Question

Large angle scattering of \(\alpha-\) particle could not be explained by (A) Thomson model (B) Rutherford model (C) Both Thomson and Rutherford model (D) neither Thomson nor Rutherford model

Short answer

The large angle scattering of alpha particles could not be explained by the Thomson model (A), in which electrons are dispersed evenly throughout the atom's volume. This model doesn't predict significant deflections due to the lack of centrally concentrated positive charge. On the other hand, Rutherford's model, with a dense, positively charged nucleus, does predict large deflections and is supported by experimental evidence.

Step-by-step solution

Understand Thomson's Model

In the Thomson model of the atom, the electrons are dispersed evenly throughout the atom's volume, nicknamed as the "plum pudding model". According to this model, positively charged alpha particles would pass through the uniformly charged negatively charged "cloud" of electrons with only little deflections due to the slight repulsions or attractions by the electrons' negative charges.

Understand Rutherford's Model

In the Rutherford model, also known as the Planetary model, the atom consists of a dense, positively charged nucleus surrounded by electrons in orbits. According to this model, the alpha particles could pass close to the nucleus, altering their path dramatically due to the intense electrostatic repulsion between the positively charged alpha particles and the positively charged nucleus.

Determine which model cannot explain large angle scattering

In Thomson's model, significant large-angle scattering of alpha particles would not be expected because there is no centrally concentrated positive charge to cause large deflection. In contrast, Rutherford's model does predict large angle deflections if alpha particles pass near the atom's nucleus. Experiments, such as Rutherford's gold foil experiment, have proven large angle scattering to be real, thereby validating Rutherford's model and invalidating Thomson's model concerning large angle scattering.

Select the correct answer

Based on the analysis above, Thomson's model could not explain large angle scattering of alpha particles. Therefore, the correct answer is: (A) Thomson model

Key concepts

Thomson's Model

Thomson's Model, often referred to as the "plum pudding model," represents an early concept of atomic structure. This model postulates that electrons are spread uniformly throughout the atom, somewhat like raisins scattered within a pudding of positive charge. Thomson envisioned the atom as a sphere of positive charge with negatively charged electrons embedded within it.

The model presents a homogeneous positive charge distribution across the sphere, which means that the internal structure of the atom is uniform. When high-energy particles like alpha particles encounter such a structure, the model predicts that these particles should pass through the atom with only minimal deflections.

Due to the lack of a concentrated positive charge, Thomson's Model does not account for considerable scattering of alpha particles. Therefore, when faced with experiments that showed large angle deflections, such as those performed by Rutherford, the "plum pudding model" could not provide an adequate explanation.

Rutherford's Model

Rutherford's Model proposed a groundbreaking shift from Thomson's framework and became known as the "Planetary Model". This model suggests that the atom is mostly empty space, but contains a small, dense, positively charged nucleus at its center. Electrons orbit around this nucleus, much like planets orbiting the sun.

The distinctive aspect of Rutherford's Model is its ability to explain large angle scattering of alpha particles observed in experiments. It hypothesizes that when alpha particles, which are also positively charged, come in close proximity to the dense nucleus, they experience a strong electrostatic force. The repulsive force between the alpha particles and the nucleus can lead to dramatic changes in their trajectory, resulting in large angle deflections.

Rutherford's interpretation was powerfully supported by experimental evidence from scattering experiments, notably the famous gold foil experiment. Here, unexpected large-angle deflections of alpha particles were observed, suggesting the presence of a concentrated nucleus, thus validating Rutherford's Model and rendering Thomson’s insufficient.

Alpha Particles

Alpha particles are fundamental in exploring atomic structure due to their unique properties. Composed of two protons and two neutrons, they are essentially helium nuclei, carrying a +2 charge. Their relatively large mass and positive charge make them ideal for scattering experiments in atomic physics.

In Rutherford's experiments, alpha particles were directed at thin metal foils to probe the internal structure of atoms. Because of their substantial energy and charge, alpha particles interact strongly with the electric fields of other charged particles inside the atom.

When alpha particles approach a positively charged nucleus, they can be significantly deflected due to electrostatic repulsion, making them instrumental in revealing the concentrated nature of the nucleus in Rutherford’s Model. Conversely, in Thomson’s Model, such large angle scattering was not anticipated, as the absence of a concentrated center precludes strong interactions leading to wide deflections.

Thus, the behavior of alpha particles in scattering experiments provides crucial evidence that helped shift the scientific view from the "plum pudding model" to the Planetary Model of the atom.