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Less Dense
Ice floats in liquid water because it is less dense. In fact, it is about nine percent less dense. This is because when ice forms, it takes up more space -- increases in volume -- than it did when it was a liquid. Ice is less dense because of hydrogen bonding -- a type of weak bonding between a hydrogen atom and an unbonded pair of electrons from a different atom.
Why Ice Is Less Dense
Since ice is the frozen form of water, its molecules are more tightly packed together than if they were in the liquid form. This effectively removes much of the space between the molecules. Though ice may look more solid, it actually is not because it is composed of several empty spaces. This is due to the ice molecules having a triangular shape, which keeps the molecules farther apart from each other. When water freezes, it decreases in density and increases in volume. Because of the triangular molecular shape, ice is therefore lighter than the water, thus having the ability to float.
Archemedes Principle
Named after the Greek scientist, the Archimedes Principle states that a substance such as ice will float if it is less dense than other elements in the mixture. According to Innovate Us, "for an object to be able to float, the weight of the fluid it displaces should be greater than its own weight." The object's density needs to be less than that of the fluid it is immersed in for that object to float.
Polarity
A water molecule is slightly polarized on either end. This is due to the molecule's atomic structure. The two hydrogen atoms form a "V" with the oxygen molecule. In doing so, the hydrogen side becomes slightly positive, while the oxygen side is slightly negative. This polarity is why water has such a strong surface tension. The positive side will attract a negative side, thus forming a strong covalent bond.