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Structure of Atom

Dalton's Atomic Theory and Laws of Chemical Combination-Group A

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

Dividing Matter

Matter cannot be divided infinite number of times.

 

For example, if we keep chopping a log of wood into smaller and smaller pieces, then we will reach a point when the wood will not be divisible any further. Minute particles of wood will remain and these will not be visible to the naked eye. This is true for all forms of matter.The same was believed by the early Indian and Greek philosophers. In India, around 500 BC, an Indian philosopher named Maharishi Kanad called matter as padarth and these smallest particles (atoms) as ‘parmanu’. The word ‘atom’ is derived from the Greek word ‘atomos’ which means ‘indivisible’. It was the Greek philosopher Democritus who coined the term. However, for these ancient thinkers, the idea of the minute indivisible particle was a purely philosophical consideration. 

 

 

By the end of the eighteenth century, scientists had begun to distinguish between elements and compounds. Two French chemists named Antoine Lavoisier and Joseph Proust observed that elements combine in definite proportions to form compounds. On the basis of this observation, each of them proposed an important law of chemical combination. The laws proposed by them helped Dalton formulate his atomic theory.

Dalton’s Atomic Theory 

In the early nineteenth century, an English chemist named John Dalton proposed a theory about atoms. Known as ‘Dalton’s atomic theory’, it proved to be one of the most important theories of science. The various laws of chemical combination also supported Dalton’s theory. Dalton asserted that ‘atoms are the smallest particles of matter, which cannot be divided further’. He published his atomic theory in 1808 in his book A New System of Chemical Philosophy. The postulates of Dalton’s atomic theory are as follows:

All matter is made up of very tiny particles. These particles are called atoms. An atom cannot be divided further, i.e., atoms are indivisible. Atoms can be neither created nor destroyed in a chemical reaction. All atoms of an compounds . In a given compound, the relative numbers and types of atoms are constant.

Know Your Scientist

John Dalton (1766−1844) was born into the poor family of a weaver in Eaglesfield, England. He was colour-blind from childhood. He became a teacher when he was barely twelve years old. By the time he was nineteen, he had become the principal of a school. In 1793, Dalton left for Manchester to teach physics, chemistry and mathematics at a college. Elected a member of the Manchester Literary and Philosophy Society in 1794, he became its president in 1817 and remained in that position until his death. During his early career, he identified the hereditary nature of red−green colour blindness. In 1803, he postulated the law of partial pressures (known as Dalton’s law of partial pressures). He was the first scientist to exp…

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