Why does the TLC tank need to be covered?

The reason for covering the container is to make sure that the atmosphere in the beaker is saturated with solvent vapour. Saturating the atmosphere in the beaker with vapour stops the solvent from evaporating as it rises up the paper.

it is extremely important that the solvent in the developing chamber be lower than the spotos on the plate, because the solvent must be drawn upward through the sample in order to draw the sample along with it. ... Finally it is important to cover the chamber to be sure that the solvent does not evaporate.

Explanation:

1. a)c-14 has two more protons

2. d)they have different numbers of protons, neutrons, and valence electrons; they are all gases at room temperature.

3. c) iron and nickel.