If atoms are mostly empty space and atoms compose all ordinary matter, why does solid matter seem to have no space within it?

At the heart of every atom lies the nucleus, an incredibly dense core composed of protons and neutrons. The number of protons defines the atomic number, which determines the element's identity, such as hydrogen, oxygen, or iron. Surrounding the nucleus, electrons whirl in a complex dance, occupying regions of space known as orbitals.

Unlike the simplistic planetary model often depicted in textbooks, the behavior of electrons in these orbitals is dictated by the principles of quantum mechanics. Electrons do not orbit the nucleus in predictable paths; instead, they exist as a probability cloud, where each point in the cloud represents the likelihood of finding an electron at that location. This quantum behavior is essential in understanding why matter appears solid, as it influences the interactions between atoms in a profound way.

The electromagnetic force is one of the fundamental interactions in nature, playing a pivotal role in how atoms bond and interact with each other. At the atomic level, it is primarily the electrons that are responsible for this force. Electrons possess a negative charge, while the nucleus is positive due to the protons.

The electromagnetic force arises from the attraction between oppositely charged particles, as well as the repulsion between like charges. When atoms come into proximity, their electron clouds can repel each other, creating a force that is strong enough to prevent the atoms from passing through each other. This seemingly robust force gives rise to the solidity we experience in everyday objects, despite the vast empty spaces within and between atoms.

The way we perceive matter is largely dependent on the human scale of observation. At our macroscopic level, matter appears solid and continuous. However, if we zoom into the nanoscopic scale, we would observe that atoms are mostly empty space. This disparity in perception is due to the limitations of our senses, which are not capable of discerning phenomena at atomic dimensions.

The solidity of matter is more about the interactions—specifically electromagnetic forces—between atoms than the presence of substantial physical structure. Even though the nuclei of atoms occupy a minuscule fraction of the space within the atom, the electron clouds define the atom's boundaries, creating the illusion of a continuous solid when observed without magnification.

Quantum mechanics is central to understanding atomic properties and behaviors. It explains the dual nature of particles and waves, energy quantization, and the uncertainty principle, all of which are fundamental to how atoms and their constituent particles interact and form matter as we know it.

For instance, the quantum model of the atom explains the existence of distinct energy levels and the shapes of electron orbitals. These quantum effects contribute to chemical bonding, where electrons are shared or transferred between atoms to achieve stability. Without the insights provided by quantum mechanics, phenomena such as the structure and hardness of diamonds, the conductivity of metals, or the colors produced by different elements when heated, would remain inexplicable.