Nitrogen and Oxygen
Molecular NitrogenMolecular nitrogen makes up 78% of the gases in the atmosphere. Let's look at this molecule more closely. Molecular nitrogen is made up of 2 nitrogen atoms. From the periodic table we see that N has an electron configuration of [He]2s22p3. It has a filled 2s orbital along with 3, half-filled 2p orbitals. One p orbital is along the z direction, one along the x direction, and the final one along the y direction.
Consider the sharing of electrons in the 2p(z) orbitals of two nitrogen atoms. This type of covalent bond, where the orbitals that share electrons are pointing directly at one another, is called a sigma bond ().
Two 2p(x) orbitals, one from each nitrogen atom, can share an electron side-by-side. The symmetry of this kind of covalent bonding is called a pi bond (). The two 2p(y) orbitals from each nitrogen atom combine in the same way.
Molecular nitrogen has a triple bond between the two atoms, one sigma bond and two pi bonds. This bond is very strong and requires 941 kJ/mol of energy to break it. This amount of energy is called the bond energy.
Let's count the number of electrons around each atom in N2. Remember that shared electrons are counted for each atom (counted twice). Each nitrogen has an unshared pair of electrons and 3 shared pair of electrons for a total of 8 electrons around each nitrogen atom. This is the same number of electrons as in the next noble gas, neon.
Molecular nitrogen is "fixed" by bacteria associated with some plant roots. This provides NH3 and NH4+ that growing plants can use to make proteins and other biological molecules. When plants decompose, other bacteria in the soil convert the nitrogen-containing molecules back into N2. The process of recycling the nitrogen is called the nitrogen cycle.
Molecular OxygenMolecular oxygen makes up nearly 21% of the atmosphere. The simple electron dot diagram indicates that O2 has a double bond (one sigma bond and one pi bond) between the two oxygen atoms. Each oxygen also has 2 non-bonding pairs of electrons. When we count the shared electrons and the non-bonding electrons for each oxygen atom, we get 8 electrons. The same number as the filled shell configuration for neon. Based on this, we would expect that O2 would be very stable and unreactive, just like N2. This is not the case. Next time we'll go into more depth on the bonding of this molecules in order to explain this.