[Chemistry Class Notes] on Electrons Pdf for Exam

Electrons are subatomic particles that, along with protons and neutrons, come to form the most fundamental part of atoms. For a long time, the atom was considered to be the final structure of an element. The term ‘atom’ being wholly representative of that thought since it comes from the Greek word meaning indivisible. However, all that changed when in 1897, the English physicist J.J. Thomson, through his now-famous cathode ray tube experiments, led to the discovery of electrons as being a part of subatomic particles, i.e., particles which make up the atom.

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Atoms are the smallest particle of an element that will have the same property of the element which can never be created or destroyed according to Dalton’s atomic theory. But when we break the atom further it will give us electrons, protons, and neutrons. Every atom is made up only of these three particles. Every atom of any given element will have the same physical properties due to the fact that it contains exactly the same subatomic particle as that of the other one. Electron is a subatomic particle that belongs to the first generation of the lepton particle family. They are thought to be elementary particles of atoms as they have no known components or structure. The electron is denoted by e- and has one negative elementary charge.

Thompson Cathode Ray Experiment

In the early 19th century physicists were trying to understand the structure of the atom and at that time J.J Thompson a British physicist conducted an experiment with the cathode ray tubes. Cathode ray tubes are sealed glass chambers with negative atmospheric pressure and have most of the gas sucked out of them. The tube has high voltage applied across the ends of it at the electrodes attached. The positively charged electrode is called Anode and the Negatively charged electrode is called Cathode. When electricity is applied to the electrodes a beam of particles flows from the cathode towards the anode giving it the name cathode ray tube. These beams when projected onto a phosphor’s screen can create a glow of spark. 

When Thompson attached two oppositely charged electrodes on the side of the cathode ray he found the rays were deviating towards the positively charged electrode and away from the negatively charged electrode. This helped Thompson to theorize that the Cathode rays were actually Negatively charged in nature. He repeated the experiment by replacing the electrode of the tube and found the results were always the same. This helped him theorize that the cathode ray from all the elements have similar properties and are actually negatively charged particles. After calculating, he found the mass of the particle is only one two-thousandth of the mass of the hydrogen atom. In the end, he theorized that all atoms consist of Negatively charged particles. This contradicts Dalton’s atomic theory that atoms were indivisible, so at first, it was not accepted by scientists around the world. But later it was accepted and the particles were named electrons.

Electrons are subatomic particles that, along with protons and neutrons, come to form the most fundamental part of atoms. For a long time, the atom was considered to be the final structure of an element. The term ‘atom’ being wholly representative of that thought since it comes from the Greek word meaning indivisible. However, all that changed when in 1897, the English physicist J.J. Thomson through his now-famous cathode ray tube experiments, led to the discovery of electrons as being a part of subatomic particles, i.e., particles which make up the atom.

Define Electrons

An electron is defined as a subatomic particle that is negatively charged and can exist either in a free state, i.e., not bound to any atom, or attached to the nucleus of an atom. In atoms, the electrons are represented as existing in spherical shells of heterogeneous radii based on the energy levels of the electron. Thus the energy contained in the electron is directly proportional to the size of the spherical shell that the electron exhibits. Electrons are usually denoted by the sign e-. Now that you are familiar with the electron definition in chemistry let us understand the properties of an electron.

Properties of Electron

Subatomic particles come with their own set of specific properties that are unique to them. Thus the properties of protons, neutrons, and electrons are, for the most part, well defined. Here are some of the properties of an electron:

• Mass of an Electron: The mass of an electron is [9.1 times 10^{-31}] kg or 0.000548579909 atomic mass units. The mass of an electron can also be represented as being 1/1840 of the mass of a hydrogen atom. Now, since the mass of a hydrogen atom is 1 u, the relative mass of an electron can also be represented as 1/1840 u

• The absolute mass of an Electron: Absolute mass of an electron is [9 times 10^{-28}] grams 

• Charge of an Electron: The electron is a negatively charged particle, and therefore the effective charge is -1.  

• Electrical Charge: Since the charge of electrons is -1; the electrical charge should be [1.602 times 10^{-19}] coulombs.

• Electrons revolve around the nucleus of the atom of the element in specific, well-defined orbits.

• The properties of an electron are unaffected and independent of the gas present in the discharge tube.

• Electrons also exhibit a two-sided nature defined as wave-particle duality, which means that the electron, which is a particle under specific circumstances, behaves as a wave. 

Electrons and Compound

Electrons were discovered in 1897. However, the electron meaning and its significance came to be redefined with the discovery of the proton in the period between 1911 – 1919 and the neutron in 1932. With the discovery of the other two subatomic particles, the entire model of atomic structure, as we know it today, was established.

Protons are defined as the positively charged subatomic particles in the nucleus of an atom. They are 99.86% as large as protons, and the number of protons present in an atom is unique for each element. For instance, hydrogen has one proton, oxygen has eight, and carbon has six.

Neutrons were already theorized by the scientist Rutherford when he discovered the proton but neutrons were discovered by Chadwick in 1932. Neutrons were subatomic particles with no charge or neutral charge, hence the name ‘neutron’. Except for the hydrogen atom, neutrons make up the constitution of all other elements of the periodic table. The neutron’s mass is larger than a proton but only slightly.

Thomson’s Cathode Ray Experiment

The discovery of electrons in chemistry was substantiated by an electric discharge in cathode-ray tubes. Thomson noted that the rays inside the tube were deflected, which was inferred as the presence of a negatively charged particle inside the vacuum tube that made such a phenomenon possible. His electron definition in chemistry came to be represented as the plum-pudding model based on the fact that there were a large number of electrons present in a form that produced an overall positive charge hence establishing an overall neutral charge. With the discovery of protons and neu
trons, this model was replaced by the modern atomic model that we see today.