Structure of Atom

Charged Particles in Matter

What happens when you comb your dry hair and then bring the comb near few small pieces of paper? Combing your dry hair will move some of the subatomic particles called electrons from hair to the comb. The charge of an electron is negative and thus the comb will also gain the charge of an electron (negative charge). Finally, when the comb is brought near the bits of paper, the bits of paper will get attracted to the comb.

Similarly, what happens when you rub a silk cloth on a glass rod and then the glass rod is brought near an inflated balloon? The electrons from the glass rod are transferred to the silk cloth. Hence, the glass rod becomes positively charged. When the positively charged glass rod is brought near the inflated balloon, it will be attracted to the glass rod.

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These activities prove two objects when rubbed together, the objects become electrically charged. However, how do the objects become electrically charged? All matter is composed of divisible particles called atoms. The atoms are composed of tiny charged subatomic particles named electrons, protons, and neutrons.

Earlier it was postulated by John Dalton that atoms are indivisible particles. However, the contribution of many scientists by the very end of 1800’s or the start of 1900’s gradually started revealing that atoms are divisible into tiny subatomic particles.

Discovery of Subatomic particles

Discovery of Existence and Charge of an Electron

charge of an electron

Thomson’s Atomic Model (Source Credit: Wikipedia)

A British Physicist named J.J Thompson in the year 1897 proposed that an atom constitutes of at least one negatively charged particle. He discovered electrons while working on an experiment. In the experiment, gas was passed at low pressure through a glass discharged tube.

Two electrodes were placed at the two opposite ends of the glass discharge tube which was further connected to high voltage supply (battery). The electrode joined to the negative end was called cathode and the electrode joined to the positive end was called anode.

Thomson observed a stream of negatively charged particles emitted from the cathode. Furthermore, the stream of particles moved towards the anode. The streams of negatively charged particles discharged from the cathode end of the discharge tube are called cathode rays. He named it as “corpuscles” which was later called “electron.”

An electron is represented by “e” and it is known to contribute to the negative charge of an atom. The absolute charge of an electron is the negative charge of 1.6×10-19 coulombs. The relative mass of an electron is 1/1836 (1/2000), thus the mass of an electron is very small and is considered as 0. Therefore, the mass of an electron is negligible and the charge of an electron is -1.

Discovery of Proton

charge of an electron

                                                                                                       Rutherford’s Gold Foil Experiment (Source Credit: Britannica)

Proton was discovered by Rutherford when he conducted the famous gold foil experiment. Initially, in 1886 Goldstein discovered the presence of positively charged rays while conducting an experiment in the discharged tube by using perforated cathode. The experiment was similar to J.J. Thomson but with different situations. This led to the emission of charged particles from the anode.

The positively charged radiations produced from the anode end of the discharge tube are called canal rays. The rays were named as anode rays or canal rays. A series of experiments led to the discovery of protons. Protons are known as the particles that contribute to the positive charge of the atom.

The mass of the particles was found to be 2000 times more than that of an electron. Protons were represented by the letter “p”. The absolute charge of a proton is the positive charge of 1.6×10-19 coulomb. The mass of a proton is 1.6×10-24 g and is considered 1 (which is mass of a hydrogen atom). The mass of a proton is considered one unit and it’s charge is considered to be +1.

Discovery of Neutron

charge of an electron

                                                                                                           Chadwick Experiment (Source Credit: Wikimedia)

 Neutron was not discovered until the year 1932. It was discovered by James Chadwick by using scattered particle to calculate the mass of the neutral particle. The subatomic particle “neutron” is present in an atom’s nucleus. Neutron is represented by the letter “n” and is considered a neutral particle.The mass of a neutron is measured as 1.6 x 10 -24g.

Gram is not a very appropriate unit for the calculation of such tiny subatomic particles. Therefore, they are alternatively calculated in Dalton or amu (atomic mass unit). A Neutron and a proton have a similar mass that is nearly 1 amu.

Summary of Subatomic Particles:

Name Representation Absolute Charge Relative Charge Mass (Kg) Relative mass
Electron e -1.60×10-19 C -1 9.1×10-31  kg 1/1840 (negligible)
Proton P+ 1.60×10-19 C +1 1.672×10-27 kg 1u
Neutron n 0.0 C 0 1.674×10-27 kg

              1u

Conclusion

The discovery of the subatomic particles raised many questions. One of them was how these subatomic particles are arranged in an atom. This led to the proposal of different models of these charged particles. These form the basis of the vast subject of “structure of an atom.”

Solved Questions For You

Q1: Which of the options represent the properties of electrons?

  1. the relative charge of -1
  2. negligible mass
  3. the charge is opposite and equal to a proton
  4. all of these

Ans: The answer is 4 (all of these), as per the defined nature of electrons.

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