Question

For each of the atomic models below, identify the strengths and weaknesses of the model as well as a diagram of the model.

(a) Quantum or Wave Mechanical Model

(b) Bohr Model

(c) Rutherford or Planetary (Nuclear) Model

(d) Thomson or Raisin Bun (Plum Pudding) Model

(e) Dalton or Billiard ball/Solid sphere Model

Answer 1

A) Quantum or wave machanical model

n = 3 n = 2 n = 1 m AE = hf +Ze Quantum or Wave Mechanical Model

Quantum mechanics is based on Schröding based wave equation and its solution. The solution of the wave equation brings the idea of ​​shells, sub-shells and orbitals. The probability of finding an electron at a point within an atom is proportional to the | ψ | 2 at that point, where ψ represents the wave-function of that electron.

Application of Schröding affecting equation to multi-electron atoms present’s some difficulty: Schröding attacking wave equation cannot be solved exactly for a multi-electron atom. This difficulty was overcome by using approximate methods.

The application of Schrödinger wave equation in determining the structure of an atom led to the formation of the quantum mechanical model of an atom.

Features of quantum mechanical model:

The energy of an electron is quantized i.e. an electron can only have certain specific values ​​of energy.

The quantized energy of an electron is the allowed solution of the Schrödinger wave equation and it is the result of wave like properties of electron.

As per Heisenberg's Uncertainty principle, the exact position and momentum of an electron cannot be determined. So the only probability of finding an electron at a position can be determined and it is | ψ | 2 at that point where ψ represents the wave-function of that electron.

An atomic orbital is the wave-function (ψ) of an electron in an atom. Whenever an electron is described by a wave-function, it occupies atomic orbital. As an electron can have many wave-functions, there are many atomic orbitals for electron. Every wave-function or atomic orbital has some shape and energy associated with it. All the information about the electron in an atom is stored in its orbital wave function ψ and quantum mechanics makes it possible to extract this information out of ψ.

The probability of finding an electron at a point within an atom is proportional to the square of the orbital wave function i.e., | ψ | 2 at that point. | ψ | 2 is known as probability density and is always positive.

Limitations of quantum model

The magnetic dichroism, i.e., the difference in the absorption coefficient for right- and left-circularly polarized electromagnetic waves, is a relativistic many-electron effect in a magnetic material. Jenkins and Strange have introduced the most simple and analytically solvable quantum mechanical model which exhibits magnetic dichroism, i.e., the relativistic one-electron atom in an external magnetic field. We have extended this model by considering the 2p – 3d transitions and by taking into account the effect of an additional crystal field. The model predicts near-zero dichroism for the 2p1 / 2–3d3 / 2 transition (L2-absorption) if the crystal field and the effect of the magnetic field on the core states are neglected, in contrast to the strong dichroism for the L2- absorption in real materials. The reason for this limitation of the model of Jenkins and Strange is discussed. The j –j mixing of the initial states by the additional crystal field potential has some weak effect on the dichroism

B ) Bohr model

-N shell M shell L shell K shell Nucleus Bohr Model

The Bohr model shows that the electrons in atoms are in orbits of differing energy around the nucleus (think of planets orbiting around the sun). Bohr used the term energy levels (or shells) to describe these orbits of differing energy. ... The energy level an electron normally occupies is called its ground state.

The Bohr model can be summarized by the following four principles: Electrons occupy only certain orbits around the nucleus. Those orbits are stable and are called "stationary" orbits. Each orbit has an energy associated with it.

limitations of Bohr model

1) The Bohr atomic model theory made correct predictions for smaller sized atoms like hydrogen, but poor spectral predictions are obtained when larger atoms are considered.

2) It failed to explain the Zeeman effect when the spectral line is split into several components in the presence of a magnetic field.

C) Rutherford model

Rutherford model

Rutherford's model shows that an atom is mostly empty space, with electrons orbiting a fixed, positively charged nucleus in set, predictable paths. This model of an atom was developed by Ernest Rutherford

Atoms nucleus is surrounded by negatively charged particles called electrons. The electrons revolve around the nucleus in a fixed circular path at very high speed. These fixed circular paths were termed as "orbits."

limitations of Rutherford model

The main problem with Rutherford's model was that he couldn't explain why negatively charged electrons remain in orbit when they should instantly fall into the positively charged nucleus.

D) Thomson model

0.0 Spherical cloud 0x of positive charge + + +0. + Electron + O Thomson's model

Thomson's model was known as the "Plum Pudding Model" (or "Raisin Bread Model.") As each atom was a sphere filled with a positively charged fluid, known as the "pudding". Scattered in this fluid were negatively charged electrons, these were the “plums” in the pudding.

Though defunct by modern standards, the Plum Pudding Model represents an important step in the development of atomic theory. Not only did it involve new discoveries, such as the existence of the electron, it also introduced the notion of the atom as a non-inert, divisible mass.

limitations of Thomson model

He argued that the plum pudding model was incorrect. The symmetrical distribution of charge would allow all the α particles to pass through with no deflection. Rutherford proposed that the atom is mostly empty space. The electrons revolve in circular orbits about a massive positive charge at the center.

Thomson's atomic model failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain an atom's stability.

The theory did not mention anything about the nucleus of an atom.

It was unable to explain the scattering experiment of Rutherford.

E) Dalton model / solid sphere model

Dalton model
The Solid Sphere Model was the first atomic model and was developed by John Dalton in the early 19th century. He hypothesized that an atom is a solid sphere that could not be divided into smaller particles. He came up with his theory as a result of his research into gases.

Because Dalton thought atoms were the smallest particles of matter, he envisioned them as solid, hard spheres, like billiard (pool) balls, so he used wooden balls to model them. ... Dalton added these so the model atoms could be joined together with hooks and used to model compounds.

limitations of Dalton model

1) Atom consists of particles like electrons, protons, and neutrons but Dalton says atoms are indivisible and they can neither be created nor be destroyed.

2) Discovery of isotopes revealed that the atoms of the same element possess different weights.