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Crystalline solids are composed of atoms, ions, or molecules that form a consistent, repeating pattern in all three dimensions. This is what is referred to as the atomic structure. Imagine a vast three-dimensional grid where each intersection point hosts an atom or an ion. This structure is fundamental to the properties and appearance of crystalline solids.

In the realm of crystallography, the orderly arrangement is key to the substance's stability and properties. The atomic structure dictates the entire physics of the solid, including how it interacts with light, heat, and other forms of energy. A significant aspect of this orderly arrangement is that it provides a high degree of symmetry, meaning that the structure looks the same from various directions. This symmetry is a hallmark of many crystalline solids, and it's directly reflected in their external appearance.

The internal pattern explains why specific angles between atoms influence the crystal's formation into regularly shaped facets. These facets manifest externally as symmetrical planar faces.

Symmetrical planar faces are a fascinating aspect of crystalline solids. They are easily visible to the naked eye, and this external manifestation often draws people to study crystals. This feature is directly connected to the internal orderly pattern of atomic structures.

The symmetry of these faces is a direct result of the inherent symmetry present at the atomic level. Each face of a crystal is an outward representation of the specific internal planes of atoms within the crystal. The regular geometric shape that most people recognize in crystals, such as cubes or hexagonal prisms, is produced because the atomic structure aligns itself in specific plane orientations.

Translation of internal symmetry to external faces means that regardless of how the crystal grows, it tries to maintain the geometric symmetry dictated by its atomic arrangement. This is why we see distinct shapes and forms in natural crystals. Just as in any well-organized system, the internal order of atoms ensures that the resulting facets are perfectly symmetrical.

The relationship between a crystal's internal and external structure is profound and significant. The internal structure, the meticulous arrangement of atoms, ions, or molecules, informs the external appearance of the crystalline solid.

The periodic atomic pattern determines both the angle and dimension of the faces that form on the crystal. As the crystal grows, the atoms continue to follow this internal blueprint, expanding the visible symmetrical faces that define the crystal’s outer form. This is a beautiful translation of the intricate, invisible world of atoms into something tangible and visible.

What's important to note is that the external symmetry is a direct mirror of the internal atomic arrangement. Thus, if the internal structure changes, it inevitably leads to a change in the crystal's external appearance. This linkage between internal compositional structure and external growth is a fundamental principle in the study of crystallography, ensuring that each crystal reflects its unique atomic blueprint in its outer form.