What are the Energy Levels?
According to the Bohr atomic model, Each orbit has a fixed amount of energy. Electrons moving in particular orbit possess the energy of that orbit. Larger orbit has a greater amount of energy. From this, you can say that electrons of the outer orbit possess more energy than the inner orbit electrons.
Energy level diagram represents the energy level of a different orbit. As shown in below diagram the first orbit represents the first energy level. The second orbit represents the second energy level and so on.
The larger the orbit of an electron have a greater energy and also higher is the energy level.
What are the Energy Bands?
In case of the single isolated atom possess the fixed amount of the energy in each orbit. But atom in solid is greatly affected by the neighbor atom. The result is that electron in any orbit has a range of energies rather than the single energy. This is known as the energy band. The range of energy possessed by the electrons in solid is known as the energy band.
In the above image, you can see the single isolated atom energy levels, Each orbit has single energy. Therefore electron can have an only single energy corresponding to the orbit in which electrons is present. When an atom is in solid have a different range of energies. In the first orbit have slightly different energies because no two electrons in this orbit have same charge environment. since there are millions of first orbit electrons, The slightly different energy level form an energy band. First orbit electrons form a first energy band, second orbit electrons form second energy band and so on.
Important Energy Bands in solids:-
As we discuss above energy levels in single isolated atoms are converted into corresponding energy band when an atom is in solid. Here are listing of some important band.
The range of energies which have valence electrons is known as the valence band.
As we know The electrons in the outermost orbit are known as the valence electrons. Valence band has the electrons of the highest energy. Valence band may be partially filled or completely filled but have an always some number of valence electrons. In the case of gases, the valence band is completely filled whereas for other material valence band is partially filled. A partially Filled electron can accommodate more electrons.
The range of energies which have free electrons known as the conduction band.
Conduction band consists of the free electrons. Materials like metals have the valence electrons are loosely bound to the nucleus of the atom. Even at the ordinary temperature valence electrons may get detached and become free electrons, These free electrons are responsible for current conduction in a conductor. That’s why they are called conduction band.
All electrons in a conduction band are free electrons. If the substance has an empty conduction band, then no free electrons present in a conduction band so current is not flow. Insulators have an empty conduction band.
The gap between the Conduction band and valence band is known as the forbidden gap.
Forbidden gap have no energy. Because electrons can not stay in this region. The width of the forbidden gap is a measure of the bondage of the valence electrons to the atom. The greater the energy gap more tightly valence electrons are bound to the nucleus of an atom. In order to push the electrons from the valence band to the conduction band, external energy is required. The amount of the external energy is equal to the forbidden energy gap must be filled.