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Chemical compounds are formed by the joining of two or more atoms. A stable compound occurs when the total energy of the combination has lower energy than the separated atoms. The bound state implies a net attractive force between the atoms … a chemical bond. The two extreme cases of chemical bonds are:

Covalent bond: bond in which one or more pairs of electrons are shared by two atoms.

Ionic bond: bond in which one or more electrons from one atom are removed and attached to another atom, resulting in positive and negative ions which attract each other.

Other types of bonds includemetallic bondsandhydrogen bonding. The attractive forces between molecules in a liquid can be characterized asvan der Waals bonds.

Covalent chemicalbondsinvolve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons inionicbonds. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.

Hydrogen gas forms the simplest covalent bond in the diatomichydrogen molecule. The halogens such as chlorine also exist as diatomic gases by forming covalent bonds. The nitrogen and oxygen which makes up the bulk of the atmosphere also exhibits covalent bonding in forming diatomic molecules.

Covalent bondsin which the sharing of the electron pair is unequal, with the electrons spending more time around the more nonmetallic atom, are called polar covalent bonds. In such a bond there is a charge separation with one atom being slightly more positive and the other more negative, i.e., the bond will produce adipole moment. The ability of an atom to attract electrons in the presense of another atom is a measurable property calledelectronegativity.

In chemicalbonds, atoms can either transfer or share their valence electrons. In the extreme case where one or more atoms lose electrons and other atoms gain them in order to produce a noble gas electron configuration, the bond is called an ionic bond.

Typical of ionic bonds are those in the alkali halides such assodium chloride, NaCl.

Ionic bonding can be visualized with the aid ofLewis diagrams.

Comparison of ionic and covalent materials.

Energy contributions to ionic bonds

The properties ofmetalssuggest that their atoms possess strongbonds, yet the ease of conduction of heat and electricity suggest that electrons can move freely in all directions in a metal. The general observations give rise to a picture of positive ions in a sea of electrons to describe metallic bonding. Such bonds are neitherionicnorcovalentsince the participating electrons are not localized on the atoms.

The general properties of metals include malleability and ductility and most are strong and durable. They are good conductors of heat and electricity. Their strength indicates that the atoms are difficult to separate, but malleability and ductility suggest that the atoms are relatively easy to move in various directions. The electrical conductivity suggests that it is easy to move electrons in any direction in these materials. The thermal conductivity also involves the motion of electrons. All of these properties suggest the nature of themetallic bondsbetween atoms.

Hydrogen bonding differs from other uses of the wordbondsince it is a force of attraction between a hydrogen atom in one molecule and a small atom of highelectronegativityin another molecule. That is, it is an intermolecular force, not an intramolecular force as in the common use of the word bond. As such, it is classified as a form ofvan der Waalsbonding, distinct fromionicorcovalent bonding.

When hydrogen atoms are joined in apolar covalent bondwith a small atom of high electronegativity such as O, F or N, the partial positive charge on the hydrogen is highly concentrated because of its small size. If the hydrogen is close to another oxygen, fluorine or nitrogen in another molecule, then there is a force of attraction termed a dipole-dipole interaction. This attraction or hydrogen bond can have about 5% to 10% of the strength of a covalent bond.

Hydrogen bonding has a very important effect on the properties ofwaterand ice. Hydrogen bonding is also very important in proteins and nucleic acids and therefore in life processes. The unzipping of DNA is a breaking of hydrogen bonds which help hold the two strands of the double helix together.

with the electrons spending more timaround mornonmetallic atom插图