MPSC State Service 2019 · CSAT · Question 21 of 72

Passage: According to Dalton՚s atomic theory (1803) “Matter is made of indivisibleparticles called atoms. ” Sir J. J. Thomson (1867) in his plum pudding model proposed that atoms are made of negatively charged electrons embedded in a gel of positive charge. Rutherford (1911) overturned Thomson՚s atomic model with his gold foil experiment. He beamed α -particles ( 2 2 He nucleus) on a thin gold foil and observed: Most of the α-particles passed straight through the thin gold foil. Some of the α-particles were deflected by some angle. A very few α-particles bounced back towards the source Conclusions : An atom has a tiny, dense, positively charged core called nucleus. Negatively charged constituent with negligible mass called electron revolves around the nucleus at distances in paths called orbits. Limitations : Electron being moving charged particles should continuously emit energy, thus keep moving closer to the nucleus and finally drop into it. But this does not happen. And also distribution of electrons in the defined orbits and their energies remained unexplained. Nobel prize winner Neils Bohr (1922) revised Rutherford՚s model as follows: Though there are many orbits, electrons are confined to only clearly defined orbits of fixed energies. Electrons could jump between these defined orbits but could not freely spiral inwards or outwards in intermediate states. Electrons must absorb or emit specific amount of energy for transition between these defined orbits. J. Chadwick (1932) discovered a neutral particle called neutron present inside the nucleus along with the proton and having a mass same as that of proton. Thus an atom consists of a nucleus which contains nucleons i.e.. protons and neutrons, and electrons revolve around it in fixed orbits. This is an electrically neutral atom which can lose one or more electrons to form cation (MN + ) or gain one or more electrons to form anion (Xn-). As the distance of defined orbit increases from the nucleus, the orbits or shells are given shell number, n = 1,2, 3,4 … and are respectively called K, L, M, N … shell. The maximum number of electrons in each shell is given by 2 n2, where n is the shell number. The atomic number ‘Z’ i.e.. the number of protons and the mass number ‘A’ i.e.. the number of nucleons can be represented symbolically as follows: A Z Symbol of the element Which of the following observations of Rutherford՚s experiment is impossible to understand scientifically with the plum pudding model? a. Most of the α-particles passed straight through the thin gold foil. b. Some of the a-particles got deflected by some angle. c. A very few α-particles bounced back towards the source.

Reading Comprehension

1. AOnly a
2. BOnly b
3. COnly cCorrect
4. Da, b and c