What would be a good model of equilibrium? (You don't have to involve chemical equilibrium.)

Try using a solid and another type of phase like water and have it at different temperatures. Once you have done that, let them sit in your location that you have them. Make sure to have their starting temperatures and their ending temperatures which should be equilibrium because of the room temperature.

Explanation:

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When a melted popsicle is put into a freezer and later the melted popsicle becomes frozen after a length of time.

Explanation: This is an example of thermal equilibrium because the substance has reached the same temperature as it's sourroundings.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.