METALLIC BOND: PROPERTIES, HOW IT IS FORMED AND EXAMPLES - CHEMISTRY - 2025

The metallic bond is the one that holds the atoms of the metallic elements tightly together. It is present in metals and defines all their physical properties that characterize them as hard, ductile, malleable materials and good conductors of heat and electricity.

Of all the chemical bonds, the metallic bond is the only one where the electrons are not located exclusively between a pair of atoms, but are delocalized between millions of them in a kind of glue or "sea of ​​electrons" that hold them tightly together. or cohesive.

Copper metal bond

For example, suppose the metal copper. In copper, its Cu atoms give up their valence electrons to form the metallic bond. Above this bond is represented as Cu ²⁺ cations (blue circles) surrounded by electrons (yellow circles). The electrons are not still: they move throughout the copper crystal. However, in metals we do not speak formally of cations, but of neutral metal atoms.

The metallic bond is verified by examining the properties of the metallic elements, as well as those of their alloys. These integrate a series of shiny, silver, tough, hard materials, which also have high melting and boiling points.

How is the metallic bond formed?

Metallic bond in zinc

The metal bond is formed only between one set or group of metal atoms. In order for the electrons to relocate throughout the metallic crystal, there has to be a "highway" that they can travel on. This is designed from the overlap of all the atomic orbitals of the neighboring atoms.

For example, consider a row of zinc atoms, Zn ··· Zn ··· Zn ···. These atoms overlap their valence atomic orbitals to create molecular orbitals. In turn, these molecular orbitals overlap with other orbitals of neighboring Zn atoms.

Each zinc atom contributes two electrons to contribute to the metallic bond. In this way, the overlap or union of the molecular orbitals, and the atoms donated by zinc, originate a "highway" by which the electrons are delocalized throughout the crystal as if they were a glue or a sea of ​​electrons, covering or bathing all the metallic atoms.

Properties of the metallic bond

Structures

The metallic bond originates compact structures, where the atoms are closely united, without much distance that separates them. Depending on the type of specific structure, there are different crystals, some more dense than others.

In metallic structures, one does not speak of molecules, but of neutral atoms (or cations, according to other perspectives). Returning to the example of copper, in its compacted crystals there are no Cu ₂ molecules, with a Cu-Cu covalent bond.

Reorganization

The metallic bond has the property of reorganizing itself. This does not happen with the covalent and ionic bonds. If a covalent bond breaks, it will not re-form as if nothing had happened. Also, the electric charges on the ionic bond are invariable unless a chemical reaction takes place.

Consider for example the metal mercury to explain this point.

The metallic bond between two adjacent mercury atoms, Hg ··· Hg, can break and re-form with another neighboring atom if the crystal is subjected to an external force that deforms it.

Thus, the bond is reorganized while the glass undergoes deformation. This gives metals the properties of being ductile and malleable materials. Otherwise they will break like pieces of glass or ceramic, even when hot.

Thermal and electrical conductivities

The property that the metallic bond has of having its electrons delocalized also gives metals the ability to conduct heat and electricity. This is because, since the electrons are delocalized and moving everywhere, they effectively transmit the atomic vibrations as if it were a wave. These vibrations translate into heat.

On the other hand, when electrons move, empty spaces are left behind that others can occupy, thus having an electronic vacancy through which more electrons can "run" and thus originate an electric current.

In principle, without addressing the physical theories behind the phenomenon, this is the general explanation of the electrical conductivity of metals.

Metallic luster

Delocalized and mobile electrons can also interact with and reject photons in visible light. Depending on the densities and surface of the metal, it can display different shades of gray or silver, or even iridescent sparkles. The most exceptional cases are those of copper, mercury and gold, which absorb photons of certain frequencies.

Electron delocalization

To understand the metallic bond it is necessary to understand what is meant by the delocalization of electrons. It is impossible to determine where the electrons are. However, it can be estimated in which region of space they are likely to be found. In a covalent bond AB, the pair of electrons is distributed in the space that separates the atoms A and B; they are then said to be located between A and B.

In an AB metallic bond, however, it cannot be said that electrons behave in the same way as in an AB covalent bond. They are not located between two specific atoms of A and B, but are diffused or directed to other parts of the solid where there are also compacted, that is, closely bonded, atoms of A and B.

When this is so, the electrons of the metallic bond are said to be delocalized: they travel any direction where there are atoms of A and B, as shown in the first image with the copper atoms and their electrons.

Therefore, in the metallic bond we speak of a delocalization of these electrons, and this characteristic is responsible for many of the properties that metals have. The theory of the sea of ​​electrons is also based on it.

Examples of metallic bonds

Some commonly used metal links in daily life are as follows:

- Metallic elements

Zinc

Metallic bond in zinc

In zinc, a transition metal, its atoms are linked by the metal bond.

Gold (Au)

Pure gold, like the alloys of this material with copper and silver, are currently highly used in fine jewelry.

Copper (Cu)

This metal is widely used in electrical applications, thanks to its excellent electricity conduction properties.

Silver (Ag)

Given its properties, this metal is widely used both in fine jewelry applications and in the industrial field.

Nickel (Ni)

In its pure state it is usually used for the manufacture of coins, batteries, foundry or various metal parts.

Cadmium (Cd)

It is a very toxic material and is used in the manufacture of batteries.

Platinum (Pt)

It is used in fine jewelry (alloys with gold), and in the manufacture of laboratory measuring instruments and dental implants.

Titanium (Ti)

This metal is commonly used in engineering, as well as in the manufacture of osteosynthetic implants, industrial applications, and jewelry.

Lead (Pb)

This material is used in the manufacture of electrical conductors, more specifically, for the manufacture of the outer jacket of telephone and telecommunications cables.

- Metallic compounds

Common steel

The reaction of iron with carbon produces common steel, a material much more resistant to mechanical stress compared to iron.

Stainless steel

A variation on the above material can be found by combining common steel with transition metals such as chromium and nickel.

Bronze

It is produced by combining copper with tin, in an approximate proportion of 88% and 12%, respectively. It is used in the making of coins, tools and public ornaments.

Mercury alloys

Various alloys of mercury with other transition metals, such as silver, copper and zinc, produce the amalgams used in dentistry.

Chrome Platinum Alloy

This type of alloy is widely used to make razor blades.

Pieltre

This alloy of tin, antimony, envelope and bismuth is commonly used for making household utensils.

Brass

It is generated by combining copper with zinc, in a proportion of 67% and 33%, respectively. It is used in the manufacture of hardware items.

Sea of ​​Electron Theory

Simple representation of a sea of ​​electrons. Source: Muskid

The image above illustrates the concept of a sea of ​​electrons. According to the theory of the sea of ​​electrons, metal atoms shed their valence electrons (negative charges) to become atomic ions (positive charges). The released electrons become part of a sea in which they are delocalized for each inch of the metallic crystal.

However, this does not mean that a metal is made up of ions; its atoms are actually neutral. We do not speak of Hg ⁺ ions in liquid mercury, but of neutral Hg atoms.

Another way to visualize the sea of ​​electrons is by assuming the neutrality of the atoms. Thus, although they grant their electrons to define the metallic bond that keep them tightly cohesive, they also instantly receive other electrons from other regions of the crystal, so that they never acquire a positive charge.

This theory explains why metals are ductile, malleable, and how bonds can be rearranged to allow the deformation of a crystal without breaking. Some people call this sea of ​​electrons "electronic cement", since it is capable of moving, but under normal conditions, it solidifies and keeps the metallic atoms firm and fixed.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
3. Wikipedia. (2020). Metallic bonding. Recovered from: en.wikipedia.org
4. The Editors of Encyclopaedia Britannica. (April 4, 2016). Metallic bond. Encyclopædia Britannica. Recovered from: britannica.com
5. Helmenstine, Anne Marie, Ph.D. (January 29, 2020). Metallic Bond: Definition, Properties, and Examples. Recovered from: thoughtco.com
6. Jim Clark. (September 29, 2019). Metallic Bonding. Chemistry LibreTexts. Recovered from: chem.libretexts.org
7. Mary Ellen Ellis. (2020). What is a Metallic Bond? - Definition, Properties & Examples. Study. Recovered from: study.com