Atoms are most tightly packed in the state of matter known as a solid. In a solid, atoms are closely arranged in a regular, ordered pattern or lattice. The interatomic forces, such as electromagnetic forces, hold the atoms together in a fixed position.
Within a solid, atoms are closely packed and have limited freedom of movement. They vibrate around their equilibrium positions but generally maintain their relative positions in the lattice structure. This close packing of atoms gives solids their characteristic rigidity and resistance to compression.
Different types of solids have different arrangements of atoms, resulting in different levels of packing efficiency. For example, in crystalline solids, such as diamond or sodium chloride, atoms are arranged in a highly ordered, repeating pattern. This arrangement allows for efficient packing and results in a high density of atoms.
Amorphous solids, like glass or certain plastics, do not possess a well-defined long-range order. However, the atoms or molecules in amorphous solids are still densely packed, although with a less regular arrangement compared to crystalline solids.
In contrast, in the states of matter such as liquids and gases, the intermolecular forces between atoms or molecules are weaker, allowing for greater freedom of movement. This leads to a more disordered arrangement and a lower packing density compared to solids.