Problem 42 - A Complete Resource Book in Chemistry for JEE Main

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 scatterd.

Step-by-Step Solution

Answer

(d) Helium nuclei, which impinged on a metal foil and got scattered.

Step 1: Analyze the Question Requirements

The question asks which type of particles was used in Rutherford's experiment to establish the nuclear model of the atom. It provides four options to choose from.

Step 2: Recall Rutherford's Experiment

Rutherford's gold foil experiment involved the use of alpha particles to probe the structure of the atom. This experiment demonstrated that most of the atom's mass is concentrated in a small nucleus. Alpha particles are, essentially, helium nuclei.

Step 3: Match Experiment Details with Options

From your understanding of Rutherford's experiment, match 'alpha particles' with the options given. 'Alpha particles' are the same as helium nuclei. Therefore, the correct option relating to particles scattering upon hitting the foil is (d).

Step 4: Verify the Correct Answer

The scattering observed in Rutherford's experiment helped establish the existence of the atomic nucleus. The description 'helium nuclei, which impinged on a metal foil and got scattered' matches the details of the experiment.

Key Concepts

Nuclear Model of the Atom

In the early 20th century, the understanding of atomic structure underwent a revolutionary change, thanks to Ernest Rutherford. The nuclear model of the atom distinctly marked this transformation. Before this model, atoms were thought to be a uniform mass, as per J.J. Thomson's plum pudding model.

Rutherford's experiment revealed a new picture: each atom consists of a small, dense center, called the nucleus. This center holds nearly all the atom's mass and is positively charged. Surrounding the nucleus are tiny electrons, which occupy the rest of the atom's volume. This nucleus-centric view replaced the idea of atoms being amorphous, homogenous spheres.

The nuclear model is crucial because it accurately explains various atomic behaviors and interactions. It explains why atoms are mostly empty space and how they can emit radiation, such as in radioactive decay. This breakthrough provided a foundation for later, more complex models, like the quantum mechanical model.

Alpha Particles

Alpha particles played a pivotal role in Rutherford's discovery. These particles are composed of two protons and two neutrons, the same as a helium nucleus. Hence, they carry a positive charge.

Key characteristics of alpha particles include:

• High mass compared to other subatomic particles.
• Relatively low penetration power; they can be stopped by a sheet of paper.
• Ability to cause ionization in materials they interact with.

In experiments, their positive charge and substantial mass made them perfect for probing the structure of atoms. When directed at thin materials, like gold foil, they interact with atomic nuclei, allowing scientists to infer the structure of the atom. Their deflection patterns revealed the existence of a concentrated atomic core.

Gold Foil Experiment

The gold foil experiment is one of the hallmark experiments in physics. Conducted by Rutherford and his assistants, Hans Geiger and Ernest Marsden, it used alpha particles to probe the nature of atoms.

Here's how it worked:

• A source emitted alpha particles towards a thin sheet of gold foil.
• A surrounding detector screen recorded where these particles landed as they passed through or deflected off the foil.
• Most particles passed straight through, highlighting the atom's mostly empty space.
• A few deflected at large angles, suggesting a dense, positively charged center: the nucleus.

This outcome was unexpected at the time, challenging existing atomic models and setting the stage for modern atomic theory. The experiment beautifully illustrated how scientific inquiry can radically alter our understanding with the right tools and questions.

Atomic Nucleus

The atomic nucleus, discovered through Rutherford's pioneering gold foil experiment, is the atom's central core. It is incredibly dense and carries a positive electric charge due to its protons.

Some key points about the nucleus:

• Composed of protons, which determine the element's identity, and neutrons, which add mass but no charge.
• Contains nearly all the atom's mass, despite being small compared to the entire atom.
• Strong nuclear forces bind the particles within the nucleus, making it stable.

Understanding the nucleus changed how scientists view chemical elements and isotopes. It also opened up paths to discovering nuclear reactions and energy. The knowledge that the atomic nucleus is massively compact and central facilitates numerous advanced fields, from nuclear energy to medicine.