**1. Definition and Classification of Heavy Metals:**
– Heavy metals are defined based on density, atomic weight, atomic number, and chemical behavior.
– Criteria for heavy metals can include density above 5g/cm³, atomic weight greater than 40, and atomic number above 20.
– The lack of consensus in defining heavy metals leads to variability in classification.
– Different fields such as metallurgy, physics, and chemistry have distinct perspectives on heavy metal classification.
– Metals like antimony, arsenic, and selenium are sometimes classified as heavy metals based on their properties.
– The density criterion guides the inclusion of elements in the heavy metal list.
**2. Characteristics and Impact of Heavy Metals:**
– Some heavy metals are essential nutrients or relatively harmless in small amounts.
– Toxic heavy metals like arsenic, cadmium, mercury, and lead pose significant health risks.
– Heavy metals are generally less reactive and have fewer soluble sulfides and hydroxides compared to lighter metals.
– Understanding the sources and effects of heavy metal exposure is essential for mitigating risks.
– Essential heavy metals like iron and zinc play crucial roles in biological systems.
– Despite their scarcity, heavy metals are widely used in various modern applications.
**3. Biochemical Classification and Origins of Heavy Metals:**
– In biochemistry, heavy metals are classified based on their Lewis acid behavior in aqueous solutions.
– Metal ions are categorized as class A, class B, or borderline metals based on their preferential bonding characteristics.
– The term ‘heavy metal’ was first used by Leopold Gmelin in 1817 to categorize elements.
– Crafting of metal ornaments, tools, and weapons due to malleability was a historical origin.
– Metals discovered until 1809 had high densities, leading to the term’s creation.
**4. Toxicity and Environmental Impact of Heavy Metals:**
– Heavy metals can cause serious toxic effects like cancer, brain damage, and death.
– Some heavy metals are toxic only in excess or specific forms.
– Heavy metal poisoning can result from sources like mining, industrial waste, and agricultural runoff.
– Heavy metals like chromium, arsenic, cadmium, mercury, and lead have high potential to cause harm.
– Heavy metal contamination examples include Minamata disease in Japan and lead in Flint, Michigan’s water.
**5. Uses and Applications of Heavy Metals:**
– Heavy metals are essential in various modern applications and industries.
– Iron is the most common refined metal, constituting 90% of all metals.
– Heavy metals serve diverse purposes due to their unique properties.
– Heavy metals like lead are used in underwater diving as ballast, and tungsten is used in military ordnance for armor plating.
– Heavy metals are used in electronics, magnets, lighting, nuclear science, and various other applications.
Heavy metals are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. The criteria used, and whether metalloids are included, vary depending on the author and context. In metallurgy, for example, a heavy metal may be defined on the basis of density, whereas in physics the distinguishing criterion might be atomic number, while a chemist would likely be more concerned with chemical behaviour. More specific definitions have been published, none of which have been widely accepted. The definitions surveyed in this article encompass up to 96 out of the 118 known chemical elements; only mercury, lead and bismuth meet all of them. Despite this lack of agreement, the term (plural or singular) is widely used in science. A density of more than 5 g/cm3 is sometimes quoted as a commonly used criterion and is used in the body of this article.
The earliest known metals—common metals such as iron, copper, and tin, and precious metals such as silver, gold, and platinum—are heavy metals. From 1809 onward, light metals, such as magnesium, aluminium, and titanium, were discovered, as well as less well-known heavy metals including gallium, thallium, and hafnium.
Some heavy metals are either essential nutrients (typically iron, cobalt, and zinc), or relatively harmless (such as ruthenium, silver, and indium), but can be toxic in larger amounts or certain forms. Other heavy metals, such as arsenic, cadmium, mercury, and lead, are highly poisonous. Potential sources of heavy metal poisoning include mining, tailings, smelting, industrial waste, agricultural runoff, occupational exposure, paints and treated timber.
Physical and chemical characterisations of heavy metals need to be treated with caution, as the metals involved are not always consistently defined. As well as being relatively dense, heavy metals tend to be less reactive than lighter metals and have far fewer soluble sulfides and hydroxides. While it is relatively easy to distinguish a heavy metal such as tungsten from a lighter metal such as sodium, a few heavy metals, such as zinc, mercury, and lead, have some of the characteristics of lighter metals; and lighter metals such as beryllium, scandium, and titanium, have some of the characteristics of heavier metals.
Heavy metals are relatively scarce in the Earth's crust but are present in many aspects of modern life. They are used in, for example, golf clubs, cars, antiseptics, self-cleaning ovens, plastics, solar panels, mobile phones, and particle accelerators.
