Chapter 21. Properties of the Elements
(We will also add the metals from Chapter 20)
First, let’s look at Periodic Table and Group numbering.
21.2 Hydrogen. The simplest atom, ~90% of all atoms in the universe. H isotope (protium) = 1
proton surrounded by e. H (deuterium) or H (tritium) contain 1 or 2 neutrons as well.
Deuterium ( H) was produced in the Big Bang—too “fragile” to survive fusion conditions in the
stars (which produce the lighter elements) or supernovas (which produce heavier elements).
Stable molecular form is H i.e. H2H. Hydrogen is the exception in the periodic table — cannot
be satisfactorily classified in any group: similarities to certain metals and nonmetals (groups 1
and 17, respectively).
Brief Summary of Hydrogen Chemistry
almost always forms covalent compounds; ionic ones rare.
high ionization energy (e close to nucleus without other e’s to shield it) and low
electronegativity (only one proton to attract es).
∴ unlike groups 1 and 17 in that H and H ions are rare
because they usually bond covalently to other things instead of being found as ions e.g.,
H O3, OH, NH , etc 4+
Very rare exception is the hydride ion, H
Ionic Hydrides (H ).
Result from H2 (g) being reduced to H ions, rather than oxidized to H
It is very difficult to reduce H (g)
H 2g) + 2 e → 2 H (g) Eº = 2.23 V
but very strong reducing agents (Na, Ca, Li, etc, with halfcell potentials below 2.23 V) can do it
to give ionic salts containing H , called the hydride ion.
2 Na (s) + H (g2 → 2 NaH (s) (ionic salt)
The hydride ion is very reactive and will either:
(1) find a H and go to H (g): 2
NaH (s) + H O (l2 → Na (aq) + OH (aq) + H (g) 2
or (2) reduce something and go to H (g): 2
TiCl 4l) + 4 LiH (s) → Ti (s) + 4 LiCl (s) + 2 H (g) 2 Covalent Hydrogen Compounds. Common and stable: CH , NH , H O and HF, etc, etc. 4 3 2
The other element has higher electronegativity than H (2.2) ∴ we think of these as containing H
(+1) oxidation state (i.e. H ) and e.g., N(3), O(2), F(1).
N 2(g) + 3 H (g2 → 2 NH (g). 3