What causes a molecule to have a net dipole moment

answer and explanations:

dipole moments occur when there is a separation of charge. Dipole moments occur due to atoms electronegativity, where one atom has the ability to attract electrons towards it giving the electrons a negative and a positive charge.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.