Arrange F, Cl, O, and N in the decreasing order of electronegativity.
F > O > N > Cl
Lets write electronic configuration of concerned elements.
F → 1s22s22p5 (2ndperiod)
O → 1s22s22p4 (2nd period)
N → 1s22s22p3 (2nd period)
Cl → 1s22s22p63s23p5 (3rd period)
Electronegativity depends on size of atom and nuclear charge.
Atomic Radius: The greater the size of the atom, the lesser the electronegativity, since the electrons being farther away from the nucleus, experience a lesser force of attraction.
Effective Nuclear Charge: The greater the nuclear charge, greater will be the electronegativity because the increase in nuclear charge causes electron attraction with a greater force.
Since the nuclear charge increases due to an increase in atomic number, electronegativity increases from left to right in a period.There is an increase in atomic number down a group, i.e., nuclear charge increases, but due to the addition of extra shells, the atomic size increases. The effect of an increase in the atomic size overcomes the effect of an increase in the nuclear charge.Hence, electronegativity decreases down the group.
F > O > N > Cl (Electronegativity)