Rutherford made a vital contribution to atomic structure. J.J. Thomson’s “plum pudding” model, which saw atoms as positively charged spheres with negatively charged electrons buried within, was the dominant theory prior to Rutherford. However, Rutherford’s model presented a radically different image, demonstrating that the atom is made up of a compact, tiny, positively charged nucleus that is encircled by electrons.
Rutherford’s Gold Foil Experiment
Gold foil experiment, carried out in 1909 by his colleagues Ernest Marsden and Hans Geiger, provided Rutherford with his insights. In this experiment, alpha particles positively charged particles released by some radioactive substances were used to attack a thin sheet of gold foil. Using a fluorescent screen, they tracked the alpha particles’ movements as they moved through the foil
Rutherford’s Nuclear Model of Atom:-
Key Observations and Inferences:-
The gold foil experiment give way three vital observations that led Rutherford to propose a new model for the atom:
1.Most alpha particles passed through the foil with minimal deflection: This implied that the majority of the atom’s volume is made up of empty space, which permits the alpha particles to flow through it without running into any major problems.
2. Some alpha particles were deflected at large angles: A few particles showed a significant deviation, suggesting that the atom contained a highly concentrated positive charge. Positively charged alpha particles were rejected by this concentrated positive charge, which caused them to deflect.
3. A few alpha particles rebounded back: It was suggested that the bulk of the atom’s mass is concentrated in a small central region since certain alpha particles bounced back straight after colliding with something very small and dense inside the atom.
Rutherford’s Nuclear Model of Atom
Based on these observations, Rutherford proposed a new atomic model, often referred to as the “nuclear model.” According to this model:
1.The Nucleus: According to Rutherford’s theory, the nucleus, atom’s center core, carries the positive charge and almost all of its mass. In relation to the size of the complete atom, the nucleus is little. According to Rutherford’s calculations, the nucleus’s diameter is roughly 1/10,000 that of the atom. The notable deflection of the alpha particles seen in the gold foil experiment is caused by this core nucleus.
2. Electrons in Orbit: Electrons are negatively charged particles that surround the nucleus. These electrons follow specific orbits around the nucleus in accordance with Rutherford’s concept. Like planets orbiting the sun without spiraling inward, this arrangement stopped the electrons from simply falling into the nucleus due to electrostatic attraction.
3. Empty Space: Rutherford concluded that the majority of the atom’s volume is empty space, which explains why the alpha particles were able to travel through the gold foil without running into resistance, as the majority of them did so without experiencing any discernible deflection.
Limitations of Rutherford’s Model
Rutherford’s model was revolutionary, it had limitations and could not explain all atomic phenomena:
1.Stability of the Atom: An electron orbiting the nucleus should continuously release energy in the form of electromagnetic radiation, according to the laws of classical electrodynamics. The atom would become unstable as a result of this energy loss since the electron would spiral inward and finally collapse into the nucleus. Atoms are naturally stable, on the other hand, indicating that Rutherford’s model was unable to adequately account for atomic stability.
2. Atomic Spectra: Measured line spectra of elements were not understandable by Rutherford’s model. Each element has its own line spectra, which are produced when energised atoms emit or absorb light in particular wavelengths. These different energy levels and their associated spectra were not explainable by Rutherford’s model, which had electrons in arbitrary orbits.
Impact and Legacy of Rutherford’s Model
In spite of its drawbacks, Rutherford’s nuclear model of the atom was a significant advancement in atomic theory and cemented the way for other models that expanded on his research. Protons and neutrons as essential parts of the atomic nucleus concepts that are central to contemporary atomic physics was made possible by Rutherford’s discovery of the nucleus.
Key points :-
Atomic number is equal to number of protons present in the nucleus of an atom and mass number is equal to sum of number of protons and neutrons (collectively known as nucleus) present in the nucleus of the atom.
Atomic number (Z) = number of protons in the nucleus of an atom = number of electrons in a neutral atom.
Mass number (A) = number of protons (Z) + number of neutrons (n).
Isotopes :- Different atoms of same element having same mass number but different atomic numbers.
Isobars :- Atoms of different elements having same mass number but different atomic numbers
Isotones :- Atoms of different elements containing same number of neutrons.
Isodiaphers :- Atoms having same isotopic number (i.e no. of neutrons – no. of protons = same).
Isosters :- Molecules having same number of atoms and electrons, e.g. CO2, N2
Isoelectronics :- Those species which have same number of electrons.
Drawbacks of Rutherford’s model of atom :
According to Rutherford’s model of atom, electrons which are negatively charged particles revolve around the nucleus in fixed orbits. Thus, the electrons undergo acceleration. According to electromagnetic theory of Maxwell, a charged particle undergoing acceleration should emit electromagnetic radiations. Thus, an electron in an orbit should emit radiation. Thus, the orbit should shrink. But this does not happen.
Calculations show that it should take an electron only 10-8 s to spiral into the nucleus. But tnis does not happen, Thus, the Rutherford model cannot explain the stability of an atom.
The model does not give any information about how electrons are distributed around nucleus and what are the energies of these electrons
Note :-
Rutherford’s work established the fundamental knowledge of atomic structure, impacting all further studies in atomic physics and chemistry, even though it was unable to adequately address the stability of atoms or explain atomic spectra. This finding transformed the discipline and influenced how we currently perceive atomic science by expanding our knowledge of matter at the atomic level and opening the door for developments in quantum mechanics and contemporary physics.
Rutherford’s model proposed that an atom consists of a tiny, dense, positively charged nucleus at the center, surrounded by electrons revolving in orbits around the nucleus, with most of the atom’s volume being empty space.
The experiment showed that most alpha particles passed through the gold foil unimpeded, while some were deflected at large angles and a few bounced back. This indicated the presence of a dense, positively charged nucleus, which led to the development of the nuclear model.
Three key observations were: most alpha particles passed through the foil, some deflected at large angles, and a few rebounded. These observations suggested that atoms have a dense central nucleus.
Thomson’s “plum pudding” model suggested that positive charge was spread out across the atom, with electrons embedded within it. Rutherford’s model, in contrast, proposed a central, dense nucleus with electrons orbiting around it, similar to planets orbiting the sun.
According to classical physics, electrons revolving around a nucleus should lose energy as they emit radiation, causing them to spiral into the nucleus. Rutherford’s model could not explain why atoms are stable despite this predicted collapse.
The model could not explain the stability of atoms or account for atomic spectra, as it lacked the concept of quantized energy levels for electrons, which were later introduced by Bohr’s model.
Rutherford’s model paved the way for Bohr’s model and quantum mechanics, introducing the concept of a dense nucleus and setting the stage for the idea of quantized electron orbits, foundational concepts in modern atomic theory.