Which statement best describes how energy is important to chemistry? A. Energy is stored in an atom's electrons.
B. Energy is not conserved during chemical reactions,
C. Energy is absorbed and released during chemical reactions,
D. Energy holds atoms and bonds together,

C. Energy is absorbed and released during chemical reactions,

Explanation:

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. Endothermic reactions absorb energy, and exothermic reactions release energy.

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.  

it is the most commonly accepted one