What is the thin structure that surrounds a cell?

That is the membrane of the cell

Explanation:

The cell membrane is made up of layers of protein and fat. It is made with a phospholipid bilayer. It allows some substances to pass into the cell while blocking others. This is known as semipermeability.

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.

they it depends on if one believes about radioactivity, they make a geiger counter beep but maybe it was going to beep anyway electronics do wierd things all the time or it was programmed to beep near certain metals.