Problem 68 - Master Resource Book in JEE Main Physics

Question

Match the following Column I with Column II Column 1 I. Thomson atomic model II. Rutherford Atom III. Bohr atom model IV. Ionisation potential Column II A. Fixed for an atom B. Stationary orbits model C. Charge and mass are distributed uniformly in a sphere D. Nucleus Codes (a) 1-A, 11-B, III-C, IV-D (b) \(1-\mathrm{D}, 11-\mathrm{B}, \mathrm{lll}-\mathrm{A}, \mathrm{IV}-\mathrm{C}\) (c) 1-C, 11-D, III-B, IV-A (d) 1-A, 11-C, 111-D, IV-B

Step-by-Step Solution

Answer

The correct option is (c): I-C, II-D, III-B, IV-A.

Step 1: Understanding the Models and Concepts

We first need to understand what each term in Column I refers to. The Thomson atomic model, also known as the 'plum pudding model,' suggests that the positive charge is spread evenly throughout the sphere of the atom. The Rutherford atom model proposed the existence of a small, dense nucleus where all the positive charge (and most of the mass) is concentrated. The Bohr atom model introduced the idea of electrons traveling in distinct orbits (or shells) around the nucleus (stationary orbits). Ionisation potential refers to the energy required to remove an electron from an atom, which is generally fixed for a given atom.

Step 2: Matching Models with Descriptions

With an understanding of what each model and concept represents, we can now match these with the correct descriptions from Column II. For Thomson's model (I), which features a uniform distribution of charge and mass in a sphere, the match is with C. Rutherford's model (II) is characterized by the presence of a nucleus, so it matches with D. The Bohr model (III), with its stationary orbits concept, corresponds to B. Finally, the Ionisation potential (IV) is related to the fixed energy requirement to remove an electron, corresponding to A.

Step 3: Verifying Against the Given Options

We now compare our matches to the given options. Our matches are I-C, II-D, III-B, and IV-A. By checking with the options, we see that option (c) aligns perfectly with our successful matches. It states 1-C, 11-D, III-B, IV-A.

Key Concepts

Thomson model

The Thomson atomic model, often fondly referred to as the "plum pudding model," was proposed by J.J. Thomson in 1897. This model imagines the atom as a large, positively charged sphere with negatively charged electrons embedded within it, much like raisins in a pudding. Here are some key points:

• The positive charge in the atom is spread uniformly over the entire sphere.
• Electrons are scattered throughout the positively charged framework, offsetting the charge to create a neutral atom overall.
• This model was a pioneer at its time but didn't explain observations related to atomic scattering experiments.

Though it was groundbreaking in introducing subatomic particles like electrons, it couldn't explain why atoms emit certain spectral lines. As science progressed, newer models began to emerge, such as the Rutherford model.

Rutherford model

The Rutherford model was conceived by Ernest Rutherford in 1911 following his famous gold foil experiment. It was a significant leap from previous models as it introduced the idea of a nucleus within the atom. Here's what makes the Rutherford model distinctive:

• It proposed that most of the atom's mass and positive charge is concentrated in a small core called the nucleus.
• Electrons orbit this nucleus, much like planets revolve around the sun, but without defined orbits.
• This model could explain the deflection of alpha particles observed in the gold foil experiment, which the Thomson model could not.

However, while the Rutherford model was a major advancement, it couldn’t explain the stability of atoms nor the specific patterns of light observed in atomic spectra. Arnold Sommerfeld later elaborated on this model, setting a foundation for the subsequent Bohr model.

Bohr model

Niels Bohr took Rutherford's groundwork a step further in developing the Bohr model in 1913. This was revolutionary in explaining atomic structure along with the discrete nature of atomic spectra. Key features of the Bohr model include:

• Electrons travel in specific, fixed orbits or 'shells' around the nucleus, known as stationary orbits.
• Each orbit corresponds to a certain energy level, and electrons can jump between these levels by absorbing or emitting energy in quantized amounts.
• This model accurately explained the hydrogen spectrum, successfully addressing gaps left by the Rutherford model.

Though the Bohr model explained many aspects of atomic behavior, it was mostly applicable to hydrogen-like atoms and lacked acceptance for more complex elements. Its insights paved the way for quantum mechanics and the modern understanding of atomic structure.

ionisation potential

Ionisation potential, also known as ionization energy, is a vital concept in atomic physics. It refers to the energy required to remove an electron from an isolated atom or molecule. This concept is crucial because it provides important insights into the reactivity and bonding tendency of elements. Important aspects include:

• Ionisation potential is usually expressed in electron volts (eV).
• For a given atom, this potential is generally fixed and depends on the effective nuclear charge experienced by the electron.
• Higher ionisation potential indicates a strong hold of the nucleus on the electrons, suggesting that the atom is less inclined to lose an electron.

Understanding ionisation potential helps explain aspects of the periodic table, such as why certain elements are more likely to form positive ions or participate in chemical reactions. It is a key concept for anyone delving into chemistry and physics.