Relationships between chemical bonding, electronic polarisability, optical basicity and coordination number are discussed for oxidic glasses. Particular attention is focused on the lithium borate glass system because of the startling observation that the basicity decreases as the initial lithium oxide content is increased. This arises owing to a very large increase in the polarising power of the boron. Only part of the increase is due directly to the coordination number change, which is shown to be responsible for Yb rising from 2.47 (the value for threefold coordinated boron) to 3.3. However, the coordination number change also results in the complete loss of Pi-bonding in the BO4 unit, and it is this factor that further increases Yb to the experimentally observed value of 4.2.
