He wondered, “Why is water liquid when ice is solid?”
Amit asked his teacher about it.
His teacher told him that this difference was due to the arrangement of atoms and molecules inside the substance.
So, let’s learn about the behaviour of atoms and molecules in liquids and solids.
Solids have very closely bound atoms while liquids have loosely bound atoms.
Therefore, intermolecular space in solid is small and that of in liquid is large.
Now, let us discuss solid and its properties in detail
As atoms in the solids are closely packed, the outer orbit of electrons come very close or may even overlap.
Each electron has a unique position and is surrounded by a different pattern of charges. Therefore, they will have different energy levels.
These energy levels with continuous energy variation form an energy band.
The energy band which includes energy levels of valence electrons is known as the valence band and energy band above it is known as the conduction band.
The gap between the conduction band and valence band is known as energy band gap.
When the gap between the conduction and the valence band is very small, the valence electrons may jump to the conduction band causing conductivity.
Conductivity is a measure of ease at which an electric charge or heat can pass through a material.
Based on the size of the energy band gap, solids can be classified into three categories.
Now let’s discuss the classification based on the energy bandgap.
They are classified into
metals,
insulators
and semiconductors
Let's discuss metals, insulators, and semiconductors in detail.
In metals, the gap between the conduction band and valence band is either very small or they overlap.
Therefore, electrons can move from valence band to conduction band making conduction possible.
Hence, metals have very high conductivity and low resistivity.
Examples of metals: iron, silver, gold, copper, etc
In insulators, the energy band gap is very high (>3 eV).
Thus, electrons can't jump to the conduction band which results in low conductivity.
The resistivity of insulators is very high as compared to that of metals. due to the high bandgap.
Examples of insulators: rubber, glass, plastic, wood, etc
In semiconductors, the energy gap is larger than metals but smaller than insulators (< 3 eV).
Therefore, at room temperature, few electrons can jump to the conduction band.
Hence, resistivity is comparatively less and conductivity is more than that of insulators.
Examples of semiconductors are silicon, germanium, etc It has a wide application in electronic industries.
Revision
Solids are divided into 3 categories based on the energy bandgap.