Doc Brown's chemistry notes on atomic and electron structure

8. Allotropes - definition - oxygen, carbon and sulfur examples explained and don't confuse with isotopes!

As explained above, isotopes are atoms of the same element with different masses due to different numbers of neutrons in the nucleus. Same protons and electrons. e.g. atomic number 6 = 6 protons = carbon, but there can be 6, 7 or 8 neutrons giving isotopes of carbon–12, 13 or 14. They are NOT allotropes.

Allotropes are defined as different atomic or molecular forms of the same element in the same physical state (gas, liquid or solid).

The different physical allotropic forms arise from different arrangements of the atoms or molecules of the element and in the case of solids, different crystalline allotropes. With gases it must be different molecular formula.

They may chemically similar, or different, but always physically different in some way e.g.

The allotropes of oxygen

Oxygen atoms usually form 'stable' O₂ oxygen molecules (also called dioxygen), BUT they can form a very reactive unstable molecule O₃ ozone (also called trioxygen). The mass of the oxygen atoms in each of the molecules is mainly 16 (99.8%), and about 0.2% of two other stable isotopes of masses 17 and 18. Whatever isotope or isotopes make up the molecule, it doesn't affect the molecular structure or the respective chemistry of the O₂ or O₃ molecules.

However, what sometimes confuses the issue is the fact that oxygen O₂ and ozone O₃ are examples of allotropes.

O₂ (oxygen, dioxygen) and O₃ (ozone, trioxygen) are both gases but have different densities, boiling points etc.

oxygen–16, 17 or 18 are isotopes of oxygen with different nuclear structures due to different numbers of neutrons, but they behave chemically in an identical manner whether they are in an oxygen or ozone molecule.

BUT O₂ and O₃ are different molecular structures of the same element in the same physical state and are called allotropes irrespective of the isotopes that make up the molecules.

 

The allotropes of carbon

Graphite, diamond, graphene and carbon nanotubes are four solid allotropes of the element carbon and have significantly different physical and in some ways chemical properties! (lots of details and diagrams on bonding page)

 

The allotropes of sulfur

Rhombic and monoclinic sulphur have different geometrical crystal structures, that is different ways of packing the sulphur atoms (which are actually both made up of different packing arrangements of S₈ ring molecules). They have different solubilities and melting points.

There is also a 3rd unstable allotrope of sulfur called plastic sulphur made by pouring boiling molten sulphur into cold water which forms a black plastic material consisting of chains of sulphur atoms –S–S–S–S–S– etc..

 

To summarise ...

It doesn't matter which isotopes make up the structure of any of an element's allotropes described above, so to summarise by one example ...