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CHEMICAL ELEMENT

The concept of chemical element is related to that of chemical substance. A chemical element is characterized by a particular number of protons in the nu­clei of its atoms. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium. However, several isotopes of an element, that differ from one another in the number of neutrons present in the nucleus, may exist.

The most convenient presentation of the chemical elements is in the periodic table of the chemical elements, which groups elements by atomic number. Due to its ingenious arrangement, groups, or columns, and periods, or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as atomic radius, electronegativity, etc. Lists of the elements by name, by symbol, and by atomic number are also available.

Hydrogen (H) is the first element in the periodic table because it has just one proton in its nucleus. Helium (He) is second, because it has two protons, and so on. The periodic table can be coloured-coded. Often, each group is given a particular colour so that it is easy to pick out all the elements that belong to a particular group. As well as a name, each element has a symbol. Often this is the first letter or two of the element's name, but it can come from a Latin name. Each also has an atomic number and a mass number.

Gallium. One element that Mendeleyev left a gap for in his periodic table was gallium (element 31). Mendeleyev called it eka-aluminium because he predicted it would have similar properties to aluminium. In 1875, French scientist Lecoq de Boisbaudran discovered gallium. It has the exact properties that Mendeleyev pre­dicted. Gallium is a soft, silvery metal with a melting point of 29.8°C (85.6°F).

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