Final answer:
Sodium is a metal due to its body-centered cubic (BCC) crystal structure and ability to form positively charged ions by losing its single valence electron, which leads to the formation of metallic bonds that are characteristic of metals.
Step-by-step explanation:
Sodium is a metal that crystallizes in a body-centered cubic (BCC) structure in its solid state. This crystal structure is characterized by its atoms being arranged at all eight corners of a cube with an additional atom at the center, making it highly efficient in terms of packing density, occupying about 68% of the total volume.
In a BCC arrangement, the coordination number of each atom is eight, meaning each atom contacts four atoms in the layer above and four in the layer below. The presence of delocalized electrons, which allows metals to conduct electricity and account for their metallic properties, is also a characteristic of sodium, as is for other metals like potassium, barium, chromium, molybdenum, tungsten, and iron at room temperature.
Referring to the octet rule, atoms like sodium attempt to achieve stability by achieving a noble gas electron configuration. For sodium, this is accomplished by losing its single valence electron, thus forming a positively charged sodium ion (Na+) and allowing the formation of metallic bonds that hold the BCC structure together.