In a molecule, such as a water molecule (H2O), the unequal sharing of electrons between atoms is primarily due to differences in electronegativity. Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond.
In the case of water (H2O), it consists of two hydrogen (H) atoms and one oxygen (O) atom. Oxygen is more electronegative than hydrogen, meaning it has a greater tendency to attract electrons. This is because oxygen has a higher number of protons in its nucleus and a stronger positive charge compared to hydrogen. As a result, the electrons in the covalent bonds between oxygen and hydrogen atoms are more strongly attracted to the oxygen atom.
Because of this unequal sharing of electrons, the oxygen atom ends up with a partially negative charge (δ-), while the hydrogen atoms have partially positive charges (δ+). This is represented as a partial negative charge on the oxygen (δ-) and partial positive charges on the hydrogens (δ+).
The unequal distribution of charge within the water molecule leads to its unique properties, such as its ability to form hydrogen bonds, which are essential for various chemical and biological processes. These partial charges are responsible for the strong intermolecular forces between water molecules, which give rise to properties like high surface tension, high heat capacity, and the ability to dissolve many substances.