This course is a continuation of From Newton to the Neutron: Matter, Energy, and Void (Part 1, Section 1). In that course, we explored mass, force, work, energy, the gas laws, charge, states of matter, valence electrons, ions, and atomic structure. This course builds on those foundations with four lessons on chemical reactions (Part 2, Section 1) and five lectures on nuclear reactions (Part 3, Section 2).

What is the nature of matter and energy? What is physics? What is chemistry? What is the difference between a chemical and a nuclear reaction? In this course, we minimize the mathematical complexity to focus on the underlying scientific concepts and their historical development.

We begin with Isaac Newton, who formulated the concept of force, defined mass as a measure of matter using inertia, and explained how forces change the motion of objects according to their mass. These ideas eventually led to the concept of energy—the ability to perform work and overcome resistance. Work, a form of energy, was defined as a force acting on an object through a distance.

Beyond mass, scientists discovered another force-related property of matter: charge. Work and heat energy were found to be derived from gases, which led to the concept of atoms. Further discoveries revealed that atoms contain subatomic particles that carry charge, leading to a quest to understand their structure. By 1932, with the discovery of the neutron, the modern concept of the atom was largely complete.

Along the way, scientists observed that some atoms undergo spontaneous radioactive decay, emitting distinct particles with different masses and charges. These processes led to the discovery of transmutation, where a parent element transforms into a daughter element, sometimes releasing radiation in the form of electromagnetic waves.

In 1933, Leo Szilard hypothesized that atoms absorbing neutrons could split into new elements while emitting additional neutrons, creating a chain reaction. The following year, Enrico Fermi demonstrated that neutron capture could induce radioactivity. In 1938, Otto Hahn and Lise Meitner discovered the first example of nuclear fission in Germany. Alarmed by the implications, Szilard drafted a letter to President Franklin D. Roosevelt, signed by Albert Einstein and other physicists, warning of the possibility of a fission-based atomic bomb. This marked the beginning of the race for nuclear weapons.

During the 1930s, another nuclear process—nuclear fusion—was proposed as the energy source of the Sun. In the 1950s, humans first harnessed uncontrolled nuclear fusion in the form of a thermonuclear bomb. Scientists continue to pursue controlled nuclear fusion as a potential source of abundant, clean energy with no radioactive byproducts, a solution to the global energy crisis.

Nuclear fusion in stars, known as nucleosynthesis, produces elements up to iron. Heavier elements, including those found in our bodies, form through cosmic neutron capture. As the famous lyrics say:

"We are stardust, we are golden, we are billion-year-old carbon, and we've got to get ourselves back to the garden."