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Atomic Structure

Thomson's Atomic Model

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

The Atomic Model 

Atom Is Divisible

Do you recall Dalton’s atomic theory? Dalton postulated in his theory that an atom is indivisible. However, the later discoveries of electrons proved this to be erroneous. 

In 1886, while carrying out an experiment in a gas discharge tube, E. Goldstein discovered positively charged radiations which led to the discovery of the subatomic particles called protons. Later, in 1897, J. J. Thomson discovered another type of subatomic particle—the negatively charged electron. Consequent to these discoveries, an atom was no longer indivisible; rather, it became a sum total of differently charged subatomic particles.

We know that an atom is neutral. It is made up of an equal number of oppositely charged particles—protons and electrons. Now, the question that arises is this:

How are the subatomic particles arranged inside an atom?

Many scientists performed varied experiments to develop different models for the structure of an atom. The first such model was proposed by J. J. Thomson. His atomic model is compared to a plum pudding and a watermelon; hence, it is known by the names ‘the plum-pudding model’.

The Plum-Pudding Model of an Atom 

Let us understand Thomson’s atomic model with the help of a slice of a watermelon. The slice consists of a red edible portion with embedded black seeds. Now, if we liken this watermelon to an atom, then (as per Thomson’s model) the positive charge in the atom is spread all over the red edible part; and the negatively charged particles, like the seeds, are embedded in this positively charged space.

In the same way, we can liken an atom to a plum pudding. In this case, the positive charge is spread all over the pudding, while the negatively charged particles are embedded like plums in this positively charged space. 

According to Thomson’s atomic model:

1. An atom consists of a positively charged sphere with electrons embedded in it.

2. The negative and positive charges present inside an atom are equal in magnitude. Therefore, an atom as a whole is electrically neutral.

  Cathode Rays

J.J Thomson discovered that there are small particle pr…

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