A helium balloon rises because its weight is lighter than the air it displaces. The helium (a gas lighter than air) inside the balloon, combined with the weight of the rubber balloon, is less than the weight of the air that takes up the same amount of space. Like a marshmallow in a cup of hot chocolate, the balloon will rise to get above the heavier air.
Balloons are not perfect and the helium in a balloon will gradually leak out. As the helium amount decreases and the size of the balloon shrinks, it takes up less space and therefore displaces less air. Because the rubber in the balloon weighs the same, the weight of the balloon plus its decreasing amount of helium eventually equals the weight of the displaced air. At this point in time, the balloon no longer rises and stays at a constant height. As more helium leaks out, the weight of the balloon plus helium now weighs more that the air it displaces. As a result, the balloon begins to fall.
Balloons are not perfect and the helium in a balloon will gradually leak out. As the helium amount decreases and the size of the balloon shrinks, it takes up less space and therefore displaces less air. Because the rubber in the balloon weighs the same, the weight of the balloon plus its decreasing amount of helium eventually equals the weight of the displaced air. At this point in time, the balloon no longer rises and stays at a constant height. As more helium leaks out, the weight of the balloon plus helium now weighs more that the air it displaces. As a result, the balloon begins to fall.