Electron and Electron Charge

What is an Electron?

An electron is a subatomic particle that is electrically negative in charge. Electrons are found in every atom apart from other particles.

Electrons are the primary element of electric current. The charge on one electron is known as a unit electrical charge. The charge of an electron is equal to the charge of the proton hole with an opposite sign. The amount of electrical charge is not determined according to each electron since it is extremely small. Instead, it is determined by the standard unit of quantity of electrical charge and represented by the coulomb (C).

C = 6.24 x 10¹⁸ electrons

Charge of Electrons

The existence of charged particles was first discussed or hypothesised by Richard Laming (1838-1851). Later, Irish physicist G. Johnstone Stoney suggested the term “electron” for the particle in the year 1891. The electron was finally discovered or identified as a particle by British physicist J.J. Thomson and his team in the year 1897.

An atom is the smallest possible component of an element. Every solid, liquid, gas and plasma are composed of atoms. Atoms are extremely small; typical sizes are around 100 picometers. Atoms are very small and it is not possible to accurately predict the behaviour of an atom. An atom also comprises neutrons and protons. Protons are positively charged, while neutrons neither have a positive charge nor a negative charge. It is a changeless particle.

Also Read: Atomic Structure

The centremost part of an atom is called its nucleus. The proton and neutron are housed inside the nucleus of the atom. Electrons, on the other hand, are found outside the nucleus.

The electrons of an atom are attracted to the protons in the nucleus of the atom. If the number of electrons is equal to the number of neutrons, then the atom is electrically neutral. If an atom has more or fewer electrons than the number of protons, then it has an overall negative or positive charge, respectively. These atoms are called ions.

Charge of Electron in Coulombs

Electrons have an electric charge of −1, and their mass is approximately about 1/2000 the mass of a neutron or proton. Electron charge is usually denoted by the symbol e. It is a fundamental physical constant that is used to express the naturally occurring unit of electric charge, which is = 1.602 × 10^-19 coulomb. So, the charge of the electron will be -1.602 x 10^-19 C.  In the centimetre-gram-second system of units (CGS), it is 4.80320425(10)×10^-10 statcoulombs.

If we trace back to the development of the system, the use of the elementary charge as a unit was first promoted by George Johnstone Stoney in 1874. This was done for the first system of natural units, which were called Stoney units. It was only at a later point in time that he decided to give the name electron for this unit. It was done so because, at the time, electrons were not discovered. What it meant was that there was no big difference between the particle electron and the unit of charge electron. Later, the name electron was decided to be assigned to a particle, and the unit of charge e had to get a new name.

Apart from the electrons, all the freely existing charged subatomic particles have an electric charge equal to the above-mentioned value or some whole-number multiple of it. Quarks, which are found in protons and neutrons, are said to have charges of 1/3 or 2/3 of the said value.

If the value is expressed in atomic units, then the elementary charge will take the value of unity, which is e = 1. Therefore, an electron’s charge can be denoted by – e. On the other hand, if we look at protons, they will have a charge of +1. As such, protons are relatively stable; their number rarely changes, only in the instance of radioactive decay. Meanwhile, if we talk about the ground state of an atom, it has an equal number of protons and electrons, which means that it will have a net charge of 0. However, as electrons can be transferred from one atom to another atom, they can become charged and are often called ions.

Oil-Drop Experiment

While J. J. Thomson was the first to discover the electron, he was only able to obtain or determine the electron’s charge to mass ratio. In particular, the first person to successfully measure the electron’s charge was an American Physicist named Robert Millikan. He was able to determine the elementary charge of an electron through his oil-drop experiment in 1909. He was awarded the noble prize in the year 1923 for his extensive work in the field.

If we talk about the oil-drop experiment, it involved using droplets of oil and ionising them in the presence of the air. Oil droplets were dropped into a chamber having a hole. As the droplets fell into the chamber, he exposed the droplets to X-rays. The goal of this step in the process was to ionise the molecules in the air. Once this occurred, the electrons were attached to the oil droplets as they were charged. The chamber was connected to a battery, and the potential difference at the top and bottom gave rise to an electric field that acted on the charged oil drops.

By keeping the voltage constant, Millikan balanced the force of gravity with the force of the electric field on the charged particles, and this caused the oil droplets to be suspended in mid-air. Then, Millikan calculated the charge on particles that were suspended in mid-air.

While conducting the experiment, he found that the force of gravity was basically equal to the force of the electric field (the product of the charge (q) and the electric field (E)). It was given as follows:

$\text{q}\cdot \text{E}=\text{m}\cdot \text{g}$

$\text{q}=\frac{\text{m}\cdot \text{g /}}{\text{E}}$

So, by finding the mass of the oil droplets and the acceleration due to gravity (9.81 m/s²), along with the energy of the x-rays that were being used, Millikan could now calculate the charge.

Millikan was not able to directly count the number of electrons on each oil droplet.  Nonetheless, he found that the common denominator between all measured charges was equal to 1.5924(17)×10-¹⁹ C. It was then concluded that this value was the charge of an electron.

Today, the measured value of an electron’s charge, which is 1.5924(17) × 10-¹⁹ C, only differs from the accepted value of 1.602176487(40) × 10-¹⁹ C by lesser than 1 per cent.

What Is the Quantum of Charge?

Most of the elementary particles (quarks included) usually have charges that are integer multiples of 1/3 e. So, we can say that the “quantum of charge” is 1/3 e.  We can also say that the”elementary charge” is 3 times as large as the “quantum of charge”.

Electron Shell (Orbitals)

An atom consists of a number of shells around its nucleus. These shells accommodate the electrons of the atom. Electron shells usually consist of one or more subshells. The electron shell decides the electronic configuration of the atom. The number of electrons that can be accommodated in each shell is given by 2n^2, where n is the number of the electron shell.

The first electron shell can accommodate only 2 electrons, and as the shell number increases, the number of electrons that can be accommodated increases. This orbital is filled first before the next orbitals.

Examples: 

1. Hydrogen has 1 electron, electronic configuration of hydrogen – 1s¹.

2. Helium has 2 electrons, electronic configuration of Helium –  1s².

The second electron shell contains one spherical shape s orbital and three dumble shape p orbitals, each can accommodate 2 electrons. After the first electron shell filling of the 1s orbital, 2s and 2p orbitals of the second electron shells are filled in.

Examples:

1. The atomic number of Li is 3, and the electronic configuration of Lithium = 1s² 2s¹.
2. The atomic number of Ne is 10, and the electronic configuration of Neon = 1s² 2s² 2p⁶ (Inert gas).

The third electron shell includes additional orbitals for large elements. Subshell d accommodates 5 orbitals, and subshell f accommodates 7 orbitals.

The 3n principal shell has orbitals of s, p, and d and can accommodate 18 electrons in it, and the 4n principal shell has s, p, d and f subshell and can accommodate 34 electrons in it.

The electrons in the outermost occupied shell determine the chemical properties of the atom; it is called the valence shell, and the electrons in the valence shell are called the valence electrons.

Each shell consists of one or more subshells, and each subshell consists of one or more atomic orbitals.

The electron shells are labelled as K, L, M, N, O, P, Q or 1, 2, 3, 4, 5, 6, 7, going from the innermost electron shell to the outermost shell. The electrons in the outer shell occupy higher average energy, and these electrons can move farther than the electrons in the innermost shell. The pull of the atom’s nucleus on the valence electrons will be less and can easily be broken. The pull of the nucleus reduces as the shell number increases, and therefore, by giving a small external force, the electrons can be moved out of the shell.

Energy Level

There are some energy bands and energy gaps in an atom. Each electron is located or included in any of the energy bands. Each atom has a valence band and a conduction band. The gap between the valence band and the conduction band is called the forbidden energy gap. The gap also decides the electrical properties of the material.

The outermost electrons are placed in the valence band. The electrons in the conduction band can move freely to produce electricity.

For a conductor, the conduction band overlaps the valence band, and without any further energy, the electrons in the valence band can be brought to the conduction band. No forbidden gap is found in conductors. Common examples of conductors are copper, silver, etc.

For a semiconductor, the gap between the valence band and the conduction band is very small. A semiconductor has a forbidden energy gap of approximately 1 electron volt (1 eV). The electrons in the valence band of a semiconductor can be moved to the conduction band by applying an external force. At normal temperatures, the semiconductor acts as an insulator. By applying an external force greater than 1 electron volt, a semiconductor can be made as a conductor. The common examples of semiconductors are germanium and silicon.

Read More: Semiconductors

For insulators, the gap between the valence band and the conduction band is very large. Insulators have a forbidden energy gap of approximately 15 electron volts (15 eV). The electrons in the valence band of an insulator cannot be moved to the conduction band, and hence insulators remain as insulators themselves. Common examples of insulators are rubber, wood, plastic, etc.

Electron Cloud

An electron cloud is an atomic model in which the atom consists of a small nucleus surrounded by a cloud of fast-moving electrons. In an electron cloud, it is not possible to find the exact position of an electron at any given time. The electron cloud model predicts the probability of the density of electrons around the nucleus of the atom.

Positron

Positrons are identical to electrons. What makes positrons different from electrons is that it carries charges of opposite sign. When a positron collides with an electron, both particles may be destroyed, producing gamma-ray photons.

Electron Diffraction

Electron diffraction is the wave nature of electrons. This is utilised to analyse the crystal structure of the matter with high accuracy.

Electron Affinity

The electron affinity of an atom or molecule is defined as the amount of energy released or spent when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion.

Chemical Bonds

The strongest bonds are usually formed by electrons in an atom. An electron further helps to maintain a chemical bond between two different atoms. However, a chemical bond is a lasting attraction between atoms or molecules that results in the formation of chemical compounds. There are three main types of chemical bonds. They are as follows:

• Covalent bond
• Ionic bond
• Metallic bond

Covalent Bond

Covalent bonds are formed by the equal sharing of electrons between two or more atoms. By the covalent bonding of atoms, new molecules are formed. The most common example of covalent bonding is H₂O.

Properties of covalent bond compounds:

1. Lower melting and boiling point compared to ionic compounds.

2. Covalent compounds are flexible compounds, not hard compounds.

3. Less flammable and does not conduct electricity in water.

4. Is not soluble in water.

Ionic Bond

Ionic bonding is the transfer of valence electrons from one atom to another. In ionic bonding, the metal loses electrons to become a positively charged cation, and non-metals accept these electrons to become a negatively charged anion.

Lithium fluoride (LiF) is an example of an ionic bond.

Properties of ionic compounds:

1. High melting and boiling point compared to covalent compounds.

2. They are hard compounds.

3. More flammable and conduct electricity in the water.

4. Soluble in water.

 Metallic Bond

Metallic bonding is a type of chemical bond formed between positively charged atoms in which the free electrons are shared among a lattice of cations. The covalent and ionic bonds form between two discrete atoms, while metallic bonding forms between the same metal atoms. Iron and cobalt are examples of metallic bonding.

Read More: Chemical Bonding

Applications

• Electron beams are used in different fields, such as in welding, treatment of tumours, electron beam lithography and electron beam processing.
• Low-energy electron diffraction
• Free electron laser
• Cathode ray tubes and vacuum tubes

Discovery of Electron