Electronegativity
The above equation only calculates the difference in electronegativity between two elements. To make a scale from the equation, a reference point has to be chosen. Hydrogen was chosen as a reference point because it bonds covalently with many elements. Hydrogen's electronegativity was first fixed at 2.1, but was later changed to 2.20. Another thing that need to be known to make an electronegativity scale is which element is more electronegative than the reference point, which is hydrogen. This is often done with the so-called "chemical intuition": in the above example, hydrogen bromide (H-Br) dissolves in water and breaks into H+ cation and Br- anion. So, it can be assumed that bromine is more electronegative than hydrogen.