<P> There are several types of structures for covalent substances, including individual molecules, molecular structures, macromolecular structures and giant covalent structures . Individual molecules have strong bonds that hold the atoms together, but there are negligible forces of attraction between molecules . Such covalent substances are usually gases, for example, HCl, SO, CO, and CH . In molecular structures, there are weak forces of attraction . Such covalent substances are low - boiling - temperature liquids (such as ethanol), and low - melting - temperature solids (such as iodine and solid CO). Macromolecular structures have large numbers of atoms linked by covalent bonds in chains, including synthetic polymers such as polyethylene and nylon, and biopolymers such as proteins and starch . Network covalent structures (or giant covalent structures) contain large numbers of atoms linked in sheets (such as graphite), or 3 - dimensional structures (such as diamond and quartz). These substances have high melting and boiling points, are frequently brittle, and tend to have high electrical resistivity . Elements that have high electronegativity, and the ability to form three or four electron pair bonds, often form such large macromolecular structures . </P> <P> Bonds with one or three electrons can be found in radical species, which have an odd number of electrons . The simplest example of a 1 - electron bond is found in the dihydrogen cation, H . One - electron bonds often have about half the bond energy of a 2 - electron bond, and are therefore called "half bonds". However, there are exceptions: in the case of dilithium, the bond is actually stronger for the 1 - electron Li than for the 2 - electron Li . This exception can be explained in terms of hybridization and inner - shell effects . </P> <P> The simplest example of three - electron bonding can be found in the helium dimer cation, He . It is considered a "half bond" because it consists of only one shared electron (rather than two); in molecular orbital terms, the third electron is in an anti-bonding orbital which cancels out half of the bond formed by the other two electrons . Another example of a molecule containing a 3 - electron bond, in addition to two 2 - electron bonds, is nitric oxide, NO . The oxygen molecule, O can also be regarded as having two 3 - electron bonds and one 2 - electron bond, which accounts for its paramagnetism and its formal bond order of 2 . Chlorine dioxide and its heavier analogues bromine dioxide and iodine dioxide also contain three - electron bonds . </P> <P> Molecules with odd - electron bonds are usually highly reactive . These types of bond are only stable between atoms with similar electronegativities . </P>

Where are the electrons in a covalent bond