
The noble-gas configuration
The noble gases of group 18 (or VIIIa) of the periodic table are extremely reluctant to react, yet the elements immediately before and after them in order of increasing atomic number are the most reactive of all elements. This discovery gave chemists a clue to a driving force of chemical reactions.
Consider fluorine, neon, and sodium—a series of elements whose atomic numbers increase in steps of one unit. Compounds of neon are unknown: it has no chemical reactivity. Fluorine, which has one less electron per atom than neon, is extremely reactive. When it reacts, it often forms compounds in which it is present as a negative fluoride ion (F–)—its atoms acquire an electron from some other element of the compound. Sodium, too, is extremely reactive, but it forms positive ions (Na+) by losing one electron. The common factor for these three elements is that their most stable forms—F–, Ne, and Na+—are isoelectronic: they all have the same number of electrons. The same effect happens around the other noble gases, so electron counts of 2 (helium), 10 (neon), 18 (argon), 36 (krypton), 54 (xenon), and 86 (radon) are unusually stable.
The valencies of the elements are largely accounted for by the number of electrons that an atom of that element must gain, lose, or share to make up a noble-gas configuration; sodium (11 electrons) has a valency of one, for example.
Electron shells and shielding
Quantum mechanics helps explain why the noble-gas configuration is so stable. Electrons arrange themselves in shells around the nucleus of an atom. In these shells, each electron experiences both an electrostatic attraction from the positively charged protons in the nucleus and a repulsion from the negative charges of the other electrons around the nucleus.
Each noble-gas configuration represents the completion of an electron shell. The first shell can hold two electrons, the second shell eight, and so on. Addition of an electron to a noble-gas configuration starts a new shell, which lies farther from the nucleus than the completed shells. Electrons in the inner shells effectively shield the outer electron from the attractive force of the nucleus.
Where an atom has a near-complete noble-gas configuration, an additional electron fits into the almost-full outer shell. The repulsive effect of the electrons alongside it in the same shell is weak. Because of the weak shielding effect, the electron experiences a relatively strong attraction to the nucleus, even if there are more electrons than protons, as is the case in a negative ion or an anion.
