Hydrogen combines with every element in the periodic tableexcept the nonmetals in Group VIIIA (He, Ne, Ar, Kr, Xe, and Rn).Although it is often stated that more compounds contain carbonthan any other element, this is not necessarily true. Most carboncompounds also contain hydrogen, and hydrogen forms compoundswith virtually all the other elements as well. Compounds ofhydrogen are frequently called hydrides, eventhough the name hydride literally describes compounds thatcontain an H- ion. There is a regular trend in theformula of the hydrides across a row of the periodic table, asshown in the figure below. This trend is so regular that thecombining power, or valence, of an element was oncedefined as the number of hydrogen atoms bound to the element inits hydride.

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Hydrogen is the only element that forms compounds in which thevalence electrons are in the n = 1 shell. As a result,hydrogen can have three oxidation states, corresponding to the H+ion, a neutral H atom, and the H- ion.

H+ = 1s0
H = 1s1
H- = 1s2

Hydrogen"sPlacement in the Periodic Table

Because hydrogen forms compounds with oxidation numbers ofboth +1 and -1, many periodic tables include this element in bothGroup IA (with Li, Na, K, Rb, Cs, and Fr) and Group VIIA (with F,Cl, Br, I, and At).

There are many reasons for including hydrogen among theelements in Group IA. It forms compounds (such as HCl and HNO3)that are analogs of alkali metal compounds (such as NaCl and KNO3).Under conditions of very high pressure, it has the properties ofa metal. (It has been argued, for example, that any hydrogenpresent at the center of the planet Jupiter is likely to be ametallic solid.) Finally, hydrogen combines with a handful ofmetals, such as scandium, titanium, chromium, nickel, orpalladium, to form materials that behave as if they were alloysof two metals.

There are equally valid arguments for placing hydrogen inGroup VIIA. It forms compounds (such as NaH and CaH2)that are analogs of halogen compounds (such as NaF and CaCl2).It also combines with other nonmetals to form covalent compounds(such as H2O, CH4, and NH3), theway a nonmetal should. Finally, the element is a gas at roomtemperature and atmospheric pressure, like other nonmetals (suchas O2 and N2).

It is difficult to decide where hydrogen belongs in theperiodic table because of the physical properties of the element.The first ionization energy of hydrogen (1312 kJ/mol), forexample, is roughly halfway between the elements with the largest(2372 kJ/mol) and smallest (376 kJ/mol) ionization energies.Hydrogen also has an electronegativity (EN = 2.20)halfway between the extremes of the most electronegative (EN= 3.98) and least electronegative (EN = 0.7) elements.On the basis of electronegativity, it is tempting to classifyhydrogen as a semimetal, as shown in the three-dimensional graphof the electronegativities of the main-group elements shownbelow.

This three dimensional graph of the electronegativities of the main-group elements helps us understand why it is difficult to classify hydrogen as a metal or a nonmetal.

Hydrogen is oxidized by elements that are more electronegativeto form compounds in which it has an oxidation number of +1.


Hydrogen is reduced by elements that are less electronegativeto form compounds in which its oxidation number is -1.


Properties andFormation of Hydrogen

At room temperature, hydrogen is a colorless, odorless gaswith a density only one-fourteenth the density of air. Smallquantities of H2 gas can be prepared in several ways.

1. By reacting an active metal with water.

2 Na(s) + 2 H2O(l) " width="17" height="9"> 2 Na+(aq) + 2 OH-(aq) + H2(g)

2. By reacting a less active metal with a strong acid.

Zn(s) + 2 HCl(aq) " width="17" height="9"> Zn2+(aq) + 2 Cl-(aq) + H2(g)

3. By reacting an ionic metal hydride with water.

NaH(s) + H2O(l) " width="17" height="9"> Na+(aq) + OH-(aq) + H2(g)

4. By decomposing water into its elementswith an electric current.

2 H2O(l) " width="17" height="9"> 2 H2(g) + O2(g)

Practice Problem 2:

Use oxidation numbers to determine what is oxidized and what is reduced in the following reactions, which are used to prepare H2 gas.

(a) Mg(s) + 2 HCl(aq) " width="17" height="9">Mg2+(aq) + 2 Cl-(aq) + H2(g)

(b) Ca(s) + 2 H2O(l) " width="17" height="9"> Ca2+(aq) + 2 OH-(aq) + H2(g)

Click here to check your answer to Practice Problem 2

The covalent radius of a neutral hydrogen atom is 0.0371 nm,smaller than that of any other element. Because small atoms cancome very close to each other, they tend to form strong covalentbonds. As a result, the bond dissociation enthalpy for the H-Hbond is relatively large (435 kJ/mol). H2 thereforetends to be unreactive at room temperature. In the presence of aspark, however, a fraction of the H2 moleculesdissociate to form hydrogen atoms that are highly reactive.

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H2(g) " width="17" height="9"> 2 H(g)

The heat given off when these H atoms react with O2is enough to catalyze the dissociation of additional H2molecules. Mixtures of H2 and O2 that areinfinitely stable at room temperature therefore explode in thepresence of a spark or flame.