Formation of soap bubbles:

A soap bubble is a very thin sheet of water sandwiched between two layers of soap molecules. The film of soapy water surrounds a bubble of air. Soap molecules have one end that repels water, and one that attracts it, and these molecules move to the inner and outer surfaces, thrusting their water-repelling ends out in the air, and their “heads” inwards. Without such molecules on the surface, the bubble would spontaneously break apart into tiny water droplets.

The spherical shape of soap bubbles:

Surface tension causes the bubble to be spherical, which is the shape that gives the minimum surface energy – the lowest ratio of surface area to volume.

Soap bubbles expand till the pressure of air in them is equal to the atmospheric pressure.