Section+22.2+-+Hydrogen

= = =**Basic Characteristics Most Common Uses of Hydrogen**= At room temperature, elemental hydrogen exists as a colorless, odorless, colorless, and tasteless //diatomic// molecule. Sadly, it is not a member of any family in the periodic table. The atomic number of hydrogen is 1, and its molar mass is approximately 1.00794g/mol.

Because hydrogen is a nonpolar molecule composed of only two electrons, attractive forces between hydrogen molecules are extremely weak. Due to this low attraction between molecules, the melting point (-259 degrees Celcius) and boiling point (-253 degrees Celcius) of hydrogen are extremely low.

A single molecule of hydrogen has very strong H-H bonds; thus, its bond enthalpy is high (approximately 436 kJ/mol). Also, due to the strong H-H bonds present within hydrogen, most reactions of hydrogen are slow at room temperatures, yet strongly affected b heat, irradiation, of catalysis. Once it is activated, hydrogen reacts rapidly and exothermically. Hydrogen creates strong, //covalent// bonds with various elements such as oxygen.

=**Isotopes of Hydrogen**= Hydrogen is the most abundant element in the universe. It’s most common isotope, 1H, accounts for more than 99.9% of all naturally occurring hydrogen. Other than 1H, hydrogen contains two other isotopes known as 2H and 3H. The chart below summarizes the basic characteristics of 1H, 2H, and 3H



The chart below summarizes the basic characteristics of 1H, 2H, and 3H
 * || Protium || Deuterium || Tritium ||
 * Protons || 1 || 1 || 1 ||
 * Neutrons || 0 || 1 || 2 ||
 * Electrons || 1 || 1 || 1 ||
 * Radioactivity || No || No || Yes / Half-life: 12.3 yrs. ||
 * Often referred to as... ||  || D (i.e. D2O) / Heavy water ||   ||
 * *Important things to note ||  || Melting point, boiling point, and density greater than protium || The isotope may be synthesized in nuclear reactors by neutron bombardment of lithium-6 ||

=**Deuterium and the Kinetic-Isotope Effect**= It is important to note the effects of an added neutron to the 2H isotope of deuterium. First off, an atom of deuterium is about //twice// as massive as an atom of protium. Thus, most of normal characteristics of hydrogen vary slightly due to the increased mass of its 2H isotope. As stated above, the melting point, boiling point, and density of 2H are slightly greater than 1H.

In addition, an interesting phenomenon takes place when protium is replaced by deuterium in chemical reactions. Due to its increased mass, deuterium can also have profound effects on the rates of reaction (varying with each separate chemical) - a phenomenon called the //kinetic-isotope effect//.

=**Isotopes of Hydrogen – Things to Note!**= 2H and 3H have proved extremely useful in studying reactions of compounds containing hydrogen. For instance, by replacing a protium atom with an atom of deuterium or tritium, a molecule can be //labeled// and traced in a chemical reaction by comparing the location of the labeled hydrogen atom in the reactants and the products. Thus, the process of labeling often times helps infer the mechanism of a particular reaction.

=**Preparation of Hydrogen**= When a //small quantity// of hydrogen is required for laboratory purposes, it is usually obtained by the reaction between an active metal, such as zinc, and a dilute strong acid such as hydrochloric acid. Zn(//s//) + 2H+(//aq//) --> Zn2+(//aq//) + H2(//g//)

//Large quantities// of hydrogen, however, are produced by reacting methane with steam at 1100 degrees Celcius. Below are two reactions that summarize this process.

CH4(//g//) + H2O(//g//) --> CO(//g//) + 3H2(//g//) CO(//g//) + H2O(//g//) --> CO2(//g//) + H2(//g//)

=**Binary Hydrogen Compounds**= __**Ionic Hydrides**__: Formed when hydrogen reacts with //alkali earth// and heavier //alkaline earth metals// (Ca, Sr, and Ba). //Example//: Ca(s) + H2(g) --> CaH2(s)

__**Metallic Hydrides**__: Formed when hydrogen reacts with transition metals. These compounds retain their metallic conductivity and other metallic properties. Interestingly enough, the ratio of many metal atoms to hydrogen atoms present in many metallic hydrides id not fixed, or in small whole numbers. //Example//: TiH1.8

__**Molecular Hydrides**__: Formed when hydrogen reacts with nonmetals of semimetals. These compounds exist as either gases or liquids under SATP. Examples of molecular hydrides are shown to the right.

=**More Hydrogen, Please!**=

Pure hydrogen was first isolated by Henry Cavendish. Approximately 75% of the known mass of the universe is made up of hydrogen, yet the element only contributes to about 0.87% of the Earth’s mass. H2O is the most abundant hydrogen compound.

For More Information, Visit:
[]

Interested in preserving our environment and contributing to a movement for a greener planet? Check out the possibility of a hydrogen economy! []

References:
Brown, Theodore L. and Eugene LeMay. //Chemistry: The Central Science//. 11th ed.Upper Saddle River, NJ: Pearson Prentice Hall, 2008. Print.