Question

Rutherford's experiment, which established the nuclear model of the atom, used a beam of (a) \(\beta\) -particles, which impinged on a metal foil and got absorbed. (b) \(\gamma\) -rays, which impinged on a metal foil and ejected electrons. (c) helium atoms, which impinged on a metal foil and got scattered. (d) helium nuclei, which impinged on a metal foil and got scattered.

Short answer

The correct answer is (d) helium nuclei, which impinged on a metal foil and got scattered.

Step-by-step solution

Identify the Correct Experiment

To solve this exercise, it is necessary to recall the details of Rutherford's experiment. Rutherford's gold foil experiment involved directing particles at a thin sheet of gold foil. The observation of how these particles were deflected allowed Rutherford to propose a model for the atomic structure.

Eliminate Incorrect Options

Options (a) and (b) can be eliminated because Rutherford's experiment did not involve either \beta -particles or \(gamma\)-rays. \(\beta\)-particles are electrons or positrons, and \(\gamma\)-rays are high-energy photons, neither of which were used in Rutherford's experiment.

Distinguish Between Helium Atoms and Helium Nuclei

The experiment did involve helium, but it is crucial to differentiate between helium atoms and helium nuclei. A helium atom includes electrons, while a helium nucleus (also known as an alpha particle) is just the core of the atom and lacks the surrounding electrons. Rutherford's experiment utilized these alpha particles (helium nuclei) for the beam.

Identify the Correct Observation

The key observation in Rutherford's experiment was the scattering of the particles, rather than absorption or ejection of electrons. Hence, the correct statement must involve 'helium nuclei' and 'got scattered'.

Key concepts

Nuclear Model of the Atom

The nuclear model of the atom is a pivotal concept in understanding the structure of matter. Developed in 1911 by Ernest Rutherford, this model arose from his famous gold foil experiment.

Thanks to Rutherford's investigations, we now visualize an atom much like a miniature solar system. At the center lies an incredibly dense nucleus consisting of protons and neutrons. Surrounding the nucleus, much like planets orbiting the sun, electrons move in the comparatively vast empty space of the atom. This radical concept replaced previous models like the 'plum pudding model', which inaccurately suggested that positive charge was spread out throughout the atom with electrons embedded like plums in a pudding.

Understanding this model is crucial because it explains why atoms are mostly empty space and determined that the majority of the atom's mass and all of its positive charge are concentrated in the nucleus. The insight that an atom has a central core rewrote the script on atomic interactions, laying the groundwork for modern chemistry and physics.

Alpha Particles

In the context of Rutherford's experiment, alpha particles played the star role. These particles are positively charged and consist of two protons and two neutrons. They are essentially helium nuclei that have been stripped of their surrounding electrons.

Because of their mass and charge, when Rutherford directed alpha particles at a thin sheet of gold foil, most passed through the empty space of the gold atoms. However, some were deflected by the dense gold nuclei, and a very few were scattered at wide angles. This scattering was key to proving the existence of a small, dense nucleus at the heart of the atom. It's interesting to note that alpha particles are a form of radiation, often emitted by radioactive substances. They have great implications in both theoretical research and practical applications like radiation therapy for cancer.

Atomic Structure

The atomic structure is the arrangement of subatomic particles within an atom. After the gold foil experiment, it became clear that atoms consist of a tiny nucleus surrounded by a cloud of electrons. The nucleus, despite being the central core of the atom, is minuscule compared to the overall size of the atom.

Atoms are mostly empty space. If one were to scale up an atom so that its nucleus was the size of a pea, the electrons would be whirling about many meters away. This structure of the atom explains many of its properties, including how atoms bond with each other to form molecules and the behavior of materials at the atomic and molecular scales. The understanding of atomic structure is a core principle of chemistry and physics and provides the basis for countless technological advancements and scientific theories.