The mass of hydrogen is 1.6726 x 10–27 kg, and the mass of helium is 6.6465 x 10–27 kg. Given this, which of the following nuclear reactions would result in a decrease of total mass, and thus a release of energy, while keeping the same number of particles involved?

A. A helium nucleus breaks up into four hydrogen nuclei.
B. Four hydrogen nuclei combine into one helium nucleus
C. Two hydrogen nuclei combine into one helium nucleus.
D. A helium nucleus breaks up into two hydrogen nuclei

B.Four hydrogen nuclei combine into one helium nucleus.

Explanation:

Mass energy which is stored in a matter itself can be very powerful. A small amount of mass can result in a very large amount of energy as the speed of light is very large.

If we consider the two elements hydrogen and helium, a hydrogen nucleus is made of one proton  and a helium nucleus is made of two protons and two neutrons.

The mass of hydrogen is 1.6726 x 10–27 kg, and the mass of helium is 6.6465 x 10–27 kg.

Four hydrogen nuclei combine into one helium nucleus would lead to decrease of total mass, and thus a release of energy such that the number of particles involved remains same.

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.