Hydrogen and its Compounds


Atomic Hydrogen

Hydrogen is the most abundant element in the universe. Earth's primordial atmosphere was probably similar to the gas cloud that created the sun and planets. It consisted mainly of hydrogen and helium, along with methane, ammonia, and water.



Atomic hydrogen has 1 proton in its nucleus and 1 electron in a 1s atomic orbital. Deuterium is a stable isotope hydrogen with 1 neutron in the nucleus along with the proton. Tritium is a radioactive isotope with 2 neutrons in its nucleus.


Molecular Hydrogen

Two hydrogen atoms can combine their electrons to form a 2-electron bond. Elemental hydrogen, H2, is the simplest of the molecules. What do we know about hydrogen?
  • Hydrogen is a lighter-than-air, flammable, diatomic gas. It is colorless and only sparingly soluble in solvents.

  • It boils at -253.5 deg C.

  • Platinum metal or heat can catalyze the reaction of hydrogen with oxygen to form water.

  • In hydrogen fuel cells, the chemical oxidation of hydrogen to water produces electricity. (We'll talk about this next time!)


Hydrogen reacts with halogens (X2) in radical chain processes to make HX. For example:
    Cl-Cl 2 Cl
    Cl + H2 HCl + H
    H + Cl2 HCl + Cl

    net reaction: H2 + Cl2 2HCl

Hydrogen adds to metals to make metal hydrides.


Electronegativity



Electronegativity is the tendency of an atom to pull the electrons in a covalent bond closer to itself.

Hydrogen and carbon are pretty close in electronegativity so their bonds are relatively non-polar. In the oxidation state formalism, we consider hydrogen bonded to a carbon to be in the +I oxidation state.

Most non-metals are more electronegative than hydrogen so their bonds with hydrogen are polarized with more electron density on the non-metal. In oxidation state formalism, hydrogen has a +I oxidation state.

Metalloids (such as B, Si, or Ge) and all metals are less electronegative than hydrogen. The H-M bonds are polarized so that more electron density is around hydrogen. In the oxidation state formalism, hydrogen is a hydride with an oxidation state of -I.




Metallic Hydrogen

Scientist at Lawrence Livermore National Laboratory used high pressure and very low temperature conditions to convert hydrogen from a diatomic insulator to a monoatomic metal. At 1.4 megabars of pressure (~1.4 x 106 atmospheres), the band gap was reduced to zero.




Acids

Hydrogen atoms bonded to very electronegative elements are acidic. That is, in water, water reacts with the compound and removes a proton from it. The strong mineral acids and some weaker acids are shown below.



The strength of an acid depends on its conjugate base. A strong acid as a very weak (highly stable) conjugate base. A base is stabilized when the excess negative charge is spread out. This can occur when:
  • electron-withdrawing groups pull some of the excess electron density away from the atoms that bears the charge through sigma bonds, or
  • there are multiple resonance structures so that the charge is partially on more than 1 atom.





Hydrides

You've already seen that element hydrides act like protected form of H-. For example, sodium borohydride adds to the electrophilic carbonyl carbon of aldehydes and ketones.




Professor Patricia Shapley, University of Illinois, 2012