All matter is composed of atoms. Each atom consists of a positively charged nucleus (populated by protons) surrounded by a negatively charged electron cloud. As a result, each atom behaves like a magnet with positive and negative charges separated by distance.

In thermodynamic processes, atoms do not appear or disappear. They persist. As discrete entities, they are in constant changing motion, attracting each other from a distance, repelling each other up close, reacting with each other when the conditions are right.

The underlying cause of these changing motions are forces. Recall Newton’s 1st Law of Motion (to paraphrase): a body at rest stays at rest and an body in motion stays in motion with the same speed and in the same direction unless acted upon by an external, unbalanced force. The constant change in motion of atoms reflects the constant presence of external forces acting upon them.

The Four Fundamental Interactions

There are four fundamental interactions in nature: gravity, electromagnetism, weak interaction, and strong interaction. The strong and weak interactions serve to maintain a stable nucleus and aren’t of concern to us here. Only the first two are relevant to thermodynamics. (Note: interactions lead to potential energy gradients and it’s these gradients that lead to forces. From here on out, I will use “force” and “interaction” interchangeably.)

Gravity

Gravity acts between masses and causes acceleration. While insignificant at the atomic scale, gravity does become important at larger scales involving, for example, planets, such as Earth. Earth’s gravitational pull on atoms leads to a range of effects, including weight, buoyancy, separation based on density, and the behavior of gases in our atmosphere.

Electromagnetic

The electromagnetic interaction is the source of all chemistry. It is responsible for electron binding to nuclei, chemical bonding between atoms, intermolecular attraction (from a distance) and repulsion (up close), elastic collisions (caused by repulsion), and photon emission and absorption. Every thermodynamic interaction between atoms—collisions, reactions, phase change, transport—ultimately traces back to electromagnetic interactions.

Energy at the atomic scale

The concept of energy plays a central role in thermodynamics. While this topic will be covered in greater depth in subsequent chapters, it makes sense to introduce it now.

At the atomic scale, energy appears in two forms: kinetic and potential. Kinetic energy is associated with physical motion such as translational motion from one place to another, rotational motion of a multi-atom molecule, and vibrational motion between atoms bound together in a molecule. Kinetic energy is directly connected with the macroscopic property of temperature.

Potential energy arises from the gravitational and electromagnetic interactions between atoms, between bound atoms in their vibrational motion, and between orbital electrons and nuclear protons.

Energy is conserved

In all of the variety of interactions between atoms, one thing doesn’t change: energy. Energy is conserved during each and every micro-interaction. If you add up the energy, both kinetic and potential, before and after each interaction, this sum remains constant. Not almost constant. Exactly constant. Because of this, the energy of the larger system containing vast numbers of atoms is also conserved. This is why energy is such an important property in thermodynamics. Just like atoms, it doesn’t appear or disappear. It persists. These facts lead to the 1st Law of Thermodynamics: matter and energy are conserved quantities.

A system of atoms self-organizes to its most probable distribution

In the ideal world we would be able to model the trajectory and interaction—attract, repel, collide, react—of each individual atom in a system. In the real world, however, given that there are about 10²³ atoms in a cubic centimeter of matter, such modeling is currently not achievable. Yes, the rise in power of molecular dynamic simulations is enabling us to get closer to this ideal goal, but we’re still a long ways away. So what do we do instead? We rely on the concept of equilibrium.

Although atomic motion appears chaotic, an underlying statistical order governs it. A system of atoms evolves naturally — without an external driving force — toward its most probable distribution, whereupon the system exists in its equilibrium state. While individual atomic positions and velocities continually change in an equilibrated system—equilibrium is considered dynamic in nature since, at the molecular level, opposing forward and reverse reactions or processes continue to occur at the same rate—the equilibrium distribution itself remains stationary. This stability permits systems to be characterized by average properties and classical thermodynamics is built upon the relations among those averages.

Summary

Macroscopic thermodynamic properties arise from the behavior of atoms, yet classical thermodynamics were formulated without explicit reference to them. This is both its power and its limitation. The classical equations were derived without assuming the existence of atoms, even though those very equations describe the collective consequences of atomic motion and interaction. One can learn to apply the equations while remaining unaware of the microscopic machinery that gives rise to them.

The objective of this book is to connect these two levels of description — the macroscopic laws and the atomic reality beneath them. This chapter initiates that connection.

We begin with several foundational statements:

• Matter is composed of atoms.
• Atoms persist; they are neither created nor destroyed in ordinary thermodynamic processes.
• Atoms move continuously and interact — attracting, repelling, colliding, and reacting.
• Energy persists; it is conserved.
• Collections of atoms evolve toward their most probable distribution, defined as the equilibrium state

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