Interactions Among Atoms

Covalent and Ionic Bonds

Electrons are shared between atoms in a compound formed by a covalent bond. They are transferred from one atom to another in an ionic bond, and a van der Waals attraction is created when a weak, noncovalent bond occurs between atoms whose outer electron clouds barely touch.
Molecular compounds are formed when two or more atoms bind to one another with a covalent bond. A covalent bond is a chemical bond that forms when valence electrons are shared between atoms. A valence electron is an electron in the outermost shell of an atom, called the valence shell. Different types of covalent bonds include single bonds, in which atoms share one pair of electrons; double bonds, in which atoms share two pairs of electrons; and triple bonds, in which atoms share three pairs of electrons.

Covalent Bonds

Oxygen gas (O2) is formed when two oxygen atoms share two pairs of electrons, forming a double bond. Since the electrons are shared equally between the two oxygen atoms, the bond is covalent.
Electrons are in constant motion around the atomic nuclei, so at any given moment, there might be a slight concentration of electrons in a particular area. This concentration of electric charge is called an instantaneous dipole, which is a dipole that forms because of a short-lived imbalance in charge distribution in an otherwise nonpolar atom or molecule, because the molecule temporarily has two poles: a negative pole where the electrons are highly concentrated and a positive pole where electrons are less concentrated or dispersed. Two molecules with instantaneous dipoles are, therefore, attracted to one another and will arrange themselves so that the opposite poles are lined up. The weak, noncovalent attractive force between two distinct molecules caused by instantaneous dipoles is called a van der Waals attraction, named for the Dutch physicist who first identified it. Electron configurations also affect the shape of a molecule. Like charges repel each other, and opposite charges attract, so molecules arrange themselves to minimize the repulsive forces between electron shells and thus minimize the bond energy. That is why the OHO{\rm O{-}H{-}O} bonds in water molecules are bent with oxygen on one side and hydrogen atoms on the other and why all four CH{\rm {C{-}H}} bonds in methane (CH4) molecules form a tetrahedron, a four-sided shape, with carbon at the center and hydrogen atoms at the vertices: These molecular shapes minimize the bond energy of the molecules' covalent bonds.
A polar bond is formed when electrons are unequally shared between atoms. Water is a polar molecule, and the oxygen "side" of the molecule is slightly negative and the hydrogen "side" is slightly positive. A nonpolar covalent bond is formed when the two atoms share the electrons equally. Such a molecule has no net charge.
When two atoms are covalently bound, they sometimes do not share the electrons equally. This creates an uneven charge distribution, with regions of partial negative and partial positive charges. It is a polar covalent bond, a covalent bond in which the electron density is more localized on one end of the bond. One end is slightly positive, and one end is slightly negative; in other words, the molecule has a permanent dipole, analogous to the instantaneous dipole that arises from random electron configurations. Water (H2O) is an example of a polar molecule. A nonpolar covalent bond, in contrast, is a covalent bond in which the nuclei of the bonded atoms exert equal or nearly equal force on the shared electrons and the charge is evenly distributed over the whole bond. Methane (CH4) has nonpolar covalent bonds. Polar covalent bonds form because some atoms attract electrons more readily than other atoms do. The tendency of an atom to attract electrons is called electronegativity. Electronegativity tends to increase toward the upper right corner of the periodic table; that is, elements in Group 17 are more electronegative than elements in Group 2, and elements in Period 2 are generally more electronegative than elements in Period 6. Periods are rows of the periodic table, and groups are columns of the periodic table. Fluorine is the most electronegative element, and oxygen is the second most electronegative. Francium and cesium are the least electronegative elements. Atoms with a greater electronegativity attract electrons more strongly, creating the unequal distribution of electrons that results in a polar covalent bond.

Electronegativity of the Elements

Electronegativity is the highest in the upper right corner of the periodic table. Oxygen (Group 16, Period 2) is highly electronegative. Cesium (Cs) (Group 1, Period 6) has minimal electronegativity.
Not all compounds are formed by covalent bonds. In a covalent bond, atoms share pairs of electrons. In contrast, an ionic bond is a chemical bond that forms when valence electrons are transferred between atoms. In an ionic bond, one or more electrons are donated from one atom to another, resulting in two ions. The atom that gains the electrons results in a negatively charged ion called an anion. The atom that loses the electrons results in a positively charged ion called a cation. Anions and cations are bonded to each other through electrostatic attractive forces, forming ionic compounds. Salts, such as sodium chloride (NaCl) and potassium chloride (KCl), are ionic compounds held together by ionic bonds. Ionic bonds can form between two atoms in which one was not the electron thief. Basically, oppositely charged ions can form ionic bonds, not just when two atoms come into contact and one steals an electron from the other.

Ionic Bond Formation

Sodium and chloride ions form when an electron is transferred from sodium to chlorine. The resulting ions bind together, forming an ionic bond and the ionic compound NaCl, or table salt.