Gold: The Ultimate Guide to Element 79

Introduction

- Gold, denoted as Au (from the Latin 'Aurum'), is a metallic element with an atomic number of 79.

- This precious metal has been sought after for centuries, not just for its brilliant, lustrous appearance but also for its unique properties, which make it useful in a range of applications, from jewelry to electronics.

- As a metal, gold is malleable, conductive, and does not tarnish, making it highly valuable in multiple domains.

Historical Background

- Gold was likely one of the first metals to be discovered by humans, with evidence suggesting its use as far back as 5000 BC.

- It wasn't "discovered" in the sense that elements like helium or oxygen were; it has been known to civilizations for thousands of years.

- The name 'gold' is derived from the Old English word 'geolu,' which means yellow, signifying its characteristic color.

Physical Properties

- Atomic weight: 196.966569 grams/mole

- Melting point: 1,064°C (1,947°F)

- Boiling point: 2,856°C (5,173°F)

- Density: 19.32 grams per cubic centimeter

- Color: Yellow

- State at room temperature: Solid

- Electrical conductivity: High

- Magnetic properties: Diamagnetic

- Isotopes: Most common is Au-197

- Other notable physical characteristics: Extremely malleable and ductile, excellent conductor of electricity and heat, resistant to tarnish and corrosion.

Chemical Properties

Gold is an interesting element in terms of its chemical properties. Unlike many other metals, it is relatively inert and does not react with many elements or compounds under normal conditions. This section aims to delve deeply into the chemical properties of gold, explaining its electron configuration, oxidation states, common compounds it forms, and its notable chemical reactions.

Electron Configuration

The electron configuration of gold is [Xe] 4f₁⁴ 5d¹⁰ 6s¹. Gold has a single electron in its outermost shell, which makes it a member of the "transition metals" group. This outer electron makes gold malleable and ductile but doesn't make it overly reactive, due in part to the electron structure and in part to relativistic effects that occur at high atomic numbers.

Oxidation States

Gold primarily exists in two oxidation states: +1 and +3. While the +1 oxidation state (aurous compounds, like AuCl) is less stable than the +3 state (auric compounds, like AuCl₃), both are useful in various chemical applications. Auric gold compounds are often more stable than aurous compounds and can be more readily synthesized.

Common Compounds

Gold forms various compounds, though it's worth noting that they aren't as numerous or as commonly encountered as compounds of more reactive elements like oxygen or iron. Some notable gold compounds include:

Notable Chemical Reactions

One of the most famous reactions involving gold is its dissolution in aqua regia, a mixture of hydrochloric acid (HCl) and nitric acid (HNO₃). In this process, the gold reacts with both acids to form chloroauric acid (HAuCl₄), according to the following reaction:

This reaction is unique because gold is generally not affected by individual strong acids. It takes the combined oxidative power of both nitric and hydrochloric acids to dissolve gold, and even then, the reaction is relatively slow. Aqua regia is one of the few substances that can dissolve gold and is used for purifying gold and concentrating it from complex mixtures.

Another crucial chemical reaction involving gold is the cyanidation process, which is the industrial method used for recovering gold from ores. In this process, gold reacts with aqueous cyanide solution in the presence of oxygen to form soluble aurocyanide complex ions:

This is followed by the separation of the aurocyanide complex from the ore residues. The gold is then recovered by reducing the complex ions back to metallic gold. It's an efficient process, but it also poses environmental risks due to the use of cyanide.

Gold can form alloys with alkali metals like sodium (Na) and potassium (K), although this is generally not of significant practical importance due to the extreme reactivity of alkali metals:

However, these alloys can be used for specialized applications, such as in experimental chemistry and materials science.

Gold is used as a catalyst in a variety of industrial chemical reactions. Although it doesn't get consumed in the process, it facilitates reactions like the oxidation of carbon monoxide (CO) to carbon dioxide (CO₂).

Stability and Complex Formation

Gold's relative chemical inactivity is also a function of its tendency to form stable complexes with ligands like cyanide (CN⁻), thiosulfate (S₂O₃²⁻), and even with large organic molecules in some medical applications. This stability is due to its electron configuration and the relativistic effects mentioned earlier.

Abundance and Sources

- Gold is relatively scarce in the Earth’s crust, estimated at about 0.005 parts per million.

- It naturally appears in vein structures and alluvial formations.

- Common ores: Often found as a native metal or in combination with silver and other metals.

- Methods of isolation or production: Extracted through mining and refined by smelting or chemical treatment.

Uses and Applications

Gold has a plethora of uses, stemming not only from its beauty and monetary value but also from its unique physical and chemical properties. Here is a more in-depth look into the different uses and applications of gold:

Industrial Uses

Medical Applications

Everyday Uses

Importance in Biological Systems

While gold itself does not play a known essential role in biological systems, its compounds are being studied for potential antimicrobial properties and as possible agents for gene therapy.

Safety

- Toxicity levels: Generally considered non-toxic, but gold compounds can be toxic and must be handled with care.

- Precautions to handle the element: Standard safety gear when dealing with gold compounds; native gold is non-reactive and safe to handle.

- Storage guidelines: Store gold compounds in a cool, dry place, away from reactive substances.

Interesting Facts

- Gold is so malleable that a single ounce can be stretched into a gold thread 5 miles long.

- The largest gold nugget ever found weighed approximately 173 pounds.

- Gold is edible and is sometimes used as a food decoration in the form of gold leaf.

- It has been discovered in outer space, and some scientists believe it to be the byproduct of supernova explosions.

Conclusion

- Gold is a unique element that combines both beauty and practicality. Its physical and chemical properties make it not just aesthetically pleasing but also incredibly useful in modern technology and medicine.

- Its rarity and the complexities involved in its extraction add to its value, making it one of the most coveted and valuable elements on Earth.