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Ok electronegativity is the basically how much the element wants an electron. What makes elements electronegative? Their desire to complete the 8 valence electrons on their outermost level. Ok. the electronegativity increases as you go from left to right. Because for Na, it has one electrons, so if it loses that one, the outermost level would disappear and the next level would be the outermost, which has 8 electrons! So Na completes octet by losing an electron and that's why it has very low electronegativity, it doesn't want to gain an electron. flourine has the highest electronegativity because it has 7 valence electrons. It only needs one more to complete 8! so it will do anything to get one electron and that's why it has the highest electronegativity.