The Foundations of Chemistry

The difference in electronegativity between Be and Cl is so large that we might expect
ionic bonding. The radius of Be^2 is so small (0.59 Å) and its charge density(ratio of
charge to size) is so high, however, that most simple beryllium compounds are covalent
rather than ionic. The high charge density of Be^2 causes it to attract and distort the elec-
tron cloud of monatomic anions of all but the most electronegative elements. As a result,
the bonds in BeCl 2 are polar covalent rather than ionic. Two exceptions are BeF 2 and
BeO. They are ionic compounds because they contain the two most electronegative
elements bonded to Be.

C. Valence Bond Theory

Consider the ground state electron configuration of Be. There are two electrons in the
1 sorbital, but these nonvalence (inner) electrons are notinvolved in bonding. Two more
electrons are pairedin the 2sorbital. How, then, will a Be atom bond to two Cl atoms?
The Be atom must somehow make available one orbital for each bonding Cl electron (the
unpaired pelectrons). The following ground stateelectron configuration for Be is the
configuration for an isolated Be atom. Another configuration may be more stable when
the Be atom is covalently bonded. Suppose that the Be atom “promoted” one of the paired
2 selectrons to one of the 2porbitals, the next higher energy orbitals.

Then there would be two Be orbitals available for bonding. This description, however, is
still not fully consistent with experimental fact. The Be 2sand 2porbitals could not overlap
a Cl 3porbital with equal effectiveness; that is, this “promoted pure atomic” arrangement
would predict two nonequivalentBeXCl bonds. Yet we observe experimentally that the
BeXCl bonds are identicalin bond length and bond strength.
For these two orbitals on Be to become equivalent, they must hybridizeto give two
orbitals intermediate between the sand porbitals. These are called sphybrid orbitals.
Consistent with Hund’s Rule, each of these equivalent hybrid orbitals on Be would contain
one electron.

The sphybrid orbitals are described as linear orbitals,and we say that Be has linear elec-
tronic geometry.

2 p

sp

2 p

2 s

Be[He] Be[He]

hybridize

2 p 2 p

2 s 2 s

Be[He] Be [He]

promote

We say that the Be^2 ion polarizesthe

anions, Cl
.

8-5 Linear Electronic Geometry: AB 2 Species (No Lone Pairs of Electrons on A) 315

Be +

2 s 2 p two sp hybrid orbitals simplified representation

of two sp hybrids

Be Be Be

As we did for pure atomic orbitals, we

often draw hybrid orbitals more

slender than they actually are. Such

drawings are intended to remind us of

the orientations and general shapes of

orbitals.

Hund’s Rule is discussed in Section

5-17.

Cl ground state configuration:

[He] 3 s 3 p