Aluminium is the most abundant of all metals and the third most abundant
element in the Earth's crust, after oxygen and silicon. It makes up about 8%
by weight of the Earth's solid surface. Aluminum is too reactive chemically
to occur in nature as the free metal. Instead, it is found combined in over
270 different minerals . The chief source of aluminium is bauxite ore.
Aluminium is remarkable for its ability to resist corrosion (due to the
phenomenon of passivation) and its light weight. Structural components made
from aluminium and its alloys are vital to the aerospace industry and very
important in other areas of transportation and building.
In an ammonium ion, the positively charged nitrogen atom forms four
covalent bonds, instead of three as in ammonia. This reaction is reversible.
The ammonium ion can act as a very weak Brønsted-Lowry acid in the
sense that it can protonate a stronger base using any one of its hydrogen (
H ) atoms and convert back to ammonia. This means that the ammonium ion is a
conjugate acid of the base ammonia. In a solution, the degree to which
ammonia forms the ammonium ion depends on the pH of the solution.
Barium is a metallic element that is chemically similar to calcium but more
reactive. This metal oxidizes very easily when exposed to air and is highly
reactive with water or alcohol, producing hydrogen gas. Burning in air or
oxygen produces not just barium oxide (BaO) but also the peroxide. Simple
compounds of this heavy element are notable for their high specific gravity.
This is true of the most common barium-bearing mineral, its sulfate barite
BaSO4, also called 'heavy spar' due to the high density (4.5 g/cm³).