Because of this, there is more repulsion between a lone pair and a bonding pair than there is between two bonding pairs. Lone pairs are in orbitals that are shorter and rounder than the orbitals that the bonding pairs occupy. In this case, an additional factor comes into play. The electron pairs arrange themselves in a tetrahedral fashion as in methane. Because the nitrogen is only forming 3 bonds, one of the pairs must be a lone pair. Each of the 3 hydrogens is adding another electron to the nitrogen's outer level, making a total of 8 electrons in 4 pairs. Nitrogen is in group 5 and so has 5 outer electrons. How this is done will become clear in the examples which follow. You know how many bonding pairs there are because you know how many other atoms are joined to the central atom (assuming that only single bonds are formed).įor example, if you have 4 pairs of electrons but only 3 bonds, there must be 1 lone pair as well as the 3 bonding pairs.įinally, you have to use this information to work out the shape:Īrrange these electron pairs in space to minimize repulsions. Work out how many of these are bonding pairs, and how many are lone pairs. Now work out how many bonding pairs and lone pairs of electrons there are:ĭivide by 2 to find the total number of electron pairs around the central atom. For example, if the ion has a 1- charge, add one more electron. (This allows for the electrons coming from the other atoms.) Add one electron for each bond being formed. ![]() That will be the same as the Periodic Table group number, except in the case of the noble gases which form compounds, when it will be 8.
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