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Electron

Subatomic Particles

What is an electron particle?

Electron, the lightest subatomic or elementary particle of atom carries a negative charge, – 1.602176634 × 10−19 coulomb or 4.8 × 10-10 esu which is demonstrated by the experiments involving the conduction of electricity through gases at low pressure. The rest mass of an electron 9.1093837015 × 10−28 g or 9.1093837015 × 10-31 kg which is approximately 1/1836 of the mass of the proton particle. Like other particles, an electron has mass, energy, and momentum (particle properties), and wavelength or frequency (wave properties) derived from the de-Broglie relation (E = hν = hc/λ).

Who discovered electron?

The electron was discovered by the British physicist J.J. Thomson in 1897 during the investigation of the properties of the cathode rays over twenty years. Gas discharge tubes contain a positive electrode (anode) and a negative electrode (cathode) with provisions of evacuation given below the picture.

Electron, the lightest subatomic or elementary particle of atom carries a negative charge and very small mass observed by conduction of electricity in gases

Under normal conditions, gases are poor conductors of electricity but at low pressure, conduction of gases occurs. When the gas discharge tubes are partially evacuated and the electrodes are connected to the source of high voltage electricity, an electric current flows through the tubes. The flow of current produces glow or rays of light which originated from the cathode surface and traveled from cathode to anode. These rays are called cathode rays. Thomson showed that the rays or electromagnetic radiation can be bent from their path both by the electric and magnetic fields.

From the above facts, he proved that the cathode rays are made up of negatively charged subatomic particles or electrons of an atom. Thomson measured the charge to mass or e/m ratio (1.76 × 10-8) of an electron but the absolute charge was determined by the American physicists Robert Millikan and Harvey Fletcher in their oil-drop experiment.

Charge and Mass

Electron is a negatively charged elementary subatomic particle of an atom. Hence the negative charge carries by an electron, e = – 4.8 × 10-10 esu = – 1.602176634 × 10−19 coulomb (C).

Mass of an electron = m and charge = e. Charge by mass (e/m) ratio for electron = 1.76 × 108 coulomb/g. From the above formula, mass of an electron particle = (1.602176634 × 10−19)/(1.76 × 108) g = 9.1093837015 × 10−28 g = 9.1093837015 × 10−31 kg.

How to calculate charge of electron?

Electrolysis of silver from an aqueous solution of silver salt is a suitable process for the determination of the charge of an electron. A constant current I (ampere) passes through the silver nitrate solution during the time t. The amount of electricity required to decompose one gram equivalent substances is called one faraday (F) = 96496 ≈ 96500 coulombs. The charge of 96500 coulombs can be considered as the charge carried by a mole of electrons. Therefore, the charge associated with an electron = (96500 coulomb)/(6.02 × 1023 electrons) = 1.60 × 10-19 coulomb = 4.8 × 10-10 esu.

Distribution of electrons in different shells

The periodic table is drawn up by the chemists primarily based on the atomic number of the chemical elements. The atomic number represents the net positive charge on the nucleus or the number of electrons surrounding the nucleus of an atom in its normal neutral condition. Rutherford model, Bohr model suggests the distribution of electrons in different shells. The distribution of atomic orbitals of an atom given below the picture,

Electron arrangement in orbital energy level

Chemical Element Atomic number Electron per shell (2n2)
K L M
Hydrogen 1 1
Helium 2 2
Lithium 3 2 1
Beryllium 4 2 2
Boron 5 2 3
Carbon 6 2 4
Nitrogen 7 2 5
Oxygen 8 2 6
Fluorine 9 2 7
Neon 10 2 8
Sodium 11 2 8 1

Modern concept of atomic structure

The electrons are distributed in the orbitals in a definite order and the detailed modern structure of an atom is obtained from the electronic configuration of the atom. The distribution of electrons in different shells not only provides the size of an atom but also suggests properties like chemical bonding, ionization energy, electron affinity, electronegativity, oxidation number or state, electromagnetic spectrum, thermal conductivity, etc. The distribution of electron for nitrogen atom given below the picture,

Electron arrangement structure of nitrogen atom

Due to the stable electron arrangement of the inert or noble gases like helium, neon, krypton, xenon are chemically inert. From helium to xenon chemical reactivity increases because the size of the atom increases and ionization energy decreases. The elements like fluorine, chlorine, bromine are highly electronegative and reactive elements but the elements like lithium, sodium, potassium are highly electropositive or reactive elements. Fluorine, chlorine, and bromine easily gaining one electron to form the next noble gas configuration, and lithium, sodium, potassium stabilizes by losing one electron from the valence shell.