Metallic Bond- Examples, Properties, Strength, Theories explaining metallic bond and Band theory

Have you ever wondered why a metal plate doesn’t break when it falls? 

To understand this, let’s first dig into the structure of the metal given below.

Here, you can see the electrostatic force of attraction between metal ions arranged in the lattice structure and free-floating electrons and this force is so strong that the whole structure is held together which is the reason why the metal plate doesn’t break easily when it falls. This type of bonding is known as metallic bonding. 

TABLE OF CONTENT

• Metallic bond
• Properties of metallic bond
• Theories explaining Metallic bond
• Practice problems
• Frequently asked questions

Metallic bond

The metallic bond is a term used to describe the collective sharing of the sea of valence electrons between several positively charged metal ions The collective sharing of the sea of valence electrons between many positively charged metal ions is referred to as the metallic bond. It can be defined as the electrostatic force of attraction between a metal kernel and a valence electron or we can say the electrostatic force of attraction between metal ions arranged in the lattice structure and free-floating electrons is known as metallic bonding.

Properties of Metallic bond 

• Electrical conductivity: As electrons movement is unrestricted, in the electron sea, any electric current passed through the metal passes through it any electric current that flows through metal also runs through it in the electron sea. So, we can say that metals are good conductors of electricity.

• Malleability and ductility: In the case of metals, the sea of electrons in the metallic bond allows for lattice deformation. As a result, when metals are pounded with a hammer, the rigid lattice is deformed rather than fractured. Because of this, metals can be beaten into thin sheets thin sheets of metal can be formed via beating (malleability) and thin wires (ductility) MalleabilityDuctility
   

  
   

  
   

• Metallic lustre: When light strikes a metallic surface, the photon's energy is absorbed by the sea of electrons. Energy absorption excites electrons, increasing their energy levels. These excited electrons return to their ground states quickly, emitting light in the process. Electrons are excited by energy absorption, which raises their energy levels. These excited electrons quickly transition back to their ground states while producing light. This light emission caused by electron de-excitation gives the metal a shiny metallic luster.
   

• High melting and boiling points: The attractive force between metal atoms is quite strong as a result of strong metallic bonding. It takes a lot of energy to overcome this force of attraction. Strong metallic bonding results in a strong attractive force between metal atoms. To resist this force of attraction requires a lot of effort. That is why metals have high melting and boiling points.

Theories explaining metallic bond

Electron sea model 

The electrostatic force of attraction between kernels and mobile (delocalised) electrons are the metallic bonds. The word kernel is used to represent the internal part of an atom, i.e., the part of the atoms other than the valence shell electrons. Kernels include inner orbital electrons and nuclei.

• Strength of metallic bonding:

From this model, it can be concluded that more the number of mobile electrons, more is the metallic bond strength 

Also, metallic bond strength decreases with increasing size of metal atom.

Generally, for transition elements;

Down the group - Strength of metallic bonding increases.

Across a period - Strength of metallic bonding first increases and then decreases.

Band theory: 

Every molecule comprises various discrete energy levels. This theory explains the way in which electrons behave inside a molecule.

When atoms come together to form a solid they are close to each other. So the outer orbits of electrons from neighboring atoms would come very close or could even overlap Atoms are near to one another when they combine to form a solid. As a result, the outer orbits of electrons from adjacent atoms would be extremely close or even overlap. This would make the nature of electron motion in a solid very different from that in an isolated atom. 

Inside the crystal, each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form which are called energy bands. 

Each electron in the crystal is in a different location, and no two electrons observe the same arrangement of surrounding charges. Each electron will thus have a unique energy level. Energy bands are formed by these various energy levels with constant energy variation.

Different categories of energy bands have been discussed below

• Valence band

This is the outermost orbital of an atom where the electrons are so tightly bound that they cannot be removed as free electrons.

• Conduction band

This is the highest energy level or orbital in the outermost shell in which the electrons are free enough to move. It is made up of those orbitals which are unoccupied by electrons either in the valence shell or higher unoccupied shell.

• Band gap

There is one energy gap that separates these two bands, the valence band and conduction band. This gap is known as the forbidden energy gap.

• Conductors

In metals, there is no separation between the bands. This helps the incited electrons to easily move from one band to another and hence, metals are good conductors of electricity.

• Semiconductors

In semiconductors, there is a small gap between the valence band and the conduction band. Hence, only a small fraction of electrons (having sufficient energy) can jump when incited. However, we can increase the conductivity of such substances by increasing the temperature or doping.

Example: Silicon, germanium.

• Insulators

In insulators, the difference between the valence band and conduction band is very high. Hence, no conductivity is shown by such substances even on increasing temperature.

Example: Glass, plastic.

Practice problems 

Q 1. Which among the alkali metals and alkaline earth metals have a higher strength of metallic bond according to the electron sea model?

Answer: In alkaline earth metals - Two electrons can be lost from the outermost shell

In alkali metals - Only one electron can be lost from the outermost shell

We know,

Strength of metallic bonding: Alkaline earth metals > Alkali metals

Q 2. How can we define semiconductor using band gap theory?

Answer: In semiconductors, there is a small gap between the valence band and the conduction band. Hence, only a small fraction of electrons (having sufficient energy) can jump when incited. 

Q 3. Mercury is a metal but it has a low melting and boiling points. Why?

Answer: Mercury has completely filled atomic orbitals due to which it becomes difficult to knock out these electrons from their orbitals and form metallic bonds. Therefore, they are soft and do not show multiple valence states unlike other transitional metals thereby decreasing its boiling point and melting point.

Q 4. Insulators do not conduct electricity. Explain this using band theory.

Answer: Because electrons in insulators are tightly bound with the nucleus, thermal energy is insufficient to push electrons into the conduction band at room temperature, and thus no electrons are available for conduction. So, we can say that in insulators, the difference between the valence band and conduction band is very high. Hence, no conductivity is shown by such substances on increasing temperature.

Frequently asked question-FAQs

Q 1. How are metallic bonds different from ionic and covalent bonds?
Answer: Covalent bonds involve the sharing of electrons in the valence shell, metallic bonds involve the delocalised electrons present in the lattice of the metals and ionic bonds involve the transferring and accepting of electrons from the valence shell.

Q 2. How is the valence band different from the conduction band?
Answer: Both valence and conduction bands are separated by some amount of energy. The major difference between them is that the conduction band holds those electrons that are responsible for conduction while the valence band specifies the energy level of electrons present in the valence shell of an atomic structure.

Q 3. Which band determines the electrical conductivity of a solid?
Answer: The forbidden band or the band gap determines the conductivity of a solid.

Q 4. How many electrons can be delocalised in an atom?
Answer: The valence electrons of metal atoms are delocalized and move freely throughout the solid rather than being confined/bound to any specific atom, according to the 'Sea of electrons' model. So, for any atom, the number of electrons delocalized in the sea of electrons model equals the number of valence electrons (electrons in the valence orbital).

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