In solution the metallic ion has a positive electric charge. Metallic ions play a fundamental role in many life processes. Iron, for example, is central to the structure of hemoglobin, the blood component that carries oxygen from the lungs. Calcium and potassium ions are necessary for the transmission of impulses along the body's nerve cells. The physical properties of most metals include high density, or high mass per unit volume, and high strength, or the ability to resist being distorted from their original shape.
Metals also have considerable elasticity.
A metal spring can be stretched, for example, but when the load is removed, it contracts to its original length. The electronic structure of metals explains many of their properties. In metals the outermost shell of the atom is generally not filled with all the electrons it can hold. Thus when many atoms come together to form a solid metal, the electrons in the outermost shells are shared among neighboring atoms and move freely from one atom's shell to another. Heat is a phenomenon involving whole atoms.