Well the first law is actually about conservation of energy, but in fact energy and mass are related via E=mc², so it’s really a law that conserves mass-energy.

In normal bench chemistry reactions not only is mass-energy conserved but also the identity of atoms is conserved. So if a reaction starts with 6 grams of say C then in the products there will be six grams of C!

This is because the energy of bench chemical reactions is so low… so that when translated to mass, the difference in mass cannot be determined…it’s very, very small.

However in nuclear reactions, the atom type are NOT conserved although the overall mass-energy of the system IS CONSERVED in keeping with the conservation of mass-energy.

To answer your question directly, nuclear reactions do not always have the same number of each type of atom enter and leaving the reaction. However, they still follow the first law of thermodynamics because the first law states that energy cannot be created or destroyed, it can only change forms.

The first law makes no guarantees about whether matter or mass is conserved. If you are familiar with Einstein’s famous E=mc² equation, you can see a simplified relationship for how energy (E) and mass (m) are related, where c is the speed of light. So even though the number and type of atoms may change during a nuclear reaction, energy is never being created or destroyed — the energy is just changing forms between things such as heat, bond energy, and matter.

Great question! As you said, both kinds of nuclear reactions (fission and fusion) obey the first law of thermodynamics. Let's look at the example of the fission of Uranium 235. U235 is a Uranium atom with 92 protons and 143 neutrons. U235 can decay along different pathways, but let's assume it decays into Barium 141 and Krypton 92. U235 can decay on it's own, but it has a half life of 7 million years, meaning that it will take 7 million years for half of a 1 kg block of U235 to decay into products. This will leave 0.5 kg of products and 0.5 kg of U235--after 7 million years! We can accelerate the decay of U235 by bombarding it with neutrons. When we do this, the fission reaction looks like:
1-U235 + 1neutron --> 1-Ba141 + 1-Kr92 + 3neutrons + energy
Let's count up the protons and neutrons in this reaction. On the left hand side, there are 92 protons and 144 neutrons (143 from Uranium and one from the neutron we added). On the right hand side, there are 56 protons from Barium + 36 protons from Krypton = 92 protons total. There are 85 neutrons from Barium + 56 neutrons from Krypton + 3 neutrons ejected = 144 neutrons total.

You can see here that there is mass balance between the left hand side and the right hand side of the decay equation. There are not the same numbers of atoms or types of atoms on each side, but all of the protons and neutrons are accounted for.

We have a slight issue still. It looks like there might not be energy balance here, since there is energy released by this fission reaction. The first law of thermodynamics says that energy is conserved in a system, so where did we get that energy? As it turns out, energy is released when protons and neutrons are bound together. This is called the nuclear binding energy.

Ba141 and Kr92 have higher binding energies than U235, meaning they release more energy when they form than U235 does when it forms, so we get some energy when those products form from fission. In this way, all energy (and mass) is accounted for in the reaction.

So the answer here is yes and no- You are correct that matter cannot be created or destroyed, but the type of atom entering a nuclear reactor is not the same type or number leaving. The total mass is still conserved, however. This is because atoms are comprised of even smaller particles- protons, neutrons, and electrons. The sum of the weights of all these components constitutes the total mass of the system, and this is what is conserved.

Specifically, in a nuclear reactor, uranium 235 adsorbs a neutron, which then splits the atom- releasing heat (this is used to make electricity), another neutron (which will hit another uranium 235 atom to continue the process), and then the remainder of the protons/neutrons/electrons are left over in what are called fission fragments, which can take the form of several atoms (strontium and xenon are examples).

Your premise is incorrect: the first law of thermodynamics tells us that ENERGY cannot be created or destroyed. Matter is a type of energy, but matter can be transformed into other forms of energy (i.e. matter is destroyed), and other forms of energy can be transformed into matter (i.e. matter is created).

Nuclear reactions do satisfy the first law of thermodynamics, but are among the reactions that can create and destroy matter. Because energy is conserved, however, any matter that is created consumes kinetic energy, and any matter that is destroyed releases kinetic energy.

Thanks for your interesting question. Here's a good answer: to read.

The atoms break apart into their protons and neutrons [and electrons] and recombine differently.