Properties of atoms

[Airmid]

Which properties of atoms do chemists use to estimate properties of compound molecules?

 

To illustrate: Let's consider O2 and N2. O2 is far more reactive than N2, yet their properties are not that different. They are close in weight and size. Both are lacking electrons to form a "perfect" configuration, and their electron negativities are not unalike. Both form dimers. As far as I can see, Oxygen dimers forms double bonds, and Nitrogen triple bonds, so there's a difference there. Is this what accounts for the lack of reactivity of N2, or are other properties in play?

 

A more complicated example: let's consider CO2 and COS. CO2 has a triple point at 195K, and COS a freezing point of 134K and a boiling point of 223K. Both are linear molecules (i.e. O=C=O and O=C=S). O is lighter than S, so logically the freezing point of CO2 should be lower than COS. CO2 is an apolar molecule, while COS is polar. Again, logically the freezing point of CO2 should be lower than COS. Except it isn't. How could I have predicted this?

 

It would be really great if you could help me out here!

Airmid.

[woelen]

If you really want to understand the differences between atoms in their formation of compounds, then you have to delve into quantum mechanics. Properties of atoms, most important for their chemistry, are the orbitals, available for bonding and how these orbitals overlap (hybridize) with other orbitals.

 

All 'rules', we learn about the chemical properties, are based on centuries of observations of all kinds of chemical compounds. Only the last 80 years or so, people are understanding the underlying physical principles to a reasonable extent and only the last few decades we have the computational power to explain all macroscopic observations on the basis of quantum mechanical models of atoms and (simple) molecules.

[Tartaglia]

O2 possesses a triplet state - ie two unpaired electrons and is therefore a diradical. This makes it substantially more reactive than N2 which is a singlet state ie all electrons are paired.

[Airmid]

Thanks! I even found some nice information on the web to explain to me what you wrote there, Tartaglia. (Dioxygen and Dinitrogen)

 

Life is so much easier if you know what to google for!

Airmid.