To predict the electron confi guration of a monatomic cation, remove the
outermost electrons in the order np, ns, and (n 1)d; for a monatomic anion,
add electrons until the next noble-gas confi guration has been reached. The
transfer of electrons results in the formation of an octet (or duplet) of electrons
in the valence shell on each of the atoms: metal atoms achieve an octet (or
duplet) by electron loss and nonmetal atoms achieve it by electron gain.
3.2 Lewis Symbols
Many of our basic ideas about the chemical bond were proposed by G. N. Lewis
in the early years of the twentieth century. Lewis devised a simple way to keep
track of valence electrons when atoms form ionic bonds. He represented each
valence electron as a dot and arranged the dots around the symbol of the element.
A single dot represents an electron alone in an orbital; a pair of dots represents
two paired electrons sharing an orbital. Examples of the Lewis symbols
of atoms are
Hz He å åN O Cl K Mg z z z
Å
Å
z z
Å
Å
a z z a
The Lewis symbol for nitrogen, for example, represents the valence electron configuration
2s22px
12py
12pz
1 (see 1), with two electrons paired in a 2s-orbital and
three unpaired electrons in different 2p-orbitals. The Lewis symbol is a visual summary
of the valence-shell electron configuration of an atom and allows us to see
what happens to the electrons when an ion forms.
To work out the formula of an ionic compound by using Lewis symbols:
• Represent the cation by removing the appropriate number of dots from the
symbol for the metal atom.
• Represent the anion by transferring those dots to the Lewis symbol for the
nonmetal atom to complete its valence shell.
• If necessary, adjust the numbers of atoms of each kind so that all the dots
removed from the metal atom symbols are accommodated by the nonmetal
atom symbols.
• Write the charge of each ion as a superscript in the normal way.
A simple example is the formula of calcium chloride. The calcium atom loses its
two valence electrons when it forms the Ca2 ion. Because each chlorine atom has
one vacancy, two are required to accept the two electrons from the calcium atom:
Cl Ca Ca2
: :
: · : Cl
: :
: · Cl
: :
: : Cl