An understanding of the chemical mechanisms that give rise to the properties of acids evolved from a number of different theories of the nature of acids. Arrhenius proposed the Arrhenius Concept of Bases that an acid is a substance which provides hydrogen ions as a result of dissociation and ionization in aqueous solution. Lewis proposed the Lewis Theory of Acids that there is a reciprocal relationship between acids and bases, and he introduced the concept of Lewis Conjugate Acid-Base Pairs. Bronsted and Lowry proposed the Bronsted Lowry Theory of Acids that acids are proton donors and that bases are proton acceptors. Allotrope An allotrope of an element is one of the forms in which the element can exist.
Sulphur can exist as five different allotropes.
Alloy An alloy is a solid solution of one metal in another. It is an example of a mixture, as no chemical bonding exists between the constituent elements in the alloy. It haves the same structure as the nucleus of a helium atom, consisting of two Protons and two Neutrons. O- Amides are volatile solids. Amines Amines are organic compounds containing carbon, hydrogen and nitrogen which are derived from ammonia by replacing one or more of the hydrogen atoms by alkyl, alkenyl, alkynyl, or aryl groups.