What does it mean? how do i know waht it is? how do i find it? And can you please give a number of examples? thanx in advance!
"Periodicity" is merely the noun form of the adjective "periodic". Periodicity refers to something which is "periodic", something which happens the same way over and over, in cycles. If a property is periodic, then we can talk about the periodicity of that property.
For instance, you can talk about the periodicity in the arrival of a newspaper or a magazine, although the period is different (1 day vs 1 month). In chemistry the properties of the elements are periodic if we arrange the elements by increasing atomic number. By arranging the elements with similar chemical properties so that they fall into vertical columns, we have the periodic table.