You have a wrong notion about moment....It seems you havent learnt about moment in physics.
Moment is the technical name given for turning effect produced by a force about a fixed point.
Lets take a simple example, a force when applied on the edge of a door causes it to turn. The direction of turning may be clockwise or anticlockwise.
In molecules, electrons are taken as the object on which turning force acts, resulting in a moment in the molecule, as a whole. Now, molecules consist of 2 or more atoms. If the charge on the nucleus is different on different atoms(a property known as electronegativity), then they attract the electron (or a pair of electrons) with unequal forces. The net force acting on the electron(s) will not be zero. So, they will be attracted towards the atom with greater positive charge. As a result, this atom will gain a partial negative charge whereas the weaker atom will develop a partial positive charge.
Hope im able to clear your misconception..... pls ask me for further doubts.