A chemical bond is composed of electrons and refers to the force that holds two atoms together. For example, water, with a formula of H2O, has two chemical bonds; each bond connects one hydrogen to the central oxygen.There are three main types of chemical bonds: ionic bonds, metallic bonds and covalent bonds. Scientists classify different bonds into one of these three categories, based on several properties like the bond's polarity and the elements involved in the bond.
A basic understanding of electron chemistry is necessary to understand bonds. The most important fact about electron chemistry is that most elements are in their most stable state when they have eight electrons in their outermost electron shell. This is why different elements bond: elements that have fewer than eight electrons in their outermost shell try to bond with elements that have more than eight electrons in their outermost shell. A successful chemical bond can even out the electrons between this pair, either by having the atoms share or by allowing one atom to donate an electron to another, so that both atoms end up with eight electrons.
Scientists define electronegativity as the ability of an atom to pull electrons towards itself in a bond. Scientists express the electronegativity of an element as a number from 1 to 4. More electronegative atoms tend to "steal" electrons in a bond, whereas more electropositive atoms tend to "give" electrons. Therefore, to even out the outer shell electrons so that each atom in a chemical bond ends up with eight electrons, many chemical compounds consist of one electronegative atom and one electropositive atom. Other compounds allow two elements of equal electronegativity to share electrons equally. Scientists classify bonds as ionic or covalent partly based on the relative electronegativities of the elements involved.
Polar and Nonpolar Covalent Bonds
Scientists define a covalent bond as a bond in which the difference between the electronegativities of the two elements involved is less than 1.8. Two subcategories of covalent bonds exist: a bond with an electronegativity difference of less than 1.2 is called "nonpolar covalent," while a bond with an electronegativity difference between 1.2 and 1.8 is called "polar covalent." For example, the respective electronegativities of hydrogen and oxygen are 2.1 and 3.4; therefore, the difference between them is 1.3. Therefore, this bond is a polar covalent bond.
Types of Elements Involved
Covalent bonds usually occur between two nonmetals. On the periodic table, most nonmetals are located to the right of the diagonal line separating metals from nonmetals, though there are some exceptions. This means that most covalent bonds are between elements like hydrogen and oxygen, or between carbon and nitrogen. Bonds that contain metals like sodium or aluminum are, by definition, not covalent bonds.