Acid and base strength

An acid is a molecule or ion capable of donating a hydronium (proton or hydrogen ion H+), or, alternatively, capable of forming a covalent bond with an electron pair (a Lewis acid).

The first category of acids is the proton donors or Br?nsted acids. In the special case of aqueous solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Br?nsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Br?nsted or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H+
In the early 1900s, an alternate definition for acids and bases was proposed by Johannes Br?nsted and Thomas Lowry to account for the fact that ammonia can neutralize the acidity of HCl even if water isn t present. This phenomenon showed them that ammonia is a base, even when there isn t water around to form hydroxide ions.
There are many different names and formulas used to describe the hydronium ion. Though the formula was shown previously as "H+", it is sometimes written as "H3O" because this is the ion formed when H+ combines with water. Another common way of referring to hydronium ions is just to call them "protons." This name comes from the fact that H+ represents a hydrogen atom (one proton and one electron) that has lost its electron, leaving only the bare proton behind.

A Br?nsted-Lowry acid is defined as a compound that gives hydronium ions to another compound—for example, hydrochloric acid gives H+ ions to compounds it reacts with. Br?nsted-Lowry bases are compounds that can accept hydronium ions—when ammonia gets a hydronium ion from HCl, it forms the ammonium ion.