Figure 1-31. The mass-abundance of the most common elements in the Earth's crust (source).
As a simple substance, oxygen exists in the form of a diatomic molecule (O2
) whose molecular mass is 32. The atmosphere (air) is approximately 21% oxygen admixed with nitrogen, N2
(78%), and roughly 1% of other gases, by volume. 1.10.2. Ozone (O3). Allotropy.
There is another, less common form of oxygen as a simple substance, ozone. Ozone is a pale-blue gas. A molecule of ozone consists of three oxygen atoms, so the formula of ozone is O3
. Ozone is much less stable and much more reactive than O2
. Also, while oxygen O2
is odorless, ozone O3
has a characteristic smell and is toxic in high concentrations. Ozone is formed in the atmosphere from molecules of O2
during thunderstorms under the action of lightning (electric discharge). That is why it is often said that the air smells of ozone after a big thunderstorm. In the upper layers of the atmosphere, there is an ozone layer. This layer is of vital importance to life on our planet because it traps hazardous ultraviolet radiation from the sun.
) is an allotropic modification
(also known as allotropic form
or just allotrope
) of oxygen (O2
(or allotropism) is the phenomenon of the existence of more than one simple substance for the same chemical element. Although allotropes are composed of atoms of the same type, they can and do exhibit distinct physical, chemical, and biological properties due to the difference in their structures. As mentioned above, oxygen O2
is nontoxic, odorless, and stable. In contrast, ozone O3
is toxic, has a characteristic smell, and much less stable.
Allotropes are known for many elements. One particularly well-known example is diamond and graphite, the two allotropes of carbon (Figure 1-32). Most people have seen diamond jewelry and used pencils, whose core, also known as pencil lead, is made of graphite. Diamond and graphite have totally different properties and appearance, despite the fact that both are composed entirely of carbon atoms. Diamond is clear and transparent, whereas graphite is black. Diamond is the hardest material known, whereas graphite is soft. Graphite conducts electricity, whereas diamond does not.