Atoms in metallic elements have low ionization energies and hold their outer electrons very loosely. These loose particles are called valence electrons, and they sit in the outermost energy levels. When metal atoms pack tightly together, their outer electron shells overlap across the whole solid.
This massive overlap creates continuous energy bands instead of separate, distinct chemical bonds. The electrons detach from their original atoms and form a highly mobile electron gas. This gas flows easily through the fixed, regular grid of positively charged metal ions.
The negative electron sea strongly attracts the positive ions and binds them all together. This widespread electrical attraction gives solid metals their incredible physical strength and toughness. The free-flowing electrons also explain why metals conduct electricity and heat so well.
These mobile particles can rapidly carry kinetic energy and electrical charge from one end to another. Furthermore, metallic bonds do not point in one specific direction like covalent bonds do. The layers of positive ions can slide past each other without breaking the overall bond. This special flexibility explains why you can easily bend, hammer, and stretch most solid metals.