Beginning with boron, all of the oxides of second-shell elements
are acids. In Chapter 4 we noticed a dramatic change in the properties
of the elements between carbon and nitrogen. Carbon is a hard solid
with four single bonds from each atom to four neighboring atoms
in the diamond lattice; nitrogen is a diatomic gas with a triple
bond between two atoms. A similar abrupt change in properties of
the oxides is found between boron and carbon. Lithium and beryllium
oxides are orderly crystals, and boron oxide is a disordered glass.
Carbon has two oxides, and both are small, gaseous molecules.
When carbon or its compounds are burned with an excess of oxygen,
one of the products is carbon dioxide, .
In this molecule, carbon makes a double bond to each of two oxygen
atoms, as shown at the top right. Each oxygen atom also has two
lone pairs, so each of the three atoms has eight electrons in its
molecule is linear; that is, the O=C=O atoms all lie along a straight
line. This is explained easily by electron repulsion or VSEPR theory.
The carbon atom has two clusters of four bonding electrons around
itself, each cluster pointing toward one of the two oxygen atoms.
The most stable arrangement of two repelling groups around a spherical
atom is at opposite poles of the sphere.