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The metallic hydrogen The metallic hydrogen There is an ambiguity in the position of hydrogen in the periodic due to its halogen like behavior and alkali metal like behavior. Hydrogen is a diatomic gas at STP and bad conductor of electricity. Interestingly when hydrogen is subjected to high pressure of around 25 GPa (250,000 atm) hydrogen atoms behave as metal atoms as they loose their hold over their electrons. At this condition, hydrogen exists as liquid rather than solid and has high electrical conductivity like alkali metals. Liquid metallic hydrogen is thought to be present in large amounts in the gravitationally compressed interiors of Jupiter, Saturn and some of the newly discovered extra solar planets. Metallic hydrogen is used as fuel in rockets because when recombining to molecular form it would release about 20 times the energy of hydrogen burning in oxygen.

Learning Objectives

After completing the topic, the student will be able to:

  • Explain the reason for ambiguity in the position of hydrogen in the periodic table.
  • List the isotopes of hydrogen and compare their characteristics.
  • Give the preparation methods of hydrogen and discuss the physical and chemical properties of hydrogen.
  • List the uses of hydrogen and describe the role of hydrogen in a fuel cell.
  • Discuss the chemical and physical properties of water.
  • Define heavy water and compare the properties of normal water and heavy water.
  • Describe the structure of hydrogen peroxide and give the preparation methods of hydrogen peroxide.
  • Illustrate the oxidizing and reducing behavior of hydrogen peroxide with examples and list the uses of hydrogen peroxide.
 Abundance of hydrogen in universe Abundance of hydrogen in universe Hydrogen is the lightest element. It is the most abundant element in the universe and makes up about 73% of the universe by weight. It is also the most abundant element in the sun.
Hydrogen and its compounds

The English chemist Henry Cavendish (1731‐1810) first isolated pure hydrogen. Because the element produces water when burned in air, the French chemist Lavoisier gave it the name hydrogen, which means 'water producer' (Greek Hydro, water; Gennao, to produce).

Hydrogen is the most abundant element in the universe. It is the nuclear fuel consumed by our sun and other stars to produce energy. Although about 70% of the universe is composed of hydrogen, it constitutes only 0.87% of Earth's mass. Most of the hydrogen on our planet is found associated with oxygen. Water, which is 11% hydrogen by mass, is the most abundant hydrogen compound. Hydrogen is also an important part of petroleum, cellulose, starch, fats, alcohols, acids and a wide variety of other materials.

Hydrogen atom consists of a nucleus with a single positive charge, surrounded by a single electron. Despite its simple structure, hydrogen is the most important element of all. In the Sun, hydrogen (H) nuclei combine to form helium (He) nuclei in a process that provides nearly all Earth's energy. Hydrogen is the most abundant element in the human body as it is the main constituent of water.

Position of hydrogen in the periodic table
Position of hydrogen in the periodic table Position of an element in periodic table depends upon its electronic configuration and properties. Hydrogen resembles the elements of group I–A, IV–A and VII–A in some respects.
Properties of hydrogen do not completely match any one of the above–mentioned groups. That is why position of hydrogen is still undecided. But its position is usually shown above the alkali metals due to the presence of one electron in its valence shell as alkali metals.
Position of hydrogen in the periodic table

Hydrogen has no perfectly suitable position in the periodic table. Because of its single valence electron and common +1 oxidation state, some think that it almost fits in Group 1A (1). However, unlike the alkali metals, hydrogen shares its electron with non‐metals rather than loosing it to them. Moreover, like a non‐metal, it has a relatively high ionization energy (IE = 1311kJ/mol) much higher than that of lithium (520kJ/mol), whose IE is the highest in Group 1A. Hydrogen has a much higher electronegativity of 2.1 with the electronegativity of Li being 1.0.

Others think hydrogen almost fits in Group 7A (17). Like the halogens, it occurs as diatomic molecules and fills its outer level either by electron sharing or by gaining one electron from a metal to form a monoatomic anion, the hydride ion H, with charge of −1. However, hydrogen has a lower electronegativity value of 2.1 which is less than any of the halogens which range from 2.2 to 4.0 and lacks their three valence electron pairs. Moreover, the H ion is rare and reactive, whereas halide ions are common and stable.

And still others think that, because of a half‐filled valence level and similarities in ionization energy, electron affinity, electronegativity and bond energies, hydrogen fits best in Group 4A (14). Hydrogen's unique behavior is attributable to its tiny size. Hydrogen has a high ionization energy because its electron is very close to the nucleus, with no inner electrons to shield it from the positive charge. It has a low electronegativity (for a non‐metal) because it has only one proton to attract bonding electrons.

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