(Chemistry Ch-2) 1. Subatomic Particles: Discovery and Characteristics

Subatomic Particles

Electron, proton and neutron

Discovery of Electron (Michael Faraday’s Cathode Ray Discharge Tube Experiment)

• Experimental Setup:

• Glass tube is partially evacuated (Low pressure inside the tube)
• Very high voltage is applied across the electrodes

• Observation:

Stream of particles move from the cathode (− ve) to the anode (+ ve) −−− known as cathode rays or cathode ray particles

• Results:

• Cathode rays move from the cathode to the anode.
• Cathode rays are not visible. They can be observed with the help of phosphorescent or fluorescent materials (such as zinc sulphide).

• These rays travel in a straight line in the absence of electric and magnetic fields.
• The behaviour of cathode rays is similar to negatively charged particles (electrons) in the presence of an electrical or a magnetic field.
• Characteristics of cathode rays do not depend upon: the material of the electrodes and the nature of the gas present in the tube

• Conclusions:

• Cathode rays consist of electrons.
• Electrons are the basic units of all atoms.

Charge to Mass Ratio of Electrons (J.J Thomson’s Experiment)

• J.J Thomson measured the ratio of charge (e) to the mass of an electron (m_e) by using the following apparatus.

• He determined by applying electric and magnetic fields perpendicular to each other as well as to the path of the electrons.
• The amount of deviation of the particles from their path in the presence of an electric or a magnetic field depends upon:

• Magnitude of the negative charge on the particle (greater the magnitude on the particle, greater is the deflection)
• Mass of the particle (lighter the particle, greater is the deflection)
• Strength of the electric or magnetic field (stronger the electric or magnetic field, greater is the deflection)

• Observations:

• When only electric field is applied, the electrons deviate to point A (shown in the figure).
• When only magnetic field is applied, electrons strike point C (shown in the figure).
• On balancing the electric and magnetic field strength, the electrons hit the screen at point B (shown in the figure) as in the absence of an electric or a magnetic field.

• Result:

To test your knowledge of this concept, solve the following puzzle.

Charge on Electron (Millikan’s Oil Drop Experiment)

• The Millikan Oil Drop Apparatus:

• Atomizer forms oil droplets.
• The mass of the droplets is measured by measuring their falling rate.
• X-ray beam ionises the air.
• Oil droplets acquire charge by colliding with gaseous ions on passing through the ionised air.
• The falling rate of droplets can be controlled by controlling the voltage across the plate.
• Careful measurement of the effects of electric field strength on the motion of droplets leads to conclusion, q = ne [Where, q is the magnitude of electrical charge on the droplets, e is electrical charge, n = 1, 2, 3 …]

• Results

Charge on an electron = − 1.6022 × 10⁻¹⁹ C

Mass of an electron

Discovery of Proton

• Electric discharge carried out in modified cathode ray tube led to the discovery of particles carrying positive charge (known as canal rays).
• These positively charged particles depend upon the nature of gas present in them.
• The behaviour of these positively charged particles is opposite to that of the electrons or cathode rays in the presence of an electric or a magnetic field.
• The smallest and lightest positive ion is called proton (it was obtained from hydrogen).

Discovery of Neutron

• Neutrons are electrically neutral.
• They were discovered by Chadwick, by bombarding a thin sheet of beryllium with α-particles.

The given table lists the properties of these fundamental particles.

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