I) But haven't masses in vacuum not the same attraction and speed.
No. Their weights are different, so they are not "attracted" / pulled in by gravity equally.
Think of this: If you find 100 heavy perfectly round stones, and you put 5 plastic balls full of air with exactly the same size in the basket with them, what will then happen when you shake them a bit? Will the lighter plastic balls fall to the bottom or "float" to the top?
They will float to the top.
The point simply is that it is easier for helium atoms to move up than for air molecules. If you shake the basket violently, the stones might jump a bit while the plastic balls can jump much higher. So on average, the helium atoms will move much higher upwards, and as soon as they do that, some oxygen molecules will take their previous location. Now they have a new location higher up, and the same happens.
Overall this causes the effect of buoyancy, sometimes called updrift, which is the force that this lighter material is pushed up with. And this upwards force is exactly the same as the force, with which the heavier materials pulls downwards - in other words, the lighter material is pushed up with the weight of the displaced heavier material, which now pushes to come back in place.
This was Archimedes' discovery.
Now to your other sub-questions:
Can be said that for airmolecules the atmosphere is a vacuum?
Well, no, a vacuum is a vacuum. If there are molecules present, it isn't vacuum, and the atmosphere isn't a vacuum.
So all together helium should have the same attraction to earth as the other airmolecules?
No, their "attraction" to Earth are different, because that "attraction" must be weight. And the helium atoms weight is lower.
II) Because helium atoms are much lighter, perhaps they could have a higher speed than fe O2 or N2?
Mass (or weight) doesn't influence possible speed. It only influences how hard it is to make them reach the speed.
Ok, but those helium atoms are in a balloon so they pushes at all sides of the balloon equal so the balloon shouldn't move at all?
If only the balloon with helium was present, and no gravity or outside atmosphere, then you are completely correct. The inside pressure cannot make the balloon move. But with gravity present, the whole thing is pulled downwards, and with the atmosphere present, there is a buoyancy force upwards as discussed above. Which-ever of these forces is greater, makes the balloon move.
III) When a balloon starts ascending from the ground there is more air (pressure) above him than beneath. So the airpressure above him should push the balloon to the ground?
Incorrect. You actually said it yourself just before: Inside the balloon, the pressure equalizes throughout so the push at any point on the balloon is the same. Same goes for this air column: All the air in the column above presses down, but the tiny bit of air below pushes up with the same force to balance out the pressure.
For example, let's say we have a big turkey. Now, we surround the turkey with air heated to 350F, that is convection cooking. The air transfers its heat by contact to the turkey.
This is what we would typically call roasting in a conventional oven. When the turkey absorbs heat from the surrounding air, the air cools down; because air is a very bad conductor of heat, the (still) air develops a temperature gradient near the turkey which means that the air that the turkey is actually in contact with is substantially below $350^\circ$.
If we equip the oven with a fan system which circulates hot air to ensure that this layer of cool air near the turkey does not develop, then this is what we would call roasting in a convection (sometimes called fan-assisted) oven. Obviously this will cook the turkey faster because the air in contact with the turkey will actually be $350^\circ$, in contrast to the conventional oven.
In both of these cases, we are not cooking the turkey via the radiant heat from the (extremely hot) heating element. The heating elements are cycled on and off to maintain the temperature inside the oven, but they spend most of their time off. If we do cook with the radiant heat from the heating element, we keep the heating element on the whole time; this type of cooking is called either broiling or grilling, depending on where you live.
To test this I put some chicken in the oven and right next to it a block of wood. Both were about 12 inches from the heating element. After some time, the surface of the chicken was 250F and the surface of the wood was 450F.
Wood is dry, chicken is not. Water has a massive heat capacity - far larger than wood or almost anything else we use in a kitchen - which means it takes a lot of energy to raise the temperature of something which has a lot of water in it. The surface of the chicken is in direct contact with the interior of the chicken, which keeps it cool.
Best Answer
I will try to explain this part with a very schematic example.
In the picture above, an air molecule collides with a compact wall, for example a crystal, separating two spatial areas, A and B. Treating the molecules as billiard balls, we will say that the air molecule will hit precisely one molecule of the wall and then bounce away.
The momentum delivered by the air molecule will result in a vibration of the molecule which was hit. Since the material is dense, this vibration will be immediately transmitted to the nearby molecules.
So, collision after collision, the vibration (temperature!) of the molecules of the wall will increase and increase, until a state of equilibrium is reached. I speak of equilibrium because also the molecules of the wall will transfer momentum to the surrounding air molecules!
So if side A was initially hotter than side B, side B will get hot in no time because of the energy transferred by the vibration of the molecules of the wall.
At equilibrium, for the equipartition theorem, the average kinetic energy of every molecule in the system (side A, side B and wall) will be
$$\langle \epsilon \rangle = \frac f 2 k_B T$$
where $f$ is the number of degrees of freedom of the molecule ($3$ for a gas particle, $6$ for a particle in a perfect 3D crystal) and $T$ is the equilibrium temperature.
Let's now consider the case depicted below:
The compact wall has now been replaced by two very thin layers containing some gas. You can see that in this case, when an air molecule hits the wall, the vibration is again immediately transmitted to the nearby molecules in the wall, but only rarely to the gas molecules contained in it.
You can then see that this time it will take much longer to get all the gas molecules contained in the wall to receive the kinetic energy and transfer it to the molecules contained in side B.
This is why solid materials have an higher thermal conductivity than gases (and liquids): because when the molecules are far apart from each other it is difficult to transfer energy.
Moreover, ordered materials (crystals) have an higher thermal conductivity than disordered materials (such as glasses), because in the former the vibration can be transferred through well defined directions, i.e. the atoms can vibrate in a synchronous way along preferred directions, while in the latter this is not possible.
Indeed, glass has a terrible thermal conductivity, about 0.8 W/m K. Water and air are even worse, with thermal conductivities of respectively 0.6 and 0.024 W/m K.
Silver, copper and gold are quite amazing thermal conductors, with conductivities of respectively 406, 385 and 314 W/ m K.
But the real hero is diamond, with an astonishing 1000 W/m K: its secret is in its perfect tetrahedral network structure of strong covalent bonds, equals in every direction, which allow it to transfer vibration efficiently from one atom to the other.