Answer: electronegativity
Step-by-step explanation:
Electronegativity is defined as the property of an element to attract a shared pair of electron towards itself.
The size of an atom decreases as we move across the period because the electrons get added to the same shell and the nuclear charge keeps on increasing. Thus the electrons get more tightly held by the nucleus.
As, the size of an element decreases, the valence electrons come near to the nucleus. So, the attraction between the nucleus and the shared pair of electrons increases and thus the electronegativity increases.