Answer: The element expected to be most electronegative is Ca.
The element expected to be least electronegative is K.
Explanation:
Electronegativity is defined as the property of an element to attract a shared pair of electron towards itself.
Down the group:
The size of an atom increases as we move down the group because a new shell is added and electron gets added up.
As, the size of an element increases, the valence electrons gets away from the nucleus. So, the attraction between the nucleus and the shared pair of electrons decreases
Hence, electronegativity decreases moving from top to bottom down a group
Across a period:
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.
Hence, electronegativity increases moving across left to right in a period.
Thus as K, Sc and Ca are arranged across a period, the electronegativity order is K< Sc < Ca.
