Positive radical other than ammonium?
Answers
nitrogen positively charged in NH4+ if it is more electronegative
Atoms are neutral particles - the number of protons and electrons is equal. If you look at the periodic table, you can find the atomic number of Nitrogen to be 7. This means that there are 7 protons in the nucleus and 7 electrons around it. The electron configuration of Nitrogen is 1s22s22p3. Electrons from electron shell 2 are called valence electrons - it is them which take part in chemical bonds. The neutral atom has five valence electrons.
Every atom is striving for a stable octet configuration, so Nitrogen needs three additional electrons. We can then calculate its formal charge. It is defined as the following sum: the number of valence electrons for the atom minus the number of unshared electrons plus half of the number of electrons taking part in a chemical bond.
Z=n(valence)−n(unshared)−12n(bonding)
What this formula means is that we take the initial number of electrons for the given atom and ignore those that would remain unshared - they belong to the given atom, so we can ignore them, as if they never were part of it. Then, out of those that take part in bonding, we only take 1/2. This is because when a covalent bond is formed, those electrons belong to both atoms. That's why they are called shared electron pairs. For Nitrogen in NH4:
Z=5−0−12x8=1.
Why I wrote those numbers? Well, Nitrogen forms four bonds, each consisting of two electrons. There are no unshared electron pairs.
In comparison, for NH3:
Z=5−2−12x6=0.
Now, on the N-H bond. As we already said, the valence electrons of Nitrogen are 2s22p3. As the energy of an s orbital is different from that of a p orbital, this suggests that Nitrogen is supposed to form two types of bonds. However, experimental data doesn't prove this guess. The lengths of all four N-H bonds in NH4 are equal (and thus their energies equal, too). One way to describe this phenomenon is through hybridization. It is a mathematical abstraction but it's very useful for teaching purposes, rather intuitive a model.
In the ammonia molecule, Nitrogen forms three bonds with Hydrogen atoms but also, there are two extra electrons that belong to it. The latter remain unchanged. They form an unshared electron pair. Hybridization suggests that the four orbitals, 2s and the three 2p, will mix with each other, forming four hybrid sp3 orbitals. The unshared electron pair occupies one of those and the rest are reserved for the electrons that form the bonds. The same line of thoughts can be applied for the ammonium cation.
