Post-Transition Metals: Definition, Properties, Uses, and Types

Post-transition metals are a group of interesting metals that appear to the right of the transition metals on the periodic table. They also neighbor the metalloids. They tend to be softer and have lower boiling points than most metals, and also form compounds more easily with nonmetals. The group includes some very popular metals such as aluminum, tin, and lead, but also some unusual and toxic metals such as thallium and polonium.

 This article will explore the characteristics of post-transition metals in more detail.

What Is a Post-Transition Metal?

The post-transition metals are a group of elements on the periodic table that appear to the right of the transition metals and left of the metalloids. There is some disagreement about how widely to define the group, but the universally accepted elements include: aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium. As a group, they are typically soft or brittle, with low melting points and low density. Chemically, they typically can form half-metal compounds and are amphoteric. This means that they can form compounds that can act as both acids and bases.

What Are the Different Properties of Post-Transition Metals?

The post-transition metals have some physical and chemical properties in common. Individual elements in the group may not follow the trend exactly, but the details below can generally be applied to the group.

1. Lower Melting and Boiling Points

Post-transition metals typically have lower melting and boiling points than other metals. Gallium’s melting point is just above room temperature, so it can be melted purely by holding it in your hand. Lead has a relatively low melting point compared to other common metals, making it easy to cast without a sophisticated furnace.

2. Softer and Lower Densities

Post-transition metals tend to be softer and less dense than the transition metals to their left on the periodic table. The hardness is generally inferred by measurements such as Mohr’s hardness scale. Metals such as tin, lead, and aluminum are well known as soft metals. In terms of the densities of the post-transition metals, even though lead and thallium have reasonably high densities of 11.35 g/cm3 and 11.85 g/cm3 respectively, they are still less dense than many transition metals such as: gold, tungsten and platinum.

3. Greater Tendency To Form Compounds With Nonmetals

Post-transition metals are still metals, but they form compounds with non-metals more readily than other metals do. Post-transition metals sit next to the metalloids on the periodic table, which have some characteristics of nonmetals. The post-transition metals tend to exhibit covalent or directional bonding, which allows them to bond more easily with nonmetals. 

4. Amphoteric Nature of Their Oxides

Post-transition metals form amphoteric oxides. This means that their oxides can react with either acids or bases to form salt and water. In other words, the oxides can either be proton donors or proton acceptors. Some other elements in the periodic table form amphoteric oxides, such as manganese and iron, but about half of all amphoteric-capable elements are post-transition metals. 

5. Partially Filled D Orbitals in Electronic Configuration

Post-transition metals have filled d orbitals and a partially filled p orbital. The transition metals have partially filled d orbitals, but the fact that post-transition metals are situated to the right of the d block means their d orbitals are full. The status of their p orbital depends on the oxidation state. Cations of the post-transition elements will lose electrons from their p orbital before losing electrons from the d orbital.

6. Wider Range of Oxidation States

Post-transition metals generally have a wide range of oxidation states. Aluminum and bismuth are examples of this — aluminum has three oxidation states (+1, +2, and +3), and bismuth has five (-3, +1, +2, +3 as the most common, plus the occasional +5). However, transition metals tend to have a wider range of oxidation states than post-transition metals.

What Are the Chemical Properties of Post-Transition Metals?

Not all element groupings on the periodic table share chemical properties — especially when loosely grouped like the post-transition metals. However, they do share some similarities. Post-transition metals tend to form covalent bonds, which in turn means that they form compounds more readily with non-metals. In addition, post-transition metals tend to have amphoteric oxides — that is, they can act as either acids or bases.

What Are the Common Methods Used for the Extraction and Refining of Post-Transition Metals?

The minor post-transition metals tend to be recovered during the refining process of more widely used metals. For instance, thallium, bismuth, and indium are produced in small amounts through the refining of lead. Ores containing lead are mined and crushed. Typically, a froth flotation process will concentrate the lead-rich material. A pyrometallurgical smelting process is then used to oxidize the lead and remove sulfur. The molten lead is then cast but still contains traces of many other metals, even valuable metals such as silver. To remove them, it’s refined, during which the raw metal is smelted again and subjected to various chemical operations to separate the different impurities. An example is the Betterton-Kroll process used to refine bismuth from lead.

What Are the Uses of Post-Transition Metals?

The uses of post-transition metals are varied. The two most widely useful are aluminum and lead, although lead has been phased out of many products due to its toxicity. Aluminum is mainly used for its good strength-to-weight ratio and its corrosion resistance. Lead has been used for its chemical properties (in paint, petrochemicals, and batteries) and for its softness and low melting point that make it easy to work and cast. Tin is used widely in solder, for joining electronic components to circuit boards. Other post-transition metals such as bismuth and gallium are used where very low melting points are valuable, such as in fusible links and fire detection systems.

What Industries Use Post-Transition Metals?

Many industries use post-transition metals. Lead is currently mostly used in lead-acid batteries, such as those found in vehicles. Aluminum is far more common now, finding applications in industries like: automotive, construction, manufacturing, and aerospace. The building industry also uses fire-fighting equipment that contains post-transition metals such as bismuth and indium for fusible links and plugs. Due to their low melting points, the plugs or links melt if the air is heated by a fire and thus provide a passive detection mechanism for fires.

What Are the Different Types of Post-Transition Metals?

The post-transition metals can be defined differently depending on who you talk to. However, they are generally accepted to include the elements listed below.

1. Aluminum

Aluminum is the third most abundant element in the earth’s crust. It is a ductile and relatively light metal that is widely used throughout the modern world. It is also corrosion-resistant and easily machined, making it ideal for many applications.

2. Bismuth

Bismuth is a silvery-white metal that is recovered primarily as a co-product of lead production. It is rarely used in its pure form but is instead added to alloys — in some cases as a non-toxic replacement for lead. Some bismuth alloys have very low melting points, even below that of water. Bismuth alloys are therefore used in fire sprinkler systems — if fire heats the air enough, it melts the bismuth plug and releases fire-suppression spray. Bismuth is a mid-value metal, being more expensive than copper but much cheaper than silver.

3. Lead

Lead is a soft and heavy metal that was once widely used because it resists corrosion and is easy to shape. It has historically been used in water piping, paint, and batteries among many other applications. However, prolonged exposure to lead is toxic, so its use has been scaled back significantly. It is a very effective radiation shield, partly due to its high density.

4. Gallium

Gallium is unusual for its low melting point — only slightly above room temperature. Therefore, gallium can melt in your hand. As a liquid, it readily coats (wets) glass and can provide a mirror finish. It is most commonly used in analog integrated circuits and also has use as a semiconductor when alloyed with arsenic. It is added into laser diodes and light-emitting diodes (LEDs).

5. Tin

Tin is a silvery, soft metal with a relatively low melting point. It is malleable and has very good corrosion resistance. Tin is primarily used as an alloying element in applications such as solder and dental fillings. It is not considered a precious metal but is more expensive than copper or iron.

6. Polonium

Polonium is a radioactive element that is very rare in the natural environment. It is commercially made through the decay of bismuth-210, but its radioactivity makes it toxic. It thus has limited uses but shows up as a source of alpha particles in measuring and research devices. Those alpha particles also produce heat which can be harnessed as an energy source for spacecraft.

7. Thallium

Thallium is a flavorless but poisonous metal. It is typically produced as a byproduct of sulphuric acid production or lead refining. Thallium sulfate was historically used as a rat poison but has since been banned due to the risk of accidental exposure to humans. It currently has niche uses in photocells and specialized glass.

8. Indium

Indium is a rare metal with a silvery-white luster that comes out of the zinc and lead production process. It is softer than lead and easily scratched. Due to its low melting point, indium is used in fire sprinkler systems and other products with fusible links.

How To Choose Which Type of Post-Transition Metals To Use

You’ll only select a post-transition metal if your application demands their specific properties. It is important to identify whether toxicity would be an issue — whether people or the environment may be exposed to the metal. If so, you can eliminate lead, thallium, or polonium as options. Secondly, to identify the best post-transition metal to use, evaluate your operating temperatures and the metals’ melting points. If a very low melting point is required, then gallium or bismuth may be suitable. However, if the metal must remain solid even under elevated temperatures, then other post-transition metals such as aluminum or tin may be better options. After that, you can evaluate other physical and chemical properties such as strength and corrosion resistance.

What Type of Post-Transition Metals Is Used in Construction?

The type of post-transition metals used in construction are aluminum and lead.

Are Post-Transition Metals Commonly Used in the Construction Industry?

As a group, post-transition metals are not often used in the construction industry. Except for aluminum, they’re usually only good for niche products, such as fusible links in fire sprinklers. Lead was once used extensively in the construction industry as a paint additive and for water piping, but these lead applications have been banned for new installations since the 1970s and 1980s respectively.

The exception to the rule is aluminum, which shows up in items such as window and door frames and extruded building trim. It is used in various items such as floating ceiling frames and gutters. Aluminum is also increasingly used in building wiring as a cheaper alternative to copper. 

What Type of Post-Transition Metals Is Used in Aerospace Manufacturing?

Aluminum is the most common post-transition metal used in aerospace manufacturing. Aluminum alloys have good strength but are almost always lighter-weight than other strong metals. Weight is always a concern for aircraft. Bismuth is another post-transition metal that finds some niche applications in aerospace, particularly in specialist electronic equipment, such as photosensitive semiconductors and solder for joining other metals.

What Type of Post-Transition Metals Is Used in Automotive Manufacturing?

Aluminum is the post-transition metal used most widely in automotive manufacturing, primarily in alloy form. Aluminum is valuable for automotive manufacturing as it is relatively light for its strength and is naturally corrosion-resistant. The low weight of aluminum components, particularly compared to steel, has fuel-efficiency benefits for vehicles. Aluminum alloys appear in many automotive components, such as: the vehicle frame, body panels, engine components, and wheels.

What Are the Advantages of Using Post-Transition Metals?

The main advantage of using post-transition metals is their low melting points. This property directly gives rise to their use in fusible links for fire detection systems. With other post-transition metals like aluminum and lead, their moderately-low melting points make them easier to work with — to melt and cast. Additionally, the low density of some post-transition metals like aluminum make them perfect for weight-sensitive structures such as aircraft and cars.

What Are the Disadvantages of Using Post-Transition Metals?

Toxicity is the primary disadvantage of using post-transition metals. In particular, lead and thallium are toxic, and polonium is radioactive. This severely limits their potential use. A secondary disadvantage is that post-transition metals tend to have lower mechanical strength when compared to other metals.

What Is the Difference Between Post-Transition Metals and Light Metals?

Light metals (as opposed to heavy metals) are those with notably low densities. Light metals include: lithium, aluminum, and magnesium. The grouping is thus defined by a specific physical property, whereas post-transition metals are defined by a chemical property. However, some elements are considered to belong to both groups, such as: aluminum, gallium, and thallium.

To learn more, see our full guide on Light Metals.

What Is the Difference Between Post-Transition Metals and Metalloids?

Metalloids are a small group of elements that make up a stepped line in the periodic table between metals and non-metals and generally show some properties of each. This group of elements is adjacent to the post-transition metals. However, the difference between metalloids and post-transition metals is that metalloids are not considered true metals, as they have some properties of nonmetals. Some metalloid allotropes have non-metallic properties, whereas other allotropes have metallic properties. An example is carbon: diamond has non-metallic properties, but graphite is electrically conductive (a metallic property).

To learn more, see our full guide on Metalloids Examples.

Summary

This article presented post-transition metals, explained them, and discussed the different types and various applications. To learn more about post-transition metals, contact a Xometry representative.

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