The Foundations of Chemistry

We can imagine that there is one electron in each of these hybrid orbitals on the Be

atom. Recall that each Cl atom has a half-filled 3porbital that can overlap with a half-

filled sphybrid of Be. We picture the bonding in BeCl 2 in the following diagram, in which

only the bonding electrons are represented.

Unshared pairs of electrons on Cl
atoms are not shown. The hybrid
orbitals on the central atom are shown
in green in this and subsequent
drawings.

316 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

Problem-Solving Tip:Number and Kind of Hybrid Orbitals

One additional idea about hybridization is worth special emphasis:

The number of hybrid orbitals is always equal to the number of atomic

orbitals that hybridize.

Hybrid orbitals are named by indicating the number and kindof atomic orbitals

hybridized. Hybridization of one sorbital and one porbital gives two sp hybrid orbitals.We

shall see presently that hybridization of one sand two porbitals gives three sp^2 hybrid

orbitals; hybridization of one sorbital and three porbitals gives four sp^3 hybrids, and so

on (see Table 8-2).

Thus, the Be and two Cl nuclei would lie on a straight line. This is consistent with the exper-

imental observation that the molecule is linear.

The structures of beryllium bromide, BeBr 2 , and beryllium iodide, BeI 2 , are similar to

that of BeCl 2. The chlorides, bromides, and iodides of cadmium, CdX 2 , and mercury,

HgX 2 , are also linear, covalent molecules (where XCl, Br, or I).

spHybridization occurs at the central atom whenever there are two regions of high

electron density around the central atom. AB 2 molecules and ions with no lone pairs

on the central atom have linear electronic geometry, linear molecular geometry, and

sphybridization on the central atom.

3 p two sp hybrids on Be

Cl Be Cl

3 p

TRIGONAL PLANAR ELECTRONIC GEOMETRY: AB 3

SPECIES (NO LONE PAIRS OF ELECTRONS ON A)

A. Experimental Facts and Lewis Formulas

Boron is a Group IIIA element that forms many covalent compounds by bonding to three

other atoms. Typical examples include boron trifluoride, BF 3 (mp
127°C); boron trichlo-

ride, BCl 3 (mp 
107°C); boron tribromide, BBr 3 (mp 
46°C); and boron triiodide, BI 3

(mp 50°C). All are trigonal planar nonpolar molecules.

FF

F

B B

F

F

F

B

F

F

120  F

120  120 

8-6

Hybridization usually involves orbitals
from the same main shell (same n).

The two X’s within one structure are
identical.

A trigonal planar molecule is a flat
molecule in which all three bond
angles are 120°.

The solid lines represent bonds
between B and F atoms.