I believe the z effective (or the effective nuclear charge) is the charge experienced by an electron due to the neutrons in the nucleus. If there are other electrons in between the nucleus and the electron is question, it lowers the amount of force felt by the electron. Mathematically it can be described as:
Z effective = N (number of neutrons) - S ( no of electrons between the nucleus and the electron in question)
Now in terms of electronegativity this means that if an electron has low effective nuclear charge (less force) it would obviously be easy to take this electron away from the atom. Thus low electronegativity. Hence, as the effective nuclear charge decreases, electronegativity decreases as well.
I hope this was helpful 🙂
