Which of the following is the type
of bond between two water molecules? (A) Hydrogen bond, (B) ionic bond,
(C) polar covalent bond, (D) nonpolar covalent bond, or (E) metallic bond.
A bond, or chemical bond, is a
strong force of attraction holding atoms or ions together. A hydrogen bond is an example of an
intermolecular bond. And ionic, polar covalent, nonpolar
covalent, and metallic bonds are examples of intramolecular bonds.
Inter- in intermolecular bond means
between. So, an intermolecular bond, such as
a hydrogen bond, is the bond or force of attraction between two separate
molecules. Intra- in the term intramolecular
bonds means within. So, an intramolecular bond is a
bond or force of attraction within an individual unit of a compound, such as a
molecule or a formula unit.
The question asks for the type of
bond between two water molecules, in other words, an intermolecular bond. Of the possible answers, only (A),
hydrogen bond, is an example of an intermolecular bond. Let’s confirm this by discussing
what a hydrogen bond is. Water, or H2O, is a bent
molecule. The electronegativity value of
oxygen is much higher than that of hydrogen. Oxygen has a value of 3.4 and
hydrogen a value of 2.2.
Electronegativity is the tendency
of an atom to attract a bonding pair of electrons towards itself. Because oxygen has a higher
electronegativity value, this means that oxygen pulls the bonding pair of electrons
more strongly towards itself than does hydrogen. This pull on the bonding electrons
results in the electrons spending more time in the vicinity of the oxygen atom. And the oxygen atom becomes Δ
negative or slightly negatively charged and the hydrogen atom Δ positive or slightly
positively charged. This slight charge distribution
will happen for every bond in the water molecules of a sample.
The slightly positively charged
hydrogen atom on one water molecule becomes electrostatically attracted to the
slightly negatively charged oxygen atom on an adjacent water molecule. This electrostatic attraction
between separate molecules as a result of the Δ positive and Δ negative charges is
called a hydrogen bond. So, the type of bond between two
water molecules is option (A), hydrogen bond. Let’s very briefly remind ourselves
of ionic, polar covalent, nonpolar covalent, and metallic bonds.
An ionic bond occurs when a metal
cation and a nonmetal anion electrostatically attract each other, for example, in
sodium chloride, Na+ and Cl−. A polar covalent bond exists
between two nonmetals when the electronegativity difference is between 0.4 and
1.8. An example is the bond within a
water molecule. A nonpolar covalent bond exists
between two nonmetals when the electronegativity difference is less than 0.4 and the
pull on the bonding electrons is equal or almost equal, for example, in Cl2 or
chlorine gas. Finally, a metallic bond exists in
metals, for example, iron, where a sea of electrons holds the positively charged
nuclei of the atoms together.
And the final answer is (A),