At standard pressure, during which physical change does the potential energy decrease?

Answer is: liquid to solid.

Cooling is change from liquids to solids.

A pure substance in the gaseous state contains more energy than in the liquid state, which in turn contains more energy than in the solid state.

Potential energy is type of stored energy.

A physical change occurs when a substance changes phase.

Physical property can be observed and measured without any changes in molecular composition. The same substance is present before and after the change.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.