N 1 in periodic system
Hydrogen is the first element in the Periodic system. It is quite difficult to place it into a certain group. Its special position is predetermined by properties of I and VII groups elements inherent to hydrogen: giving up 1 е- as IA group metals do and accepting an electron like non-metals of VIIA group. For this reason Н is situated in VIIA group and in brackets in group IA or vice-versa. At the same time there are many differences in its properties with elements of these groups.
Н is an element with the unique special properties. The only unambiguous thing is that hydrogen always behaves as a monovalent element in all compounds.
Oxidation states |
Compounds |
H-1 |
NaH, MgH2 |
H0 |
H2, PdH0.5 |
H+1 |
H2O, H3O+, NH3, B2H6, C2H4, HF |
Occurrence
Hydrogen accounts for about 73 percent of the observed mass of the universe and is the most common element in the universe. It is contained in gaseous nebulas, in interstellar gas, and in stars. Inside the latter, hydrogen atom nuclei transform into helium atom nuclei. This process occurs with the release of energy.
Hydrogen is the 10th most common element on Earth (and less than 1 percent of Earth's total mass). It is usually found in compounds. Pure hydrogen gas rarely occurs in nature, although volcanoes and some oil wells release small amounts of hydrogen gas.
Hydrogen is present in nearly all organic compounds, or compounds that contain carbon. In a free state H2 is practically absent on Earth, since this gas is very light and nonactive it rises in the high layers of atmosphere, and then abandons the planet.
Н2О is the main compound of hydrogen on Earth. Water occupies the greater part of the earth surface, being in 3 aggregate states. Its enormous masses are contained in minerals and mountain rocks. The body of a man contains about 60% of water. An organism of a man weighing 70 kg contains about 7 kg (3500 mol) of hydrogen in the forms of Н2О and organic compounds.
Table 1.1
Selected properties characterizing the hydrogen atom and molecule
Property |
|
Ionization energy of atom, eV |
13.60 |
Affinity of atom to electron, eV |
0.75 |
Relative electronegativity |
2.1 |
Radius of atom, nm |
0.046 |
Internuclear distance in molecule, nm |
0.0741 |
Standard enthalpy of dissociation of molecules at 25 °C, kJ/mol |
436.1 |
Physical properties
Н2 has no smell and color. The formation of molecules from atoms is accompanied by the release of plenty of heat:
2Н = Н2; Но = –435.1 kJ.mol-1
that is an evidence of strong H—H bond and its high energy. Small Н2 molecules are not capable of considerable deformation. Noticeable forces of interaction between them are absent which proves extraordinarily low boiling point –252.8оС (20.4 К). Therefore, Н2 was liquefied for the first time only in 1898 by physicist Dewar (130 years after its discovery), whose name was later adopted for double walled vacuumed mirrored tubes used for storage of liquefied gases.
Н2 solubility in water is low (only 1.85 volumes per 100 volumes of water at 18оС). Hydrogen has high ionization energy (13.6 eV) and low electron affinity (0.715 eV), which confirm low chemical activity of this element.
Н2 is the lightest from the known gases: 1 m3 of Н2 weighs 0.09 kg (1 L of He is twice more heavy). The lifting force of Н2 = 1.2 kg/m3, therefore balloons are filled with Н2.
Н2 molecules are mobile and have the highest capacity of penetration through many substances. H2 passes even through quartz at high temperature. The penetrated through metallic plates hydrogen has practically absolute purity. Н2 dissolves in many metals. In particular, 1 Pd volume dissolves 900 volumes of Н2. Amazingly, dissolved in palladium hydrogen creates pressure that is close to its pressure in the liquid state at such conditions.
The names protium, deuterium, and tritium come from the Greek words for the first, second, and third, respectively. Protium and deuterium are stable. Tritium is a radioactive isotope with 12.5 year half-life. The ratio of deuterium and protium atoms in compounds of natural origin makes up 1: 6800. Tritium is found in nature in negligibly small amounts.
Nuclear-spin isomers in diatomic molecules. Spins of para-hydrogen are anti-parallel and the sum is 0 leading to a singlet state. Spins of ortho-hydrogen are parallel and the sum is 1 resulting in a triplet state.
Table 1.1
Properties of isotopic hydrogen and water
Para-hydrogen has a lower energy state and it is stable at low temperatures and the ratio of ortho-hydrogen increases at higher temperatures: the content of para-hydrogen is exactly 100% at 0 K and about 25% at room temperature.
Gas chromatography and rotational lines in the electronic band spectrum of H2 can distinguish two hydrogen isomers.
Hydrogen bonds. Hydrogen can also form a unique type of bond known as a hydrogen bond. Hydrogen bonds can be formed between hydrogen and oxygen (O), nitrogen (N), or (less commonly) fluorine (F). The most electronegative elements have ability to attract electron pair of covalent bond with hydrogen maximally. It gives opportunity of additional intermolecular interaction of the electron pair of neighbouring donor molecule with hydrogen atom that practically has no any electron:
