Question

Which element is the most electronegative among C, N, O, Br, and S ?

Answer 1

Answer
Electronegativity increase ACROSS a Period, but decreases down a Group.
Explanation :
As we go across the periodic table from left to right, we add a proton (a positive nuclear charge) to the nucleus and an electron to the valence shell. It turns out that electron-electron repulsion is inferior to the nuclear charge, and as we cross the Period from left to right ATOMS get markedly smaller, due to the increased nuclear charge
Now electronegativity is conceived to the ability of an atom in a chemical bond to polarize electron density towards itself (please note that I cannot speak of the electronegativity of individual atoms on this basis, I can only speak of the electronegativity of atoms involed in chemical bonds!). There are various scales, which draw on various parameters, of which the Pauling scale is the most famous.
You will have to look up the Pauling scale yourself, but the O atom, first row, and rightmost on the table, will be most electronegative, followed by nitrogen and bromine.