When you talk about how heavy a metal is, you’re actually talking about how dense it is. Density is a measure of how tightly packed matter is. When you look at the density of different metals, you may be surprised. You may think that lead is very dense, but many other metals have a much higher density.

Density vs. Atomic Weight

When talking about heavy metals, it is necessary to distinguish between density and atomic weight. The density of a material is mass per unit volume. Density is measured in kilograms per cubic meter (kg/m³) or grams per cubic cm (g/cm³). Density affects the way different materials interact. For example, many types of metals sink in water because the metal has a higher density (i.e., it is denser) than water.

On the other hand, atomic weight is the average mass of atoms of an element. An atomic weight unit, which is dimensionless, is based on one twelfth (0.0833) the weight of a Carbon-12 atom in its ground state. In other words, a carbon-12 atom is assigned 12 atomic mass units. Atomic weight is more commonly known as relative atomic mass to avoid confusion because atomic mass is not exactly the same as atomic weight, and “weight” implies a force exerted on a gravitational field, measured in units of force such as newtons.

Most Dense Metals

Osmium and Iridium are the densest metals. In other words, its atoms are more closely packed in a solid form than other metals. With a density of 22.6 g/cm³ and 22.4 g/cm³ respectively, Osmium and Iridium are about twice as dense as Lead, which has a density of 11.3 g/cm3. Osmium and Iridium were discovered by the English chemist Smithson Tennant in 1803. Osmium is rarely used in its pure form and is mostly mixed with other dense metals such as platinum to create very hard and strong surgical equipment. Iridium is mainly used as a hardening agent for platinum alloys for equipment that must withstand high temperatures. Platinum measures a density of 21.45 g/cm³. It does not readily mix with other elements and in its pure form is used in catalytic converters, laboratory equipment, dental equipment, and jewelry.

Heaviest Metal by Relative Atomic Mass

The heaviest naturally occurring element is Plutonium (atomic number 94, relative atomic mass 244.0). Other heavy metals in terms of relative atomic mass are Uranium (atomic number 92, relative atomic mass 238.0289), radium (atomic number 88, relative atomic mass 226.0254), and Radon (atomic number 86, relative atomic mass 222, 0). Oganesson (atomic number 118) is the heaviest element on the periodic table, but it is a synthetic element that cannot be observed in nature. Lithium (atomic number 3, relative atomic mass 6.941) is the lightest metal in terms of relative atomic mass.

Definition of Heavy Metals

Heavy metals are defined as metallic elements that have a relatively high density compared to water [1]. With the assumption that heaviness and toxicity are inter-related, heavy metals also include metalloids, such as arsenic, that are able to induce toxicity at low level of exposure [2]. Any of these metal can be called a heavy metal – lead, mercury, arsenic, cadmium, cesium, chromium, selenium, silver, nickel, copper, aluminum, molybdenum, strontium, uranium , cobalt, zinc and manganese, all of which exist naturally on Earth.