150 ❯ STEP 4. Review the Knowledge You Need to Score High
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 2 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
electron, 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
nonmetals, electrons are shared, not lost and gained. No ions are formed. It is a covalent
bond that holds the atoms together. Covalent bonding is the sharing of one or more pairs
of electrons. The covalent bonds in a molecule often are represented by a dash, which rep-
resents a shared pair of 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, N≡≡N. The same driving force forms a cova-
lent 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.Al^3+
O^2 Al 2 O 3−Figure 11.3 Using the crisscross rule.TIP