It's not quite accurate to say a molecule "becomes more negative." Instead, we talk about changes in electron density within a molecule, which can make certain parts of the molecule more electronegative than others. Here's a breakdown:

* Electronegativity: This is a measure of an atom's ability to attract electrons towards itself in a chemical bond. Atoms with higher electronegativity tend to pull electrons closer, creating a partial negative charge on themselves.

* Polar Molecules: When two atoms with significantly different electronegativities bond, the electrons are pulled towards the more electronegative atom. This creates a polar molecule, where one end has a partial negative charge (δ-) and the other end has a partial positive charge (δ+).

* Electron Density: The "more negative" aspect you're referring to is related to the concentration of electrons in a molecule. When electrons are pulled towards a particular atom or region of a molecule, the electron density increases, making that area more electronegative.

Here's a simple example:

* Water (H2O): Oxygen is more electronegative than hydrogen. This causes the shared electrons in the O-H bonds to be pulled closer to the oxygen atom, giving it a partial negative charge (δ-) and the hydrogens a partial positive charge (δ+).

Important Note: Molecules don't gain or lose electrons to become more negative. Instead, the distribution of electrons changes due to differences in electronegativity, creating areas of higher and lower electron density within the molecule.