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 outer shell.

The 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.