Any of a class of substances with, to some degree, the following properties: good heat and electricity conduction, malleability, ductility, high light reflectivity, and capacity to form positive ions in solution and hydroxides rather than acids when their oxides meet water. About three-quarters of the elements are metals; these are usually fairly hard and strong crystalline (see crystal) solids with high chemical reactivity that readily form alloys with each other. Metallic properties increase from lighter to heavier elements in each vertical group of the periodic table and from right to left in each row. The most abundant metals are aluminum, iron, calcium, sodium, potassium, and magnesium. The vast majority are found as ores rather than free. The cohesiveness of metals in a crystalline structure is attributed to metallic bonding: The atoms are packed close together, with their very mobile outermost electrons all shared throughout the structure. Metals fall into the following classifications (not mutually exclusive and most not rigidly defined): alkali metals, alkaline earth metals, transition elements, noble (precious) metals, platinum metals, lanthanide (rare earth) metals, actinide metals, light metals, and heavy metals. Many have essential roles in nutrition or other biochemical functions, often in trace amounts, and many are toxic as both elements and compounds (see mercury poisoning, lead poisoning).