Atomic Structure (Models of the Atom)

 Atomic Structure (Models of the Atom)

 Atomic Structure (Models of the Atom)

Here you will learn about different atomic models that completely describe the structure of the atom, these are Dalton’s atomic theory, J.J. Thomson, Ruther Ford, Bohr, and the Quantum mechanical model of the atom.

Rutherford Atomic Model

Rutherford Atomic Model - J. J. Thomson's "plum pudding" model was unable to account for some experimental findings related to the atomic structure of elements. British scientist Ernest Rutherford carried out an experiment, and using the results of this experiment, he proposed Rutherford's Atomic Model and explained the atomic structure of the elements.

Rutherford's model revolutionized the understanding of atomic structure by introducing the idea of a dense, positively charged nucleus and the existence of empty space within atoms.

In an experiment, Rutherford bombarded a thin sheet of gold with -alpha particles and then tracked the paths taken by the particles after they made contact with the foil.

In his experiment, Rutherford fired high-energy beams of -particles at a 100 nm-thick gold sheet, coming from a radioactive source. He surrounded the thin gold foil with a fluorescent zinc sulfide screen in order to analyze the deflection the -particles experienced. Rutherford stated certain findings that were at odds with Thomson's atomic model.

Rutherford atomic model 

Postulates:

The Rutherford atomic model states:

The atom consists of a tiny, positively charged nucleus at the center.

The nucleus contains most of the atom's mass, but occupies very little space.

Electrons, which are negatively charged, orbit around the nucleus in circular paths.

Most of the atom is empty space.

An atom's positive charge and the majority of its mass are packed into a very small volume. Using the term "nucleus," he described this area of the atom.

According to Rutherford's theory, an atom's nucleus is surrounded by electrons that are negatively charged. He also asserted that the electrons that surround the nucleus travel in a circular pattern at extremely high speeds. He gave these elliptical routes the name orbits.

A powerful electrostatic force of attraction holds the negatively charged electrons and the positively charged mass of particles that makes up the nucleus together.

Limitations:

Despite being founded on experimental evidence, the Rutherford atomic model proved unable to account for some phenomena.

According to Rutherford, the electrons go along predetermined routes known as orbits as they circle the nucleus. An electron rotating around the nucleus should create electromagnetic radiation because, according to Maxwell, accelerating charged particles emit such radiation. This radiation would transport energy from the electron's speed at the expense of the orbits getting smaller. In the nucleus, the electrons would eventually collapse. According to calculations, an electron would enter the nucleus according to the Rutherford model in less than 10-8 seconds. So the Rutherford model could not account for the stability of an atom and did not agree with Maxwell's theory.

One of the problems with the Rutherford model was that it was an incomplete theory because it said nothing about how the electrons were arranged in an atom.

The early atomic models, albeit erroneous and unable to account for some experimental findings, served as the foundation for later advances in the field of quantum mechanics.

 

Bohr atomic model:

Neil Bohr put forth the Bohr model of the atom in 1915. It was created by the modification of Rutherford's atomic model. In his nuclear model of an atom, Rutherford showed how a positively charged nucleus is surrounded by negatively charged electrons.

Bohr's hypothesis is applicable to entities that are similar to hydrogen but only have one electron, such Li2+. Atoms similar to hydrogen are amenable to Bohr's hypothesis (single electron system). Only one electron makes up Li2+ and the H atom. He and He2+ each have two and zero electrons.

The Bohr model has undergone numerous revisions, most notably the Sommerfeld model, often known as the Bohr-Sommerfeld model, which proposed that electrons orbit a nucleus in elliptical orbits rather than the circular orbits predicted by the Bohr model. The Bohr-Sommerfeld system was fundamentally inconsistent, which led to a number of paradoxes.

Bohr's model was significant as it explained the discrete emission or absorption spectra observed in certain experiments, such as the hydrogen line spectrum. It introduced the concept of quantized energy levels and provided a framework for understanding the behavior of electrons within the atom.

Postulates

Electrons move in specific circular orbits around the nucleus, known as energy levels.

Electrons can jump between energy levels by absorbing or emitting energy.

Electrons in stable orbits do not emit radiationIn an atom, negatively charged electrons travel in fixed circular paths around the positively charged nucleus that are referred to as orbits or shells.

These circular orbits are referred to as orbital shells because each orbit or shell has a defined energy.

The quantum number (n=1, 2, 3, etc.), which is an integer, is used to indicate the energy levels. The lowest energy level in this quantum number range, n=1, lies on the nucleus side. When an electron reaches the lowest energy level, it is referred to as being in the ground state. The orbits n=1, 2, 3, 4... are allocated as K, L, M, N.... shells.

The energy needed to transfer an electron in an atom from one energy level to another is acquired, and the energy needed to move an electron from one energy level to another is lost.

Limitations

Zeeman Effect could not be explained by Bohr's atomic model (effect of magnetic field on the spectra of atoms).

The Stark effect was also not adequately explained (effect of electric field on the spectra of atoms).

The Heisenberg Uncertainty Principle is broken.

The spectra obtained from bigger atoms could not be explained by it.

Quantum mechanical model

The quantum mechanical model, developed in the 1920s, is the most advanced and widely accepted model of the atom. It is based on quantum theory. 

Schrödinger's wave equation and its solution serve as the foundation for quantum mechanics. The concept of shells, subshells, and orbitals is introduced by the solution of the wave equation. The |ψ|2 at that site, where ψ denotes the wave function of that electron, determines the likelihood of finding an electron at that location within an atom.

Schrödinger's wave equation cannot be precisely solved for a multi-electron atom, making its application to multi-electron atoms challenging. Approximation techniques were used to get around this problem.

The quantum mechanical model of an atom was developed as a result of using the Schrödinger wave equation to ascertain the structure of an an atom.

This model accurately predicts the behavior and properties of atoms, including electron configurations, chemical bonding, and spectral lines. It considers the probabilistic nature of electron distribution and provides a more comprehensive understanding of atomic structure compared to earlier models.

Postulates:

Electrons are not confined to specific orbits but occupy regions of space called orbitals.

An orbital is a mathematical function that describes the probability of finding an electron in a particular location around the nucleus.

Orbitals are organized into energy levels and sublevels, based on their energy and shape.

The behavior of electrons is described by wave-particle duality, where they exhibit both particle-like and wave-like properties.

An electron can only have a fixed range of energy values since its energy is quantized.

The Schrödinger wave equation's permitted solution, the quantized energy of an electron, results from the electron's wave-like characteristics.

Heisenberg's Uncertainty Principle states that it is impossible to pinpoint an electron's precise position or momentum. As a result, it is possible to calculate the probability of finding an electron at a given position, which is |ψ|2 at the location where ψ denotes the electron's wave function.

The wave function (ψ) of an electron in an atom is known as an atomic orbital. When an electron fills an atomic orbital, it is described by a wave function. There are numerous atomic orbitals for the electron since it can have a wide range of wave functions. Every wave function and atomic orbital has a specific shape and energy. The orbital wave function of an atom's electron contains all the information about that atom's electron, and quantum mechanics allows us to extract this information from it.

The square of the orbital wave function, or |ψ|2, at a given place within an atom, determines the likelihood of finding an electron there. Probability density, which is always positive, is denoted by |ψ|2.

First two Daltons and Bohr atomic model