Question

Why does the electronegativity increase as you move across a period?

Answer 1

Because of increased nuclear charge, i.e. increasing `Z_"the atomic number"`

Explanation:

Two factors underlie the structure of the modern Periodic Table: (i) `Z_"the atomic number"`; and (ii) shielding by other electrons.

Incomplete electronic shells shield the nuclear charge VERY ineffectively, and this results in the well-known diminution of the radii of atomic nuclei, from LEFT to RIGHT as we face the Table.

And as a result elements towards the right of the Periodic Table, fluorine, oxygen, nitrogen, should have the most effective nuclear charge...and as a consequence, they should tend to polarize electron-density towards themselves when they are chemically bound to other atoms. And here we have described the concept of electronegativity.