Structure of Atom — Chemistry STD 11 Science — Question

Read the passage given below and answer the following questions from (i) to (vi).
The atomic theory of matter was first proposed on afirm scientific basis by JohnDalton, a British schoolteacher in 1808. His theory, called Dalton’s atomictheory, regarded the atom as the ultimate particle ofmatter Dalton’s atomic theory was able to explainthe law of conservation of mass, law of constantcomposition and law of multiple proportion verysuccessfully. However, it failed to explain the results ofmany experiments.In mid 1850s many scientists mainlyFaraday began to study electrical dischargein partially evacuated tubes, known ascathode ray discharge tubes.Electrical discharge carried out in the modifiedcathode ray tube led to the discovery of canalrays carrying positively charged particles. Thecharacteristics of these positively chargedparticles are listed below.

1. Unlike cathode rays, mass of positivelycharged particles depends upon thenature of gas present in the cathode raytube. These are simply the positivelycharged gaseous ions.
2. The charge to mass ratio of the particlesdepends on the gas from which theseoriginate.
3. Some of the positively charged particlescarry a multiple of the fundamental unitof electrical charge.
4. The behaviour of these particles in themagnetic or electrical field is opposite tothat observed for electron or cathoderays.

The smallest and lightest positive ion wasobtained from hydrogen and was called
proton. This positively charged particle wascharacterised in 1919. Later, a need was feltfor the presence of electrically neutral particleas one of the constituent of atom. Theseparticles were discovered by Chadwick (1932)by bombarding a thin sheet of beryllium byα-particles. When electrically neutral particleshaving a mass slightly greater than that ofprotons were emitted. He named theseparticles as neutrons.J. J. Thomson, in 1898, proposed that an atom possesses a spherical shape (radiusapproximately 10–10 m) in which the positivecharge is uniformly distributed. The electronsare embedded into it in such a manner as togive the most stable electrostatic arrangementMany different names are given tothis model, for example, plum pudding, raisinpudding or watermelon. This model can be visualised as a pudding or watermelon ofpositive charge with plums or seeds (electrons)embedded into it. An important feature of thismodel is that the mass of the atom is assumed to be uniformly distributed over theatom.Rutherford and his students (Hans Geiger andErnest Marsden) bombarded very thin gold foilwith α–particles. Rutherford’s famous α–particle scattering experiment.The observations of Scattering experiment are as follows-:

1. most of the α–particles passed throughthe gold foil undeflected.
2. a small fraction of the α–particles wasdeflected by small angles.
3. a very few α–particles (∼1 in 20,000)bounced back, that is, were deflected bynearly 180°.

On the basis of observations andconclusions from this experiment, Rutherford proposed the nuclearmodel of atom. According to this model:

1. The positive charge and most of the massof the atom was densely concentrated inextremely small region. This very smallportion of the atom was called nucleusby Rutherford.
2. The nucleus is surrounded by electronsthat move around the nucleus with a veryhigh speed in circular paths called orbits.Thus, Rutherford’s model of atomresembles the solar system in which thenucleus plays the role of sun and theelectrons that of revolving planets.
3. Electrons and the nucleus are held together by electrostatic forces of attraction.

1. The atomic theory of matter was first proposed on afirm scientific basis by:

1. John Dalton
2. Ernest Rutherford
3. J.Thomson
4. Henry Moseley

2. The cathode rays start from … and move towards the….

1. Anode, Cathode
2. Centre, Anode
3. Cathod, Anode
4. Cathod, Centre

3. Negativelycharged particles in atoms, called…

1. Protons
2. Electrons
3. Neutron
4. Positron

4. The smallest and lightest positive ion wasobtained from …. and was called proton.

1. Oxygen
2. Nitrogen
3. Carbon
4. Hydrogen

5. Electrically neutral particles having a mass slightly greater than that of protons, these particles termed as:

1. Protons
2. Electrons
3. Neutron
4. Positron

6. J.J. Thomson’s atomic model is also named as:

1. Plum pudding
2. Raisin pudding
3. Watermelon
4. All the above