This topic is weird to think about, so I
completely get where you're coming from. Hopefully
I can do my best to help everyone understand.
To verify what you read, the gravitational
attractive force between the earth and air atoms
is greater than with earth and helium atoms
(as long as they are the same distance away from
the center of earth.) Gravitational force between
two objects (be it the earth and air, earth and
helium, or any other two objects) depends on
masses of both objects, as well as the
distance between those objects. This is the
equation (I know, bare with me) to describe
F = G * m1 * m2 / r2
F = gravitational attraction
G = gravity constant
m1 = mass of object 1
m2 = mass of object 2
r = distance between the two objects
This equation puts into context how gravity
works, and why masses of heavier objects like
those in air (oxygen, nitrogen, argon) will have a
stronger gravitational force exerted upon
them by the earth when compared to helium.
However, like you may also understand, helium
rises in air also because of the buoyancy
effect. All liquids and gases in the presence
of gravity exert an upward force—called
buoyancy—on any object immersed in them. If the
object is less dense (mass per volume) than the
liquid or gas, buoyancy will make it float. For
example, a cork floats in water because it is less
dense than a cork-size volume of water. But it
won’t float in air because it is denser than the
same volume of air.
Both gravitational attraction, as well as
the effects of buoyancy, are important when
considering why a helium balloon floats. If
you stress why gravity works, as well as
why something can float (based on density),
that should be a good way to get the message
across to your students.
Hopefully this helps.
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