Page 102 - Physical chemistry understanding our chemical world

CREATING FORMAL CHEMICAL BONDS 69

Why does salt form when sodium and chlorine react?

Bond formation with ions

Ionic interactions are electrostatic by nature, and occur between ions of opposite charge.
The overwhelming majority of ionic compounds are solids, although a few biological
exceptions do occur. Table 2.7 lists a few typical properties of ionic compounds.
It is generally unwise to think of ionic compounds as holding together with physical
bonds; it is better to think of an array of point charges, held together by the balance of
their mutual electrostatic interactions. (By ‘mutual’ here, we imply equal numbers of
positive and negative ions, which therefore impart an overall charge of zero to the solid.)
Ionic compounds generally form following the reaction of metal-
lic elements; non-metals rarely have sufficient energy to provide Care:chlorINEisan
the necessary energy needed to form ions (see p. 123). elemental gas; chlo-
rIDE is a negatively
The structure in Figure 2.14 shows the result of an ionic reac-
charged anion.
tion: sodium metal has reacted with chlorine gas to yield white
crystalline sodium chloride, NaCl. Each Na atom has lost an elec-
tron to form an Na cation and each chlorine atom has gained an
+
electron and is hence a Cl − anion. In practice, the new electron Thechlorideion has
anegativecharge
possessed by the chloride came from the sodium atom.
because, following ion-
The electron has transferred and in no way is it shared. Sodium
ization, it possesses
chloride is a compound held together with an ionic bond, the
more electrons than
strength of the bond coming from an electrostatic interaction bet- protons.
ween the positive and negative charges on the ions.

Why heat a neon lamp before it will generate light?

Ionization energy

Neon lamps generate a pleasant pink–red glow. Gaseous neon within the tube (at
low pressure) is subjected to a strong electric discharge. One electron per neon atom

Table 2.7 Typical properties of ionic compounds
Property Example
High melting point We need a blast furnace to melt metals
High boiling point A lightning strike is needed to volatilize some substances
Physically hard Ceramics (e.g. plates) can bear heavy weights
Often physically brittle Table salt can be crushed to form a powder
High electrical conductivity in Using a hair dryer in the bath risks electrocution
solution
Dissolve in polar solvents Table salt dissolves in water
Insoluble in non-polar solvents We dry an organic solvent by adding solid CaCl 2