The most common examples are the covalent compounds of beryllium and boron. These stable compounds have less than eight electrons around an atom in the molecule. Although NO is a stable compound, it is very chemically reactive, as are most other odd-electron compounds.Įlectron-deficient molecules represent the second violation to the octet rule. The Lewis electron dot diagram for NO is as follows:Īlthough the O atom has an octet of electrons, the N atom has only seven electrons in its valence shell. Examples of stable odd-electron molecules are NO, NO 2, and ClO 2. With an odd number of electrons, at least one atom in the molecule will have to violate the octet rule. Although they are few, some stable compounds have an odd number of electrons in their valence shells. Odd-electron molecules represent the first violation to the octet rule. There are three violations to the octet rule. As with many rules, there are exceptions, or violations. This does not mean that the octet rule is useless-quite the contrary. Always make sure all valence electrons are accounted for and each atom has an octet of electrons except for hydrogen (with two electrons).Īs important and useful as the octet rule is in chemical bonding, there are some well-known violations.This will not change the number of electrons on the terminal atoms.If the central atom has fewer electrons than an octet, use lone pairs from terminal atoms to form multiple (double or triple) bonds to the central atom to achieve an octet. We will explain later that some atoms are able to accommodate more than eight electrons.Ħ.If any electrons are left over, place them on the central atom.

These electrons will usually be lone pairs.ĥ.Beginning with the terminal atoms, add enough electrons to each atom to give each atom an octet (two for hydrogen). In H 2O, for example, there is a bonding pair of electrons between oxygen and each hydrogen.Ĥ.Place a bonding pair of electrons between each pair of adjacent atoms to give a single bond. Hydrogen and the halogens are almost always connected to only one other atom, so they are usually terminal rather than central.ģ.Chemists usually list this central atom first in the chemical formula (as in CCl 4 and CO 3 2−, which both have C as the central atom), which is another clue to the compound’s structure. When there is a central atom, it is usually the least electronegative element in the compound.Arrange the atoms to show specific connections. If the species is a polyatomic ion, remember to add or subtract the number of electrons necessary to give the total charge on the ion.įor CO 3 2−, for example, we add two electrons to the total because of the −2 charge.Ģ.(Recall that the number of valence electrons is indicated by the position of the element in the periodic table.) Add together the valence electrons from each atom.Determine the total number of valence electrons in the molecule or ion. How-to: Constructing Lewis electron structuresġ.

Valence electrons periodic table