Contents
The study of Physics Topics can help us understand and solve real-world problems, from climate change to medical imaging technology.
What are the Constituents of an Atom?
Early nineteenth century, John Dalton had proposed that all matter is made up of tiny, indivisible particles called ‘atoms’. This statement along with other similar statements was later proved to be wrong and discarded.
Towards the end of the nineteenth century, William Crookes, Joseph John Thomson, Philipp Lenard and others, while studying the silent electric discharge in gases at a low pressure paved the way for the discovery of electrons. Among them, JJ Thomson is credited with the discovery of electron, a subatomic particle. After the discovery of electrons, the myth of indivisibility of ‘atom’ was dispelled.
Electron: It has been possible to show emission of electrons from almost all matters through suitable experimental arrangements. Electrons are negatively charged particles carrying a charge which is denoted by e or e-1.
Millikan determined the amount of charge of an electron,
e = 1.6 × 10-19 coulomb = 4.8 × 10-10 esu
J J Thomson determined the specific charge of an electron, i.e., charge to mass ratio for an electron is \(\left(\frac{e}{m_e}\right)\) is 1.76 × 1011 C ᐧ kg-1.
Therefore, mass of electron,
me \(=\frac{\text { amount of charge of an electron }(e)}{\text { specific charge of an electron }\left(\frac{e}{m_e}\right)}\)
= 9.1 × 10-31 kg = 9.1 × 10-28 g
Positive charge: Most materials are electrically neutral. From various electron emission experiments it was proved that (since matter is neutral) there must be some positive charge also in the atom. Later experiments confirmed the presence of positive charge on an atom.
Rutherford’s Atomic Model
Alpha particles scattering experiment: In 1911, Rutherford and his co-workers performed the famous alpha particle scattering experiment. Alpha particles are emitted from a radioactive source with considerable energy. These α -particles were collimated into a narrow parallel beam which was made incident on a thin foil of a heavy metal like gold (Z = 79), silver (Z = 47) etc. [Fig.].
These metals being ductile, can be easily drawn into a thin foil of width about 107 m. The thinness ensured that each α-particle could interact with a single atom in each collision. Due to collision with the foil, the α-particles were scattered in different directions which were detected by a fluorescent detection screen. Very few α-particles were scattered in backward direction without penetrating the foil, but being deflected through angles greater than 90°.
Observation and inference:
i) Most of the α-particles passed straight through the metal foil without suffering any deflection [Fig.]. This observation lead to the conclusion that most of the space inside the atom is empty.
ii) Low angle scattering: Some of the alpha particles were scattered through small angles i.e., the scattering angle θ ≈ 1° [Fig.]. Here, it is assumed that this scattering takes place due to coulomb attraction between an alpha particle of charge +2e and an electron of charge -e. The deflections of α-particles are so small as an α-particle is about 7000 times heavier than an electron. From this, it is concluded that electrons are embedded discretely inside the atom.
iii) Large angle scattering: Some α-particles, though very few in number suffered deflection by 90° or larger angles. Some of these α-particles were even deflected through 180° [Fig.]. Rutherford quantitatively analysed the
number of these large angle deflections. He argued that, to deflect the α-particle backwards, it must experience a large repulsive force which was possible if the entire positive charge and almost total mass of atom were concentrated in a small space. This confirmed the presence of the nucleus. Thus the strong electrostatic repulsion between an α-particle of charge +2e and nucleus of charge +Ze was the cause of these large deflections. By quantitative analysis, it was also concluded that the diameter of the nucleus is about 10-14 m which is about \(\frac{1}{10000}\) times of atomic diameter of 10-10 m. Hence the volume of nucleus is only about 1 in 1012 part of the atom.
Rutherford’s atomic model is often compared with the solar system, but there are more dissimilarities than resemblances. Similarities are only marginal.
Similarities:
Solar system | Rutherford’s atomic model |
1. The sun is at the centre of the solar system and the planets orbit around sun. | 1. The nucleus is at the centre of an atom and the electrons orbit round it, though this view is not supported by the modern theory. |
2. The size of the sun is negligible compared with the size of the solar system. | 2. The size of the nucleus is negligible compared with the size of an atom. |
3. Most of the space in solar system is empty. | 3. Most part of an atom is empty. |
Dissimilarities:
Solar system | Rutherford’s atomic model |
1. Gravitational force acts between the sun and the planets. | 1. Electrostatic force acts between the nucleus and the electrons. |
2. There can be only one planet in an orbit. | 2. There can be more than one electron in an orbit. |
3. Planets are of different sizes and of different masses. | 3. All electrons are identical. |
From the observations, Rutherford proposed his atomic model. The sailent features of the model are: nucleus exists at the centre of an atom and electrons orbit round the nucleus in circular paths. The necessary centripetal force for the orbital motion is provided by the electrostatic attraction between the opposite charges of the nucleus and the electron.[Fig.]
Drawbacks of Rutherford model
Instability of the atom: An orbiting electron in an atom is subjected to centripetal acceleration. According to Max-well’s classical electromagnetic theory, any accelerated charged particle emits electromagnetic radiation. Since, orbiting electrons are accelerated, they should also emit radiations. If this were to happen, the energy of the orbiting electron would keep on decreasing. It would follow a spiral path and ultimately collide with the nucleus [Fig.]. Theoretical calculat-ions
show that under this condition no atom would survive for more than 10-8 s. However, matter is stable implying that atom too cannot be unstable.
Continuous atomic spectrum: If electron energy in an atom had converted Into radiant energy, we would get a continuous spectrum. Interestingly, atoms of hydrogen, helium etc. produce line spectrum instead of continuous spectrum.