Anonymous
Anonymous asked in Science & MathematicsChemistry · 5 months ago

# Why is helium lighter than air?

Relevance
• Because an atom of helium weighs less than any one of the several molecules that make up air.

• Density=mass÷volume

Hence density is directly proportional to mass,so suppose you have taken a)1 litre of air and(b) 1 litre of Helium,You will find that weights.You will weigh them as air has other heavier molecules like Nitrogen ( Proton +neutron =14 amu) Where as Helium will be lighter as atomic mass (Proton+ neutron=4)

Hence 4 is less than 14,so helium is lighter than air.

Note Apart from Nitrogen 78% air contains 21%Oxygen and.04%CO2,If we take these gases into account then for air it will be more than 14 also

• it's density is less. so it rises. why is it's density less?

.. PV = nRT

.. . .n = mass / mw

.. PV = (mass / mw) RT

.. mass / V = mw * P / RT

since density = mass / V

.. density = mw * P/RT

at the same P and T... (we're assuming the gas is ideal and thoroughly mixed)

.. density He = mw He * RT/P

.. density air = mw air * RT/P

.. density He / density air = mw He / mw air

.. density He = density air * mw He / mw air

since air is 79% N2 and 21% O2

.. mw air = 0.79 * 28 + 0.21 * 32 = 29 g/mol

and we know molar mass He = 4 g/mol so..

.. density He = (4 / 29) * density air

***********

Helium is "lighter" than air because the particles of of He have lower mass then the particles of air which results in lower density for He

• Or, cutting to the chase, for the same reason that wood floats on water and rocks sink.

• The molar mass (weight) of Helium is lower than that of N2. Gas at low pressures such as the atmosphere are very diffuse and behave pretty close to ideality (as independent particles that have a unit density that is identical for any gas at the same pressure and temperature). Thus, the number of atoms/molecules per unit volume are essentially the same without regard to what particular gas we are discussing. This means that the mass per unit volume (density) varies solely as the unit mass of the particle, and helium has a unit mass of 4 compared to a unit mass of 28 for N2. Even if you double the unit mass of He to offset the diatomic nature of N2 (presume that there are twice as many "particles" of He as N2 in a given volume), the unit mass would be 8.

8 is a lot smaller than 28.

• Air is a mixture of elements. Helium is just one element. Hence air is heavier.

• MysteryGuy
Lv 5
5 months agoReport

you are the dr. elaborate instead of keeping me waiting

• Because it consists of isolated atoms, not reacting with anything and therefore unable to bond with each other, of four nucleons and two electrons, whereas air is a mixture of gases, mainly nitrogen and oxygen, consisting of two atoms per molecule, each of which has fourteen or sixteen nucleons (assuming the most common isotopes) and seven or eight electrons, so it's around seven or eight times heavier.

• In any liquid or gas, molecules having the less "weight" will float over the others with the heavier "weight". A good example is oil that floats over water in a container or even the sea (during oil leakages).

Helium is made of 2 protons and 2 neutrons in its nucleus, and 2 electrons spinning around the nucleus. It is the second lightest element after hydrogen, the latter being the lightest with only 1 proton and 1 electron.

And as we know, objects with smaller densities than water will float on the water (like wood─not soaked for long periods in the water otherwise they will submerge too).

Helium, therefore, will just go up and up to pass all the heavier molecules in the atmosphere that surround it.

• ANDY
Lv 5
4 months agoReport

You're right. Just wanted to give an example how lighter objects go above heavier objects.

• Air is 78% nitrogen, 21% oxygen, 0.9% argon and then other gases and vapors make up the other 0.1%. If you take the same number of molecules of any gas and give it the same pressure and temperature, they will take up the same volume. So, one mole of Hydrogen gas has a mass of about 2 grams. One mole of Helium has a mass of about 4 grams. One mole of Nitrogen has a mass of 28 grams. One mole of Oxygen has a mass of 32 grams. If 2 objects take up the same amount of space, then the object with the greater mass will settle lower than the other object. Helium isn't floating up as much as it is pushed up by the heavier gases that settle as close to the earth's center of mass as they can get. Because Helium doesn't bond with other elements, there's no real force to keep it bound close to the planet's surface. It just keeps getting pushed up higher and higher until it reaches a point in the upper atmosphere where it just kind of expands into the vast emptiness that is space, never to return.

So think about how precious of a resource it is, and how necessary it is for medical and industrial applications. And then question why the average person can purchase it so readily and cheaply at a party supply store, given that we can't really make any more of the stuff (not until we have fusion reactors up and running, that is), so what little we have, we should be conserving. And then ask yourself why there's no market for Helium futures, despite the fact that we know that it will only become more and more precious as time moves forward. You can buy futures in precious metals, but not in helium?

There's a whole lot of stuff to unpack once you start asking questions.

• Anonymous
5 months ago

because the hydrogen and helium atoms are lighter than a nitrogen atom. ... Approximately the same number of atoms of each of these elements fills approximately the same amount of space. Therefore, the gases made of lighter atoms are lighter