Structure of Atom

Sub-Atomic Particles

The word ‘atom’ comes from the Greek word ‘a-tomio which means ‘uncuttable’ or ‘non-divisible’. Scientists believed that atoms were indivisible for the longest time. However, in the early 20th century, some scientists showed that atoms can be further divided into smaller parts such as electrons, protons, and neutrons. These are called sub-atomic particles. Want to know how these sub-atomic particles were discovered? Let’s dive in!

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Discovery Of Electron

Dalton’s atomic theory successfully explained the following laws – conservation of mass, constant composition and multiple proportions. However, it failed to explain certain other observations like the generation of electricity on rubbing glass or ebonite with silk or fur. These observations propelled the discovery of sub-atomic particles in the 20th century. Let’s learn about the discovery of the first sub-atomic particle – Electron.

Cathode Ray Tube Experiment

Initial experiments by Michael Faraday suggested the particulate nature of electricity. His later experiments also gave an insight into the structure of the atom. Before we learn the results of these experiments, we need to remember a basic rule – ‘Like charges repel each other, while unlike charges attract each other’.

Many scientists, especially Faraday began to study electrical discharge through gases in special tubes called cathode ray discharge tubes. These tubes are made of glass and contain two thin pieces of metal called electrodes. The electrical discharge through gases is visible only at very low temperatures or very low pressures. Alteration of the pressures of different gases is possible through evacuation.

Experiment

On applying a high voltage through the electrodes, current begins to flow from the negative (cathode) to the positive (anode) electrode through a stream of particles. These are cathode rays or cathode ray particles.

Further, they made a hole in the anode and coated the tube behind with phosphorescent material like zinc sulfide. Now, when the rays pass through the anode and strike the coating, a bright spot is seen on it.

Cathode Ray Tube Experiment

sub-atomic particles

[Source: Wikimedia Commons]

Results Of The Experiment

  • The cathode rays start at the cathode and travel towards the anode.
  • These rays are not visible as they are, but are visible with phosphorescent materials which glow when the rays hit them.
  • Cathode rays travel in straight lines in the absence of electrical or magnetic fields.
  • These rays behave like negatively charged particles in the presence of electrical or magnetic fields. This suggests that cathode rays consist of negatively charged, sub-atomic particles – electrons.
  • The characteristics of the cathode rays or electrons are not dependent on the nature of the gas in the tube or the material of the electrodes.

These results ultimately led to the conclusion that electrons are the basic constituent of all atoms. In this manner, one of the first sub-atomic particles of an atom came to light.

Did you know that televisions also works like cathode ray tubes? The picture that you see is due to fluorescence on the television screen that is coated with fluorescent or phosphorescent materials!

Charge To Mass Ratio Of an Electron

British physicist J. J. Thomson, conducted experiments to determine the ratio of electrical charge (e) to the mass of the electron (me). He did this using cathode ray tubes and applying electrical and magnetic fields perpendicular to each other and also to the path of the electrons.

Thomson argued that in the presence of an electrical or magnetic field, the amount of deviation of the particles from their path depends on:

  • The magnitude of the negative charge on the particle. Greater the magnitude of charge on the particle → greater the interaction with the electrical or magnetic field → greater the deflection of the particle.
  • The mass of the particle. Lighter the particle → greater the deflection.
  • The strength of the magnetic or electrical field. Increase in voltage across the electrodes → greater the deflection of electrons from its original path.

On application of only electric field or only magnetic field, the electrons deviate from their path and hit opposite points in the tube. By balancing the electrical and magnetic field, one can bring the electron back to the path it would follow in the absence of the two fields. Thomson determined the charge to mass ratio by calculating the amount of deflection of electrons under both fields. It is given as follows:

me = 1.758820 × 1011 C kg–1

Here, me – the mass of electron in kg, e – the magnitude of charge on the electron in Coulomb (C). Since electrons are negatively charged, the charge on them is -e.

Charge On The Electron

To determine the charge on electrons, scientist R. A. Millikan conducted the oil drop experiment. Through this experiment, he found that the charge on the electron is 1.6 × 10–19 C. Today, the accepted value of electrical charge is – 1.6022 × 10–19 C. The mass of the electron is obtained using this value and Thomson’s value of  e/mratio:

me = e / (e/me) = (1.6022 x 10-19 C) / (1.758820 × 1011 C kg-1) = 9.1094×10–31 kg

Discovery Of Protons And Neutrons

Proton

The cathode ray tube experiment also led to the discovery of positively charged particles or canal rays. Their characteristics are as follows:

  • Unlike cathode rays, they depend on the nature of the gas in the cathode ray tube. These are the positively charged gaseous ions.
  • The charge to mass ratio of these particles depends on the gas from which they originate.
  • Some positively charged particles carry a multiple of the fundamental unit of electrical charge.
  • These particles behave opposite to cathode rays in an electrical or magnetic field.

The lightest and smallest positively charged particle was obtained from hydrogen and called proton. This was characterized in 1919.

Neutron

Later, scientists also discovered an electrically neutral constituent of the atom. In 1932, the scientist Chadwick conducted an experiment where he bombarded a thin sheet of beryllium with α particles. He observed the emission of electrically neutral particles with a mass slightly greater than that of protons. As a result, these were named neutrons.

Important Properties of Sub-atomic Particles

Name Symbol Absolute charge/C Relative charge Mass/kg Mass/u Approx. mass/u
Electron e -1.6022 x 10-19 -1 9.10939 x 10-31 0.00054 0
Proton p +1.6022 x 10-19 +1 1.67262 x 10-27 1.00727 1
Neutron n 0 0 1.67493 x 10-27 1.00867 1

Solved Examples For You

Question: Calculate the charge of one mole of electrons.

Solution: 1 mole = 6.022 x 1023 atoms, charge of one electron = 1.6022 x 10-19 C

Therefore, charge of one mole of electrons = 1.6022 x 10-19 x 6.022 x 1023  = 9.65 x 104 C.

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