How does electronegativity change as effective nuclear charge increases?

Effective nuclear charge is how attracted on particular electron is to the nucleus. It can be estimated by

#Z_(eff) = Z - S#

where #Z_(eff)# is the effective nuclear charge, #Z# is the atomic number and #S# is the number of "shielding" electrons.

Shielding electrons are the electrons in shells closer to the nucleus than the electron you're interested in. Because they are all negative, if you have a lot of electrons between one electron and the nucleus, the one electron will be repelled by the electrons as well as attracted by the nucleus, which will cancel out, and there will not be a great effective nuclear force. This is known as the shielding effect.

Electronegativity is how much an atom pulls in electrons. This, as you can see, is due to its effective nuclear charge, which is how strongly a particular electron is attracted to the nucleus. If there is a great effective nuclear charge, there will be a great attraction, and so a great electronegativity. If there are a lot of shielding electrons, outer electrons won't be drawn towards the nucleus due to the low effective nuclear charge, and so electronegativity will be low.

Effective nuclear charge is directly proportional to electronegativity. An increase in one means an increase in the other.