Question

define periodicity? and define one nucleon?

Answer 1

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the quality or character of being periodic; the tendency to recur at intervals.
a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines an isotope's mass number (nucleon number). ... Nucleons sit at the boundary where particle physics and nuclear physics overlap.

Answer 2

Similarity in properties of elements after particular intervals in the periodic table is called periocity.
One nucleon is the sum of one proton and one neutron.

Hope this helps.