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. Hydrogen
Atomic Symbol: H
Atomic Number: 1
Atomic Weight: 1
Melting Point: -14.01 K
Boiling Point: 20.28 K
Density: 0.09 grams per liter
Phase at Room Temperature: Gas
Element Classification: Gas
Period Number: 1
Group Number: 18
Group Name:
Depends... (Noble Gas?)
Greek word
for water forming, hydro genes
Sounds like
A colourless, odourless gas that burns and can form an explosive mixture with air. Some see hydrogen gas as the clean fuel of the future - generated from water and returning to water when it is oxidised.

The first person to write about making hydrogen gas was a 16th century alchemist named Theophrastus von Honhemheim, better known as Paracelsus. A century later, in 1671, another alchemist, Robert Boyle, also described hydrogen gas as a colourless gas given off when he added acid to iron filings.

In 1766 a scientist called Henry Cavendish collected this colourless gas and burnt it. He observed that water was made when the gas burnt in the air. It was a French chemist, called Antoine Lavoisier, who named the gas hydrogen.

Hydrogen combines with every element in the periodic table except the nonmetals in Group VIIIA (He, Ne, Ar, Kr, Xe, and Rn). Although it is often stated that more compounds contain carbon than any other element, this is not necessarily true. Most carbon compounds also contain hydrogen, and hydrogen forms compounds with virtually all the other elements as well. Compounds of hydrogen are frequently called hydrides, even though the name hydride literally describes compounds that contain an H- ion.

Hydrogen is the only element that forms compounds in which the valence 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.

Because hydrogen forms compounds with oxidation numbers of both +1 and -1, many periodic tables include this element in both Group 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 the elements 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 of a metal. (It has been argued, for example, that any hydrogen present at the center of the planet Jupiter is likely to be a metallic solid.) Finally, hydrogen combines with a handful of metals, such as scandium, titanium, chromium, nickel, or palladium, to form materials that behave as if they were alloys of two metals.

there are equally valid arguments for placing hydrogen in Group 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), the way a nonmetal should. Finally, the element is a gas at room temperature and atmospheric pressure, like other nonmetals (such as O2 and N2).

It is difficult to decide where hydrogen belongs in the periodic table because of the physical properties of the element. The first ionization energy of hydrogen (1312 kJ/mol), for example, 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 classify hydrogen as a semimetal.


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