Video Transcript
In the gas phase, beryllium
chloride exists as single molecules of BeCl2. However, as a solid, beryllium
chloride forms an extended linear structure. The Lewis structure of BeCl2 and
the structure of the solid are shown below. How many coordinate covalent bonds
does each atom of beryllium have in the solid state?
In this question, we are given
information about the bonding in the compound beryllium chloride. We want to determine the number of
coordinate covalent bonds formed in the structure of solid beryllium chloride for
every beryllium atom.
A conventional covalent bond is a
chemical bond that forms when two nonmetal atoms share one or more pairs of
electrons and at least one electron comes from each bonding group. It is worth mentioning that
although beryllium is a metal, it can engage in covalent bonding due to its atypical
high electronegativity.
A coordinate covalent bond is a
covalent bond whose bonding pair of electrons comes from one bonding group only. The figure shows how a coordinate
covalent bond forms, as atom A donates both of the electrons in the shared pair to
form a bond with atom B.
Beryllium chloride forms an
extended linear structure in the solid state. We want to determine the number of
coordinate covalent bonds present for each beryllium atom. Let’s use dot-and-cross diagrams to
analyze the bonding in this structure. The dot-and-cross diagram
represents the bonding structure of a simple gas-phase beryllium chloride
molecule.
The electrons of the beryllium atom
are crosses, and the electrons of the chlorine atoms are dots. We can see that the molecule has
two covalent bonds but no coordinate covalent bonds, as each shared pair of
electrons has one from each bonding atom. As the compound transitions from
the gas phase to the solid phase, many beryllium chloride molecules combine to form
the extended linear structure.
The second figure shows the
dot-and-cross diagram for the extended linear structure of beryllium chloride. We can see that each chlorine atom
donates an electron pair to a beryllium atom. A chain forms as this electron
donation process happens many times. This process eventually makes a
long linear structure that has the formula BeCl2n. The n represents the number of
BeCl2 units in the chain.
Each beryllium atom ends up having
two coordinate covalent bonds and a total of four covalent bonds in this
structure. So, each atom of beryllium in the
extended linear structure of beryllium chloride in the solid state has two
coordinate covalent bonds.
