Antimony ($Sb$)
The unsociable element of antiquity—a brittle metalloid that shields our homes from fire and anchors the chemistry of early alchemy.
Antimony is an element known since at least 3100 BCE, when its sulfide ore, stibnite, was used as a cosmetic (Kohl) for the eyes in ancient Egypt. Its chemical symbol Sb comes from the Latin stibium. The name "Antimony" has a more mysterious origin; it likely comes from the Greek anti (not) and monos (alone), suggesting that the metal is rarely found native and is always "unfriendly" or paired with other elements.
Located in Group 15 and Period 5, antimony is a metalloid—possessing properties of both metals and non-metals. It sits directly below arsenic and above bismuth. While it looks like a silvery, lustrous metal, it is so brittle that it can be easily crushed into a fine powder with a hammer, making it useless for structural purposes but invaluable for specialized chemical and industrial needs.
Atomic & Physical Properties
Antimony is characterized by its poor thermal and electrical conductivity compared to true metals. Its chemistry is dominated by the $+3$ and $+5$ oxidation states.
| Property | Value |
|---|---|
| Atomic Number | 51 |
| Standard Atomic Weight | 121.76 |
| Electron Configuration | $[Kr] 4d^{10} 5s^2 5p^3$ |
| Common Oxidation States | -3, +3, +5 |
| Melting Point | 903.78 K (630.63 °C) |
| Boiling Point | 1908 K (1635 °C) |
| Density | 6.68 g/cm³ |
The Faces of Antimony: Allotropy
Like its neighbor phosphorus, antimony exists in several allotropic forms:
- Metallic Antimony: The most common and stable form. It is a blue-white, lustrous, and brittle crystalline solid.
- Yellow Antimony: A non-metallic, unstable form produced by the oxidation of stibine ($SbH_3$) at very low temperatures.
- Black Antimony: Formed by rapidly cooling metallic antimony vapor. It is much more reactive than the metallic form.
- Explosive Antimony: An amorphous form produced by electrolysis. When scratched or heated, it transforms violently back into the metallic form, releasing a cloud of white smoke.
The Expansion Property
Antimony is one of the few substances (along with water, bismuth, and gallium) that expands when it freezes. This unique property was historically used in Type Metal for printing presses. Adding antimony to the lead alloy caused the metal to expand as it cooled in the mold, forcing it into the fine corners of the letter-engravings and resulting in sharp, clear printed text.
Chemical Reactivity
Antimony is relatively unreactive at room temperature but burns brilliantly when heated in air to form Antimony Trioxide ($Sb_2O_3$).
1. Reaction with Oxygen
2. Reaction with Halogens
Antimony reacts vigorously with halogens, such as chlorine, to produce halides. It can even ignite spontaneously when dropped into chlorine gas.
3. Formation of Stibine
Antimony reacts with nascent hydrogen to form Stibine ($SbH_3$), an extremely toxic and flammable gas analogous to arsine.
The Safety Guard: Flame Retardants
The single largest use of antimony today is in the form of Antimony Trioxide ($Sb_2O_3$). It is used as a synergistic additive in flame retardants for plastics, textiles, and electronics. While it doesn't stop fire on its own, it works with halogenated compounds to "starve" the chemical chain reactions of a fire, significantly slowing its spread.
Alloys & Modern Energy
Antimony is a vital alloying agent. Because lead is too soft for many industrial uses, antimony is added (in amounts ranging from 1% to 15%) to create Hard Lead. This alloy is used in:
- Lead-Acid Batteries: Antimony hardens the lead plates in automotive batteries, improving their structural strength and charging characteristics.
- Pewter: Modern pewter is an alloy of tin, copper, and antimony.
- Bullets: Hardens the lead core of projectiles.
This is the fifty-first part of our "Elements and Their Properties" series. We are moving into the heavier realms of the p-block! To master the concepts of metalloid bonding and the unique physical properties of the Group 15 elements, visit our Success Blueprint.
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