What does it mean for an atom to be electronegative? how does this affect the shape and overall charge of a water molecule?

Electro-negativity is the tendency of an atom to attract the shared pair of electron in a covalent bonding while forming a molecule.

Explanation:

In a covalent bonding the atoms share the electrons mutually without actually transferring it to one of the atom to gain a stable configuration. In such a case when one atom is more electro-negative than the other then the shared pair of electron gets slightly shifted towards the more electro-negative developing a denser cloud of electrons near the nucleus of that atom which we denote as partial negative () in the superscript.

In water molecules we have a similar case where the oxygen atom is more eletro-negative (value=3.44) than the hydrogen atom (value=2.2). This leads to a higher electron density near the oxygen atom.  A part from the shared pairs, the oxygen atom also possesses 2 lone pairs of electrons which tend tp makethe shape a tetrahedron in a 3D structure. This results in molecular shape being bent with an H-O-H angle of 104.5°.

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