Ionic compounds are formed from ionic bonds, whereby an electron is transferred from the metal cation to the nonmetal anion. Ions form solid lattices of ions. Covalent compounds form solids through the attraction of two covalent molecules. Since the attraction between two covalent molecules is weak compared to the ionic bonds holding an ionic compound together, ionic compounds tend to have higher melting points.
Which of the following compounds has the highest boiling point?
When two bonded atoms attract electrons with equal strength, the result is a nonpolar covalent bond. A polar covalent bond is one in which the electrons are unequally shared between the atoms. An ionic bond results when the sharing is so unequal that fully charged ions form. Electronegativity difference can be used to predict bond type. If electronegativities differ by more than 2 units, the bond is substantially ionic; if they differ by less than 2 units, the bond is polar covalent; and if the values are equal, the bond is nonpolar covalent.
If you are not given electronegativity values, you can still predict the bond type using the periodic table. Metals have low electronegativity compared to nonmetals. So in general, we can predict that any metal–nonmetal combination will be ionic and any nonmetal–nonmetal combination will be covalent. If electronegativity values aren't given, you should assume that a covalent bond is polar unless it is between two atoms of the same element.
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