Protium would have the highest electronegativity value followed by Deutrium & then finally tritium ..
The electronegativity decreases in the order..
This is because electronegativity is the tendency to attract the shared pair of electrons towards a particular atom..Now for Protium(1H1) it has one proton(+ve charged) & so it could attract the electrons effectively towards itself but in deutrium there is an added neutral charge in the form of a neutron which increases the size thus reduces the electronegativity tendency of the deutrium atom & similarly for tritium..!!
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