Q.From Rutherford's alpha particle scattering by thin gold foil experiment, what conclusion regarding atom was made?
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Rutherford's nuclear model, built directly on the Geiger-Marsden scattering results, places essentially the entire positive charge and 99.9% of an atom's mass into an extremely small, dense central nucleus (about 10−5 times the size of the whole atom), around which the much lighter electrons revolve in orbits -- much as planets orbit the Sun under gravity. This picture correctly explains why most alpha particles pass through a thin foil almost undeflected (travelling through mostly empty space between the nucleus and the electron orbits) while a rare few, passing very close to a nucleus, are scattered through large angles by its concentrated charge. …
Because most alpha particles passed through the gold foil undeflected while a small fraction bounced back at large angles, Rutherford inferred how an atom's mass and positive charge must actually be arranged. …
Rutherford's scattering experiment showed that an atom's positive charge and mass are concentrated in a tiny central nucleus, with the rest of the atom being mostly empty space.
In Rutherford's experiment, a beam of alpha particles was directed at a thin gold foil, and the scattering angles were observed:
- Most alpha particles passed straight through the foil with little or no deflection ⟹ most of the atom's volume is empty space.
- A small fraction were deflected through large angles, and a very few (about 1 in 8000) bounced back almost along their original path ⟹ there must be a very small, extremely dense region within the atom that repels the alpha particles strongly.
- Since the deflecting region caused such large deflections/repulsion of the positively charged alpha particles, this central region must carry a large positive charge concentrated in a very small volume — Rutherford called this the nucleus. …
- CBSE 2025Set ANNUAL1 markMCQQ.Rutherford’s model of the atom was unstable, because(a) nuclei will break down(b) electrons move in circular orbits(c) orbiting electrons radiate energy(d) electrons are repelled by nucleus.
›Reveal solutionSolution
Classical EM theory says an accelerating charge radiates energy; an electron in a circular orbit is accelerating, so it should continuously lose energy and spiral into the nucleus — this instability is Rutherford's model's fatal flaw.
In Rutherford's model, electrons revolve around the nucleus in circular orbits under the Coulomb force of attraction, which provides the centripetal acceleration needed for circular motion. But according to classical electrodynamics, any accelerated charged particle must radiate electromagnetic energy continuously.
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- CBSE 2024Set ANNUAL1 markMCQQ.The net charge on an atom of atomic number Z as a whole is :(a) +Ze(b) – Ze(c) Zero(d) +(Z – 1)e
›Reveal solutionSolution
An atom has Z protons (each +e) balanced by Z orbital electrons (each −e), so the net charge cancels exactly.
An atom of atomic number Z has a nucleus with Z protons, each carrying charge +e, giving total positive nuclear charge +Ze. Surrounding the nucleus are Z electrons, each carrying charge −e, giving total negative charge −Ze.
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- CBSE 2023Set B1 markQ.Write True or False: Atom is a positive particle.
›Reveal solutionSolution
An atom contains a positively charged nucleus (protons) surrounded by an equal number of negatively charged electrons, making the atom overall electrically neutral.
An atom consists of a small, dense, positively charged nucleus (made of protons and neutrons) surrounded by negatively charged electrons occupying the much larger surrounding space. In a neutral atom, the number of electrons exactly equals the number of protons, so the net charge of the atom is zero. Only when an atom loses or gains electrons does i …
- CBSE 2021Set NC1 markQ.What would happen, if the electrons in an atom were stationary?
›Reveal solutionSolution
Without circular motion, nothing counteracts the electron's electrostatic attraction toward the nucleus — it would simply be pulled straight in, collapsing the atom.
Reasoning
An electron orbits the nucleus because the Coulomb attraction between electron and nucleus supplies exactly the centripetal force needed for circular motion: r2ke2=rmv2. If the electron were stationary (no orbital velocity), there would be no centripetal requirement to balance against — the unbalanced attractive Coulomb force would simply accelerate the electron straight toward the nucleus, and the electron wo …
- CBSE 2020Set 55/1/11 markMCQQ.Which of the following statements is not correct according to Rutherford model ? (A) Most of the space inside an atom is empty. (B) The electrons revolve around the nucleus under the influence of coulomb force acting on them. (C) Most part of the mass of the atom and its positive charge are concentrated at its centre. (D) The stability of atom was established by the model.
›Reveal solutionSolution
Rutherford's nuclear model successfully explained the concentration of mass and charge but failed to account for atomic stability because accelerating electrons should radiate energy and spiral into the nucleus. The answer is (D).
The Rutherford Model and Its Achievements
Rutherford's 1911 gold-foil experiment led to a revolutionary picture of the atom: a tiny, dense nucleus containing most of the mass and all the positive charge, with electrons orbiting in the surrounding space. This planetary model explained several observations beautifully but left one critical question unanswered.
Let's examine each statement against what the model actually proposed and what it could explain.
Evaluating Each Statement
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Statement (A): Most of the space inside an atom is empty.
This was Rutherford's central conclusion. The fact that most alpha particles passed straight through the gold foil with little or no deflection meant that atoms are mostly empty space. Only the rare head-on collisions with the tiny nucleus caused large-angle scattering. The model explicitly requires this.
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Statement (B): The electrons revolve around the nucleus under the influence of coulomb force.
Rutherford proposed that electrons orbit the nucleus much like planets orbit the sun, with the electrostatic attraction F=r2ke2 providing the necessary centripetal force. This was the mechanism he invoked to keep electrons from flying away. The model directly states this.
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Statement (C): Most part of the mass and positive charge are concentrated at the centre.
The scattering data demanded a nucleus: a region of radius ∼10−15 m (compared to the atomic radius ∼10−10 m) containing essentially all the positive charge and mass. This concentration was the model's defining feature.
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Statement (D): The stability of atom was established by the model. …
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- CBSE 2020Set ANNUAL1 markMCQQ.The size of the atom in Thomson's model is _____ the atomic size in Rutherford's model.(a) much greater than(b) not different from(c) much less than
›Reveal solutionSolution
Thomson's and Rutherford's models both give the atom an overall radius of about 10−10 m; what differs is where the positive charge sits, not the size of the atom itself.
In Thomson's model, the atom is a sphere of positive charge (radius ∼10−10 m) with electrons embedded in it like plums in a pudding — the positive charge is spread over the whole atomic volume.
In Rutherford's model, Rutherford's alpha-scattering experiment showed that the positive charge and almost all the mass are concentrated in a tiny central nucleus (radius ∼10−15 to 10−14 m), while the electrons orbit at distances that still make the overall atom about 10−10 m in radius.
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- CBSE 2019Set HE2341 markMCQQ.The atom is:(i) Positive charged(ii) Negative charged(iii) Uncharged(iv) Not certain
›Reveal solutionSolution
An atom is electrically neutral because the positive charge of its protons exactly balances the negative charge of its electrons.
An atom consists of a small, dense, positively charged nucleus (containing protons and neutrons) surrounded by negatively charged electrons revolving around it. The number of electrons orbiting the nucleus equals the number of protons in the nucleus (the atomic number Z). Since the magnitude of charge on a proton (+e) equals the magnitude of charge on an electron (−e) …
- CBSE 2019Set ANNUAL1 markMCQQ.Who discovered the nucleus?(a) Thomson(b) Bohr(c) Rutherford(d) de Broglie
›Reveal solutionSolution
Ernest Rutherford discovered the atomic nucleus via his gold-foil (alpha-scattering) experiment.
In 1911, Rutherford's team fired alpha particles at a thin gold foil. Most passed straight through, but a small fraction scattered at large angles — some even bounced back. This could only be explained if almost all the atom's mass and its entire positive ch …
- CBSE 2018Set ANNUAL1 markQ.Write one drawback of Rutherford's atomic model.
›Reveal solutionSolution
Rutherford's model predicts atoms should collapse in an instant — it doesn't.
In Rutherford's nuclear model, electrons revolve around a small, dense, positively-charged nucleus under electrostatic attraction, in a manner analogous to planets orbiting the Sun. However, an electron moving in a circular orbit is constantly accelerating (centripetal acceleration towards the centre). According to classical electromagnetic theory, an accelerating charge must continuously radiate electromagnetic energy. As the electron loses energy, its orbital radius should continuously shrink, spiralling into the nucleus within a fraction of a second — predicting the atom is unstable and should collapse almost instantly. This directly contradicts the observed long-term stability of atoms. (Related drawback: this classical picture also predicts a contin …
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