During an exothermic chemical reaction,
a. a system becomes warmer, and the chemical substances undergo a decrease in potential energy.
b. a system becomes warmer, and the chemical substances undergo an increase in potential energy.
c. a system becomes cooler, and the chemical substances undergo a decrease in potential energy.
d. a system becomes cooler, and the chemical substances undergo an increase in potential energy.
e. a system becomes warmer, and additional heat is gained from the surroundings.

B.

Explanation:

When an exothermic chemical reaction occurs the substance becomes warmer and it gains potential energy

the answer is a and have a great day

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.