Classification of Metals

What is Classification of Metals?

From the computer you’re reading this on, to the bolts in your plumbing, everything is connected; and there are over 80 different types of metals in use. In nature, a wide number of metals are available. Depending on which quality or characteristic you use as a yardstick, metals can be classified into different categories. 

Classification Based on Conductivity

The three categories are:

• Metals 
• Insulators
• Semiconductors

The relative values of electrical conductivity (σ) and resistivity (ρ = 1/σ) are used to classify solids into the following categories:

• Metals – Solids with extremely low resistance or extremely high conductivity fall under this category. Hence,

σ ~ 102 – 108 S/m

ρ ~ 10-2 – 10-8 Ωm

• Insulators – If you are looking to know what is the difference between a conductor and an insulator, then remember that insulators are solids with extremely high or extremely low resistance or conductivity. Hence,

σ ~ 10-11 – 10-19 S/m

ρ ~ 1011 – 1019 Ωm

• Semiconductors – Solids with resistivity or conductivity values that fall between metals and insulators. Hence,

σ ~ 105 – 10-6 S/m

ρ ~ 10-5 – 106 Ωm

It’s crucial to remember that these figures are only estimates, and they could fall outside of the range. Semiconductors can be classified into extrinsic semiconductors and intrinsic semiconductors;

• Intrinsic Semiconductor: Chemically, an intrinsic semiconductor material is created to be exceedingly pure. The most prevalent intrinsic semiconductor elements are germanium (Ge) and silicon (Si). At very high temperatures the extrinsic semiconductors become intrinsic because band to band transition dominants over impurity ionisation. 

The intrinsic semiconductor becomes an insulator at 0K(−273∘C). 

• Extrinsic Semiconductor: By introducing a tiny number of acceptable replacement atoms known as IMPURITIES, the conductivity of semiconductors can be considerably increased. DOPING is the process of introducing impurity atoms into a pure semiconductor. Under Extrinsic Semiconductor, there’s a dependence on temperature as well as on the amount of impurity.

Classification Based on Energy Bands

According to Bohr’s atomic model, the electrons revolve around the nucleus in immobile orbits. The energy of an electron is dictated by the orbit in which it revolves, according to Bohr’s atomic model. This is correct for a single atom. Atoms in a solid are close together. As a result, electrons from neighbouring atoms approach very close together, and at times even overlap. As a result, the velocity of an electron in a solid differs from that of an isolated atom.

Valence electrons are electrons that are filled in the innermost orbits of an atom. Conduction electrons, on the other hand, are electrons in the outer orbits that do not fill the shell.

Every electron in a crystal has a distinct location and a varied pattern of surrounding charges. As a result, each electron has a unique energy level. The Energy bands are made up of these energy levels with a constant variation in energy. The valence band is the energy band that encompasses the valence electrons’ energy levels. In addition, the energy band above it is referred to as the Conduction band. The valence electrons remain in the valence band if no external energy is given.

When the lowest level in the conduction band is lower than the highest level in the valence band, electrons flow from the valence band to the conduction band. The conduction band is usually empty.

Based on Band Energy- metals, insulators, and semiconductors are classified as-

• Metals– In metals, the conduction and valence bands partially overlap, and there is no prohibited energy band gap between the conduction and valence bands. Electrons can easily migrate from the valence band to the conduction band since there is no forbidden energy band gap and a large number of electrons available for conduction.

Consequently, metals have low resistance and a good heat conductor.

• Insulators– Between the valence band and the conduction band in insulators, there is a substantial energy gap. As a result, electrons in the valence band find it difficult to travel into the conduction band.

The conductivity of insulators is very low because there is no electron for transfer in the conduction band.

• Semiconductors– There is a discrete but very modest energy gap between the valence band and the conduction band in semiconductors. The electrons must acquire energy and leap from the valence band to the conduction band at room temperature.

As a result, the semiconductors are more conductive at higher temperatures.

Electric Current

Current is caused by electron mobility in conductors. Electrons and positive ions carry the charge in gases. Positive and negative ions carry the charge in liquids. The transport of charge in semiconductors is caused by electrons and holes.

Currents are of many types. The following are the several types of electric current: 

(i) Steady current: A steady current is one whose magnitude does not change over time. For a stable current, the variances between current(I) and time(t) will be a straight line.

(ii) Changing current: A varying current is a current whose magnitude changes over time.

(iii) Alternating current: An alternating current is a current whose magnitude changes over time and whose direction changes regularly. A sine or cosine curve is used to represent such a current.

Things to Remember: 

• Solids can be categorised into three categories based on their resistivity, conductivity, and energy bands.
• Metals, Insulators, and Semiconductors are the three categories.
• According to Bohr’s atomic model, the electrons revolve around the nucleus in stationary orbits.

Conclusion

Materials can be classified into pure semiconductors, conductors, and insulators based on conductivity! 

Here, we’ve learned about the classification of metals based on conductivity, types of semiconductors, and more. When you have the proper understanding of the topic, you are likely to score well in the examination.