Electrons that occupy the outermost orbit of an Atom are called Valence Electrons. This post will discuss in detail about Valence Electrons, Electron Dot Diagrams, Characteristics, Energy Bands.
What are Valence Electrons
Valence Electrons occupy the orbit that is farthest from the Nucleus of an Atom and this orbit is called a Valence Shell. These Electrons involve or participate in forming compounds’ ionic bonds or chemical bonds. The properties of the atom are determined by the number of Electrons in the valence shell.
Richard Abegg was a pioneer in Valence theory and in the year 1904, he proposed Abegg’s Rule which stated that, the difference of the maximum positive and negative valences of an element is usually eight. This theory was later used by Gilbert Lewis and he came up with “Octet Rule”. Octet Rule states that an Atom is more stable when it has 8 Electrons. Octet can be achieved by its own Electrons or by sharing. Atoms continue to form bonds until it reaches Noble gas electron configuration i.e., 8 Electrons in the outermost shell.
Fig. 2 – (a) Gilbert Lewis (b) Richard Abegg
Electron Dot Diagrams
The electrons in the outermost shell of an Atom can be represented by using dots around the symbol of an element. This representation is called as Lewis Electron Dot Diagram. Number of dots indicates number of atoms of an element. Any element can have maximum of 8 Electrons and hence the number of dots also should be 8. Fig. 2 shows Electron Dot Diagrams.
Fig. 3 – Electronic Dot Diagram of Different Elements
Characteristics of Valence Electrons
The characteristics are:
- An Atom which has less than 4 Valence Electrons (VE), tend to lose/give up electrons to form positive Cations.
- An Atom which has more than 4 VE, tends to gain/borrow electrons to form negative Anions.
- For atoms with exactly 4 VE, it can go either way.
- For any Atom with 8 VE, is considered as the most stable form.
Fig. 4 – Periodic Table
Determining VE using Periodic Table
The position of the Elements in the Periodic Table reflects the number of Valence-Electrons. Fig. 4 shows Periodic Table in which the elements within the same group or in the same column will have the same number of Valence-Electrons and they also have similar chemical properties. If you observe the Periodic Table carefully, you can see that the number of Valence Electrons in groups 1 and 2 and 13 to 18 increases by one from one element to the next. The number is represented in Fig. 5.
How do Valence Electrons Conduct Electricity
Since these Electrons are far from the Nucleus, they are loosely held in the orbit. And also, they require minimal force to leave the Atom and occupy the Valence-shell of the other Atom. This “flow” of “Electrons” is known as Electrical Current. It is measured in Amperes and the force required by the Electron to exit from its Valence Shell is called as EMF (Electro-Motive Force) and is measured in Volts.
Fig. 6 – Electrons Conduct Electricity
Energy Band
Energy Bands can be defined as the group of individual energy levels of electrons surrounding each atom. The concept of energy bandgap can be understood better by knowing two different energy levels namely:
- Valence Band
- Conduction Band
Valence Band
This band consists of Valence Electrons present in the Valence Shell of an Atom. It requires minimal energy to free itself from the outer shell and become free electrons. These Free Electrons move further to Conduction Band.
Conduction Band
The free electrons that are responsible for conduction are present in Conduction Band. The Electrons that leave the valence Band reach higher energy levels or conduction band in order to help in Conduction. This band is above Fermi level. Fermi level or Fermi energy is defined as the highest energy level that the electron occupies at zero temperature.
Fig. 7 – Representation of Energy Band
Band Gap or energy gap is different for conductors, insulators and semiconductors as shown in the Fig. 7. Band gap is very less for conductors and hence the Electrons move easily to conduction band from Valence Band. For semi-conductors, the energy gap exists between the two bands and more energy is required to free Valence electrons. For Insulators, there exists large gap between Conduction Band and Valence Band.
Valence electrons are the highest energy electrons in an atom whereas Core Electrons are tightly bound and are unaffected by chemical reactions as they are closer to the Nucleus. Hope you found this article worth reading!
Also Read: Bohr's Atomic Model - Postulates of Neil Bohr Atomic Model & Limitations Rutherford's Atomic Model - Gold Foil Experiment, Results & Applications Flywheel as Energy Storage Device, Calculations and Rotor Requirements
