Helium rises because it is less dense than the air. If it is less dense then it will rise above the air until it meets a gas lighter than it. There are other gases lighter than air. You should...
The helium ballon rises in air because the density of helium inside the balloon is less than that of the surrounded air that is displaced by the balloons volume. Ice cubes float in a glass of liquid...
Helium is used in balloons because it is much less dense than air, allowing the balloons to float and is completely nontoxic and nonflammable unlike hydrogen, which is otherwise a superior lifting...