| Chemical Bonds |
Chemical Bonds
Seeking to explain how atoms in elements combine
to form molecules, American chemists
Gilbert Lewis and
Irving Langmuir developed the theory of electron valence in 1916. They
proposed that chemical bonds form between electrons residing in the outermost,
or
valence, shell of each bonding atom. When two atoms share a pair of valence
electrons, they form a chemical bond.
The Langmuir-Lewis theory provided insight into
Mendeleyev�s periodic law by stating that an element�s reactivity is largely
determined by the number of electrons in the outer shell of its atoms. Because
elements in the same group (or column) on the periodic table all have an equal
number of valence electrons, the Langmuir-Lewis theory explains why elements
within each group share similar reactivities and properties. Moving left to
right across the periodic table, element groups have increasingly filled outer
shells. For example, Group 1 elements (alkali metal elements) each contain only
a single valence electron, while Group 18 elements (noble
gases) have completely filled outer shells. As a result, the alkali metal
elements are extremely reactive, and the noble gases are extremely stable and
unreactive, or inert.
Twentieth-century scientists observed that in
order to achieve the energetic stability of the noble gases, elements seek to
fill their outer shell with electrons. To become more energetically stable,
atoms often borrow or share electrons from other atoms, forming ionic or
covalent chemical bonds.
Ionic Bonds
Atoms form ionic bonds when they gain or lose
electrons and subsequently become electrically charged. An atom that gains an
electron is known as a negative
ion, and an atom that loses an electron is known as a positive ion.
Ionic bonds form between elements having atoms
that are close to completing their valence shell and elements having atoms that
hold few electrons in their valence shell. For example, chlorine (Cl) is only
one electron short of filling its valence shell, so it has a strong affinity for
electrons. It can easily pull an electron away from sodium (Na), which only has
one loosely held valence electron. As a result of this electron exchange, two
ions form: a negative chlorine ion (Cl-), and a positive sodium ion
(Na+). These oppositely charged ions attract each other, combining in
equal proportions to form common table
salt: Na+ + Cl- ? NaCl.
Covalent Bonds
Covalent bonds form between atoms that have a
tendency to share valence electrons to complete their outer shell. Such atoms
form electrically neutral groups of atoms called
molecules. Many familiar substances are composed of molecules. Oxygen atoms
are two electrons short of filling their outer shell. Oxygen bonds with two
hydrogen atoms (each possessing a single electron) to form
water (H2O). Chlorine (Cl), which is one electron short of
filling its outer shell, shares a valence electron with another chlorine atom to
form Cl2, thereby filling the outer shells of both atoms. Nitrogen
(N), which is three electrons short of filling its outer shell, bonds with three
hydrogen atoms to form ammonia (NH3). Most bonds that occur in
compounds are actually a combination of covalent and ionic bonding. Generally,
however, bonds in which one or more electrons remain with one atom for most of
the time are called ionic, while bonds in which the electrons are equally shared
for most of the time are called covalent.
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