If it lost that one, the valence shell, now energy level 2, would be full (a more common way
of showing this is with zero electrons). Chlorine, having seven valence electrons, needs to
gain one more in order to complete its octet. So an electron is transferred from sodium to
chlorine, completing the octet for both.
If magnesium, with two valence electrons to be lost, reacts with chlorine (which needs
one additional electron), then magnesium will donate one valence electron to each of two
chlorine atoms, forming the ionic compound MgCl 2. Make sure the formula has the lowest
whole number ratio of elements.
If aluminum, with three valence electrons to be lost, reacts with oxygen, which needs
two additional electrons to complete its octet, then the lowest common factor between
3 and 2 must be found—6. Two aluminum atoms would each lose 3 electrons (total of
6 electrons lost) to three oxygen atoms, which would each gain two electrons (total 6 elec-
trons gained). The total number of electrons lost must equal the total number of electrons
gained.
Another way of deriving the formula of the ionic compound is the crisscross rule. In
this technique the cation and anion are written side by side. The numerical value of the
superscript charge on the cation (without the sign) becomes the subscript on the nonmetal
in the compound, and the superscript charge on the anion becomes the subscript on the
metal in the compound. Figure 11.3 illustrates the crisscross rule for the reaction between
aluminum and oxygen.If magnesium reacts with oxygen, then automatic application of the crisscross rule would
lead to the formula Mg 2 O 2 , which is incorrect because the subscripts are not in the lowest
whole-number ratio. For the same reason, lead(IV) oxide would have the formula PbO 2 and
not Pb 2 O 4. Make sure the formula has the lowest whole-number ratio of elements.
Ionic bonding may also involve polyatomic ions. The polyatomic ion(s) simply
replace(s) one or both of the monoatomic ions.Covalent Bonding
Consider two hydrogen atoms approaching each other. Both have only one electron, and
each requires an additional electron to become isoelectronic with the nearest noble gas, He.
One hydrogen atom could lose an electron; the other could gain that electron. One atom
would have achieved its noble gas arrangement; but the other, the atom that lost its elec-
tron, has moved farther away from stability. The formation of the very stable H 2 cannot be
explained by the loss and gain of electrons. In this situation, like that between any two non-
metals, electrons are shared, not lost and gained. No ions are formed. It is a covalent bond
that holds the atoms together. Covalent bondingis the sharing of one or more pairsof elec-
trons. The covalent bonds in a moleculeoften are represented by a dash, which represents
a shared pairof electrons. These covalent bonds may be single bonds, one pair of shared
electrons as in H–H; double bonds, two shared pairs of electrons H 2 C=CH 2 ; or triple
bonds, three shared pairs of electrons, NN. The same driving force forms a covalent bond
as an ionic bond—establishing a stable (lower energy) electron arrangement. In the case of
the covalent bond, it is accomplished through sharing electrons.Bonding 149Al^3+
O^2 Al 2 O 3−Figure 11.3 Using the crisscross rule.TIP