|
| Physical Properties |
Atomic Number: 1
Mass Number: 1.0079
Electron Configuration: 1s1
First Ionization Energy: 1312
kJ/mol
Atomic Radius: 37
pm
Ionic Radius: 35 pm (H+), 208
pm (H-)
| Occurrence and Properties |
Hydrogen is the most abundant element in the universe and forms more compounds than any other element. While some hydrogen does exist in the upper atmosphere, most has long since escaped into space and almost all of the hydrogen found on Earth is combined with other elements.
Hydrogen is the simplest element and has an atomic number
of one. At room temperature, hydrogen is a colorless and tasteless gas
and occurs as diatomic nonpolar molecules.
| Compounds of Hydrogen |
Hydrogen forms bonds with other elements in different ways. First is that hydrogen can accept an electron, forming the hydride ion, H-. Hydrogen is unique in this capacity; none of the other alkali metals or alkaline earth metals can form a negative ion. Hydrides are generally of two types, saline hydrides and metallic hydrides. The saline hydrides are formed with the alkali metals and the heavier members of the alkaline earth metals. These are are best described as ionic substances consisting of positive metal cations and negative hydride ions. Saline hydrides will dissolve in molten alkali halides and electrolysis of these solutions results in the formation of hydrogen at the anode, or site of oxidation. This is in contrast to the electrolysis of aqueous solutions, where hydrogen is observed at the cathode, or site of reduction. Metal hydrides are moisture sensitive and will react explosively with water to yield hydrogen gas. Calcium hydride is less reactive and can be used as a source of hydrogen.
The metallic hydrides are quite different. These compounds are often nonstochiometric and it is believed that the hydride ions may occupy the intersticies between the atoms. For this reason these compounds are referred to as the insterstitial hydrides. Most compounds are black or grayish black. They are also electrically conductive.
Hydrogen also combines with many nonmetals to form compounds that are best described as covalent. Hydrogen forms an extensive series of compounds with hydrogen called hydrocarbons. Petruleum and most substances derived from petroleum are mixtures of hydrocarbons. The entire field of organic chemistry is devoted to the study of compounds of which carbon and hydrogen are the major components. Hydrogen is also found combined with oxygen in compounds such as water and hydrogen peroxide. Hydrogen combines with nitrogen to form ammonia and with phosphorous to form phosphine. Hydrogen also forms an extensive series of compounds with boron called boranes. Unlike hydrocarbons, boranes are examples of electron-deficient structures. Hydrogen chloride, bromide, and iodide can also be classified as molecular compounds. Hydrochloric acid is actually a concentrated aqueous solution of hydrogen chloride.
Some covalent hydrogen compounds
| Boron | Carbon | Nitrogen
Phosphorous |
Oxygen
Sulfur |
Fluorine
Chlorine Bromine Iodine |
| BH4-(borohydride ion)
B2H6 (diborane) B4H10(tetraborane) |
CH4 (methane)
C2H4 (ethane) C3H8 (propane) SiH4 (silane) |
NH3 (ammonia)
PH3 (phosphine) |
H2O (water)
H2S (hydrogen sulfide) |
HF
HBr HCl HI |
| Laboratory Preparation of Hydrogen |
Hydrogen can be readily produced in the laboratory by several different means. Many metals such as Mg, Ca, Al, and Zn will react with acids to produce hydrogen gas. Many of the alkali metals and alkaline earth metals will react with water to produced hydrogen has and the corresponding metal hydroxide. For most of the alkali metals, this reaction occurs too quickly and is too dangerous to be used as a source of hydrogen. Magnesium reacts slowly with water but the reaction with calcium is more vigorous. Hydrogen can also be prepared between certain metals such as aluminum and a solution containing the hydroxide ion.
M + 2H2O => M(OH)2 + H2
M + 2HCl => MCl2 + 2H2
