Why are the molecules of hydrocarbons nonpolar?

Hydrocarbon means the molecule is made of nothing but hydrogen and carbon. These are both nonpolar atoms, unlike other atoms like oxygen, nitrogen, and fluorine. These three are very polar and can take away some of the electrons from other atoms in a molecule

Jujab3ar isn't really right there. Atoms aren't classed as polar or non-polar - they are however ranked in order of electronegativity, which affects the polarity of the chemical bonds they make with each other. In the periodic table,, electronegativity increases up the table and along towards Fluorine, the most electronegative atom. The reason hydrocarbons are non-polar is twofold: firstly hydrogen and carbon have very similar electronegativities, so the C-H bond (and consequently C=H etc) is virtually non-polar. Also the shape around each carbon atom in an alkane is tetrahedral, with the bonds arranged such that any bond dipoles cancel each other out across the molecule (trigonometry planar shapes in alkenes also exhibit this in general). I hope this helps you :)

Copper, silver, aluminum, cobalt, chromium, mercury