How does the size of oxygens nucleus affect the distribution of electrons in the water molecule

The oxygen atom's nucleus is composed of eight neutrons and eight protons. The protons that are present have a positive charge, which attracts the electrons within the water molecule and pulls them closer.
This way, the negative charge of the water molecule is concentrated around the oxygen atom, creating a partial negative charge. Meanwhile, the hydrogen atoms are left with a partial positive charge as electrons have moved away from them.

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