why are there 120° in a water molecule?*

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*Between the center of the oxygen atom and the center of the two hydrogens.

In: Chemistry
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The oxygen in the middle has a lone pair of electrons, so the hydrogen repel those electrons. Which causes the angle of 120

There are four positions around an oxygen atom that will hold electrons or bonds with other atoms (in this case, hydrogen). Since the atom itself is in a 3D space, the angle between each pair of these spaces is 120 degrees. So when there are two hydrogens bonded to an oxygen, they will sit in two of those spots, making a 120 degree angle. The other two spots are “occupied” by two pairs of oxygen’s native electrons.

Actually it’s an angle of 104.5°. u/Moscau50 has the 3D geometrical picture correct, but because two pairs of oxygen electrons repel the bonding electrons more strongly (think – take up more space) the two O-H bonds get squashed together a little so the angle is a bit less than the 120° that a symmetrical structure would adopt.