Scattering of a-particles by a thin gold foil suggests the presence of
positively charged nucleus at the centre of an atom
The question asks what the scattering of alpha (α) particles by a thin gold foil suggests about the structure of an atom. This refers to the famous Rutherford scattering experiment, a landmark experiment that revolutionized our understanding of the atom.
Understanding the Rutherford Gold Foil Experiment
Ernest Rutherford and his associates performed experiments where they bombarded a thin sheet of gold foil with a beam of fast-moving alpha particles. Alpha particles are positively charged particles emitted by certain radioactive materials.
Key Observations of the Gold Foil Experiment:
- Most of the alpha particles passed straight through the gold foil without any deflection.
- A small fraction of the alpha particles were deflected by small angles.
- A very few alpha particles (about 1 in 20,000) were deflected back, almost retracing their path.
Interpreting the Scattering Results
These observations led Rutherford to draw significant conclusions about the atom's structure:
- The fact that most alpha particles passed straight through indicated that most of the space within an atom is empty.
- The small deflections suggested that there was a positively charged region in the atom causing repulsion with the positively charged alpha particles. However, this region must be very small since only a few particles were deflected.
- The rare large-angle deflections and the bouncing back of a few alpha particles indicated that the positively charged region was not only small but also very dense and contained most of the atom's mass.
Conclusion: The Nuclear Model
Based on these findings, Rutherford proposed the nuclear model of the atom. This model states that:
- An atom consists of a tiny, dense, positively charged region called the nucleus at its center.
- Most of the atom's mass is concentrated in this nucleus.
- The electrons, which are negatively charged, revolve around the nucleus in orbits.
- The size of the nucleus is very small compared to the size of the atom.
Analyzing the Given Options
Let's evaluate each option in light of the Rutherford scattering experiment:
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Option 1: electron in an atom
While electrons are present in an atom, they are very light and distributed in the space around the nucleus. They would not cause the large deflections observed in the positively charged alpha particles. The scattering pattern is primarily due to the interaction with the positive charge and mass concentrated at the center.
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Option 2: proton in an atom
Protons are the positively charged particles located within the nucleus. The scattering is indeed due to the repulsion between the alpha particles and the protons. However, simply stating "proton in an atom" doesn't capture the crucial finding that these protons (and neutrons which contribute mass) are concentrated in a tiny, central region. The scattering pattern specifically points to the presence of a positively charged nucleus.
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Option 3: positively charged nucleus at the centre of an atom
This option directly aligns with the main conclusion drawn from the Rutherford gold foil experiment. The scattering pattern, especially the large-angle scattering and backscattering, provided compelling evidence for the existence of a small, dense, positively charged nucleus located at the center of the atom. The repulsion between the positively charged alpha particles and the positively charged nucleus is what caused the observed deflections.
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Option 4: isotopes of gold
Isotopes are atoms of the same element with different numbers of neutrons. The presence of isotopes does not explain the characteristic scattering pattern of alpha particles. The deflections are primarily due to the electrostatic interaction between the alpha particle's charge and the positive charge of the atom's nucleus, and the concentration of mass. While gold foil contains different isotopes of gold, the scattering experiment's primary conclusion is about the fundamental structure of the atom (the nucleus), not isotopic composition.
Therefore, the scattering of alpha particles by a thin gold foil most strongly suggests the presence of a positively charged nucleus at the centre of an atom.
| Observation | Interpretation (What it suggests about the atom) |
|---|---|
| Most alpha particles pass straight through | Atom is mostly empty space |
| Some alpha particles deflected at small angles | There is a small, positively charged region causing repulsion |
| A few alpha particles deflected at large angles or bounce back | The positive charge and most of the mass are concentrated in a very small, dense region (the nucleus) |
Revision Table: Atomic Models
| Model | Key Features | Experiment(s) |
|---|---|---|
| Dalton's Atomic Theory | Atoms are indivisible, all atoms of an element are identical, atoms combine in fixed ratios | Laws of Chemical Combination |
| Thomson's Plum Pudding Model | Atom is a sphere of uniform positive charge with electrons embedded in it | Discovery of Electron (Cathode Ray Tube Experiment) |
| Rutherford's Nuclear Model | Dense, positively charged nucleus at the center; electrons orbit the nucleus; most of atom is empty space | Alpha Particle Scattering Experiment (Gold Foil Experiment) |
| Bohr's Model | Electrons orbit the nucleus in specific energy levels or shells | Atomic Spectra of Hydrogen |
Additional Information: Why Alpha Particles?
Alpha particles were chosen for the scattering experiment because:
- They are relatively heavy compared to electrons, ensuring that the deflection is mainly due to the force from the atom's constituents rather than the alpha particle being easily pushed around.
- They are positively charged (charge of +2e), allowing them to interact electrostatically with the charged parts of the atom.
- They have high kinetic energy, allowing them to penetrate the gold foil.
Gold foil was chosen because gold is malleable and can be made into a very thin sheet (allowing most alpha particles to encounter only a few atoms) and it has a high atomic number, meaning its nucleus has a relatively large positive charge, which helps in observing deflections.
Important Questions from Structure of Atom
Identify the element having zero valency
The atomic number of an element is 8. How many electrons will it gain to form a compound with sodium?
An atom of carbon has 6 protons. Its mass number is 12. How many neutrons are present in an atom of carbon?
What is the atomic number of nitrogen?
What are isobars?
