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Polarity is determined by the "vectors" of electronegativity of the atoms in the compound. Electronegativity is theoretically related to the pull of an atom's nucleus for the electrons in its outer most energy shell. In practice, it means how much an atom wants to pull additional electrons towards it within a compound. Electronegativity is a relative measure and is important in compounds for determining polarity. If there are 2 atoms in a compound with different electronegativities, the compound will be polar because the pull for electrons is geometrically imbalanced towards one atom or the other. In more complex compounds, you have to take into account the 3 dimensional geometric arrangement of the atoms. This is because electronegative atoms pull electrons in general directions. If there are 4 atoms in a compound and 3 of the atoms surrounding a central atom have a greater electronegativity, they each pull electrons away from the central (less electronegative) atom. If these atoms are arranged in such a way that they are pulling electrons in equal but opposite directions, the compound will be non-polar (ex: tetrahedral symmetry). If the pull is imbalanced, the compound will be polar.
Polar likes polar, and nonpolar likes nonpolar. There's actually a joke about it: "Do you know why the bear dissolved in water?" Answer: "It was a POLAR BEAR!"