Chapter 10: Hydrogen
• Hydrogen ranges in character from being a strong Lewis base to being a
strong Lewis acid.
• Hydrogen atoms can form bonds to more than one other atom simultaneously.
• The proton, H+, is always found in combination with a Lewis base and is
highly polarizing; the hydride ion, H-, is highly polarizable.
o 3 Isotopes of Hydrogen: hydrogen (1H), deuterium (D, 2H) and tritium
o The hydride ion (H-) is highly polarizable because two electrons are
bound by just one proton.
• Hydrogen has unique atomic properties that place it in a special position in
the periodic table; dihydrogen is an inert molecule and its reactions require a
catalyst or initiation by radicals.
o Hydrogen is placed at the head of Group 1 because it has only one
electron in its valence shell, like the alkali metals.
o Hydrogen requires only one electron to complete its valence shell, like
o The H2 molecule has a high bond enthalpy or strength (inert molecule)
and a short bond length.
o Hydrogen has a high ionization energy and a low but positive electron
o It is normally assigned the oxidation number -1 when combined with
metals and +1 when combined with nonmetals).
• Compounds formed between hydrogen and other elements vary in their
nature and stability.
• In combination with metals, hydrogen is often regarded as a hydride;
hydrogen compounds with elements of similar electronegativity has low
o Molecular Hydrides: exist as individual, discrete molecules; they are
usually formed with p-block elements of similar or higher
electronegativity than H (covalent E-H bonds) like CH4, NH3 and H2O.
o Saline Hydrides: known as ionic hydrides are formed with the most
electropositive elements like LiH and CaH2.
o Metallic Hydrides: non-stoichiometric, electrically conducting solids
with a metallic luster; it is formed with many d- and f-block elements.
• In the s and p blocks, strengths of E-H bonds decrease down each group but in
the d block, strengths of E-H bonds increase down each group.
o All s-block hydrides are exergonic (Δ Gf° < 0) except for BeH2, so they
are thermodynamically stable with respect to their elements at room
o For Group 13, AlH3 is the only exergonic at room temperature.
o For the rest of the p block, the simple hydrogen compounds of the first
members of the groups are exergonic but become progressively
unstable down the group.
• The reactions of binary compounds of hydrogen fall into three classes,
depending on the polarity of the E-H bond.
o If E and H have similar electronegativities, cleavage of the E-H bond
tends to be hemolytic, producing, intitially, an H atom and a radical,
each of which can go on to combine with other available radicals.
o If E is more electronegative than H, heterolytic cleavage occurs,
releasing a proton (H atom is protonic) = E-H E- + H+. o If E is less electronegative than H, heterolytic bond cleavage also
occurs but an H- ion is transferred to a Lewis acid (H atom is hydridic)
= E-H E+ + H-.
• Three hydrogen isotopes H, D and T have large differences in their atomic
masses and different nuclear spins, which give rise to easily observed