PHY 464 Assignment Questions and Answers

Please read carefully before you copy and also note Q1, means Questions 1 and A1, Means answer to Questions 1.

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ANSWERS

Q1: According to Bohr’s model of the atom, which orbit has the minimum energy?

A1: In Bohr’s model of the atom, the orbit with the minimum energy is the first orbit (n = 1), which is also called the ground state. This orbit lies closest to the nucleus. Since the electron in this orbit is strongly attracted by the positive charge of the nucleus, it possesses the least amount of energy compared to electrons in higher orbits. As we move to higher orbits (n = 2, 3, 4, and so on), the energy of the electron increases. Therefore, the minimum energy always belongs to the first orbit.

Q2: Write on J. J. Thomson’s idea of the atomic model stating their limitations.

A2: J. J. Thomson proposed the plum pudding model of the atom in 1904. According to him, the atom was imagined as a sphere of positive charge in which negatively charged electrons were embedded, like “plums” inside a pudding. This model suggested that the atom as a whole is electrically neutral because the positive and negative charges balance each other.

Limitations of Thomson’s model:

1. It could not explain the results of Rutherford’s gold foil experiment, which showed that atoms have a small, dense, positively charged nucleus.
2. It did not explain how the electrons were arranged or why they stayed fixed inside the atom.
3. It failed to account for the stability of the atom.
4. It could not describe the experimental observations of atomic spectra.

Because of these limitations, Thomson’s model was replaced by Rutherford’s nuclear model.

Q3: Write on Rutherford’s atomic model stating their limitations.

A3: In 1911, Ernest Rutherford proposed the nuclear model of the atom after his famous gold foil experiment. He discovered that when alpha particles were directed at a thin sheet of gold foil, most passed through, but some were deflected at large angles. This led him to conclude that the atom has a small, dense, and positively charged nucleus at its center.

According to Rutherford’s model:

• The atom is mostly empty space.
• All the positive charge and most of the mass of the atom are concentrated in the nucleus.
• Electrons revolve around the nucleus in circular paths, much like planets orbit the sun.
• The negative electrons balance the positive charge of the nucleus, making the atom neutral.

Limitations of Rutherford’s model:

1. It could not explain why electrons revolving around the nucleus do not lose energy and spiral into the nucleus, which should make the atom unstable.
2. It failed to explain the stability of the atom.
3. It did not account for the line spectra of elements — especially the hydrogen spectrum.

Because of these limitations, Rutherford’s model was later modified by Niels Bohr, who introduced the concept of fixed energy levels (orbits).

Q4: Summary of Bohr’s model from your own understanding and its limitations.

A4: Niels Bohr improved Rutherford’s model in 1913 by introducing the idea of quantized energy levels. According to Bohr, electrons do not just move randomly around the nucleus. Instead, they revolve in fixed circular orbits called energy levels or shells, represented by the values of n = 1, 2, 3….

From my understanding, this model explains that:

• Each orbit has a certain fixed energy, and electrons in that orbit do not radiate energy.
• An electron can jump from a lower energy orbit to a higher one by absorbing energy, and it can move back to a lower orbit by releasing energy in the form of light.
• The lowest orbit (n = 1) is the most stable, with minimum energy.

Limitations of Bohr’s model:

1. It could explain only the hydrogen atom (single-electron system) but failed for larger, multi-electron atoms.
2. It could not explain fine details of atomic spectra.
3. It did not fully follow the principles of quantum mechanics discovered later.
4. It could not explain the wave nature of electrons.

Even with these limitations, Bohr’s model was a big step in atomic theory and helped scientists move closer to the modern understanding of the atom.