 in chemistry, a lone pair is a valence electron pair which is not shared with another atomiupac gold book definition: lone (electron) pair and is sometimes called a non-bonding pair. lone pairs are found in the outermost electron shell of atoms. they can be identified by using a lewis structure. electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding. thus, the number of lone pair electrons plus the number of bonding electrons equals the total number of valence electrons around an atom. lone pairs are a concept used in vsepr theory which explains the shapes of molecules. they are also referred to in the chemistry of lewis acids and bases. however not all non-bonding pairs of electrons are considered by chemists to be lone pairs. examples are the transition metals where the non-bonding pairs do not influence molecular geometry and are said to be stereochemically inactive. == examples ==  a single lone pair can be found with atoms in the nitrogen group such as nitrogen in ammonia, two lone pairs can be found with atoms in the chalcogen group such as oxygen in water and the halogens can carry three lone pairs such as in hydrogen chloride. in vsepr theory the electron pairs on the oxygen atom in water form the vertices of a tetrahedron with the lone pairs on two of the four vertices. the hâoâh bond angle is with 104.5Â°, less than the 109Â° predicted for a tetrahedral angle, and this can be explained by a repulsive interaction between the lone pairs.organic chemistry marye anne fox , james k. whitesell 2nd edition 2001organic chemistry john mcmurry 5th edition 2000concise inorganic chemistry j.d. lee 4th edition 1991 == angle changes == the pairs often exhibit a negative polar character with their high charge density and are located closer to the atomic nucleus on average compared to the bonding pair of electrons. the presence of a lone pair decreases the bond angle between the bonding pair of electrons, due to their high electric charge which causes great repulsion between the electrons. they are also used in the formation of a dative bond. for example, the creation of the hydronium (h3o+) ion occurs when acids are dissolved in water and is due to the oxygen atom donating a lone pair to the hydrogen ion. this can be seen more clearly when looked at in two more common molecules. for example, in carbon dioxide (co2), the oxygen atoms are on opposite sides of the carbon, whereas in water (h2o) there is an angle between the hydrogen atoms of 104.
Theoretical_chemistry