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Corrosion, corrode
Corrosion, corrode
The oxidation of metals in air
and water (see: Oxidise). The most important form of corrosion is the rusting of iron.
(For elements resistant to
corrosion see: Cadmium; Chromium; Copper;
Gold; Lead; Manganese; Molybdenum; Nickel; Palladium; Platinum; Tantalum; Tin; Titanium.)
Crystalline
Made of crystals that have not developed geometric shapes.
 Crystalline – This is crystalline halite, a sodium salt.
 Corrosion – This iron horseshoe shows a common form of corrosion known as rusting.
Curium (Cm)
Element 96 on the periodic table. A silvery transition metal belonging to the actinide series.
Curium is artificial and was discovered in 1944 by Glenn T. Seaborg, Ralph A. James and Albert Ghiorso. It is a transuranium element, radioactive and is used in space vehicles to provide a compact, long-lived source of electricity. It was named for Marie Curie (see: Radium).
Covalent bond
The most common form of strong chemical bond, it occurs when two atoms share electrons.
Many non-metal atoms bond in this way, for example, hydrogen atoms bond covalently to make the molecule hydrogen (H2). By sharing electrons, the outer shell of each hydrogen atom (which has only one electron) becomes full
(it contains two electrons) and so
is stable. The same is the case for chlorine, which has seven electrons in its outer shell, but becomes stable when it has eight electrons.
Covalent bonding can also occur in compounds. For example, water
is made of two atoms of hydrogen covalently bonded to one atom of oxygen to make the water molecule (H2O); methane gas (CH4) is a carbon atom covalently bonded to four hydrogen atoms.
Covalent bonding produces small, easily separated molecules. That is why substances that are gases are covalently bonded. Liquids or solids with low melting and boiling points are also likely to be covalent. Most covalently bonded molecules are also soluble in organic solvents but are generally insoluble in water. Common exceptions include
oxygen, chlorine, ammonia, sugar and alcohol.
(See also: Ionic bond and Metallic bond.)
 Covalent bond – Certain elements exist as molecules in which their atoms are held together by covalent bonds, for example, oxygen (O2).
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