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Structure of Atom

Subatomic Particles : Discovery and Characteristics

Subatomic Particles 

Electrons, protons and neutrons are the three main subatomic particles that form an atom.

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). These particles are known as 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 an electric or magnetic field.

The behaviour of cathode rays is similar to that of the negatively charged particles (electrons) in the presence of an electrical or 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 (me) 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 magnetic field depends upon: 1. the magnitude of the negative charge on the particle (greater the magnitude on the particle, greater the deflection) 2. the mass of the particle (lighter the particle, greater the deflection) 3. the strength of the electric or magnetic field (stronger the electric or magnetic field, greater the deflection) Observations:

When only electric field is applied, the electrons deviate to point A (as shown in the figure).

When only magnetic field is applied, the electrons strike point C (as shown in the figure).

On balancing the strength of electric and magnetic fields, the electrons hit the screen at point B (as shown in the figure) as in the absence of an electric or magnetic field. Result:

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

Charge on Electron (Millikan’s Oil-Drop Experiment)

Millikan's Oil-Drop Apparatus

Atomiser forms oil droplets.

The mass of the droplets is ascertained by calculating 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 observation of the effects of electric field strength on the motion of droplets leads to the conclusion that q = ne. Here, q is the magnitude of electrical charge on the droplets, e is the electrical charge and n is 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 a modified cathode ray tube led to the discovery of particles carrying positive charge; these are 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 magnetic field.

The smallest and lightest positive ion is called a proton (obtained from hydrogen).

Discovery of Neutron

Neutrons are electrically neutral.

They were discovered by Chadwick, by bombarding a thin sheet of beryllium with alpha 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

Macroscopic objects have particle character, so their motion can be described in terms of classical mechanics, based on Newton’s laws of motion.

Microscopic objects, such as electrons, have both wave-like and particle-like behaviour, so they cannot be described in terms of classical mechanics. To do so, a new branch of science called quantum mechanics was developed.

Quantum mechanics was developed independently by Werner Heisenberg and Erwin Schrodinger in 1926.

Quantum mechanics takes into account the dual nature (particle and wave) of matter.

On the basis of quantum mechanics, a new model known as quantum mechanical model was developed.

In the quantum mechanical model, the behaviour of microscopic particles (electrons) in a system (atom) is described by an equation known as Schrodinger equation, which is given below:

Where,

= Mathematical operator known as Hamiltonian operator

ψ = Wave function (amplitude of the electron wave)

E = Total energy of the system (includes all sub-atomic particles such as electrons, nuclei)

The solutions of Schrodinger equation are called wave functions.

Hydrogen atom and Schrodinger equation

After solving Schrodinger equation for hydrogen atom, certain solutions are obtained which are permissible.

Each pe…

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