To check which bond is most polar, you must first find an electronegativity table (easily found by googling). You will then identify 2 atoms that make the bond, and find their electronegativity difference. Whoever has the highest electronegativity difference, is most polar.
Ex: Which bond is most polar H-Cl or C-Br?
Answer: H-Cl because it's electronegativity different is 3-2.1=0.9 which is great then that for C-Br (2.8-2.5=0.3)
Note that if a bond is made of two identical atoms such as O2, their electronegativity difference will always be zero, making the bond always non-polar.
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