Atomic Models: Introduction, Discoveries of Different Models

Atomic Models: We are all aware that everything in our environment is made out of matter. We also know that electrons, protons, and neutrons are the three subatomic particles. But where do all these subatomic particles reside in an atom, and what is an atom’s structure? All of these issues will be answered in this article, as well as how key Atomic Model discoveries were made.

Since the beginning of atomic structure research, a multitude of models has been developed and used to better understand how an atom behaves. As a result, there are five basic Atomic Models that have aided us in better defining and understanding the structure of the atom. 5 Atomic Models by John Dalton, J.J. Thomson, Ernest Rutherford, Niels Bohr, and Erwin Schrödinger will be discussed here.

Continue reading to know more details on Atomic Models, characteristics, etc.

Atomic Models

The earliest belief about the structure of the atom was attributed to Dalton, who suggested that matter was made of tiny indivisible particles called ‘Atoms.’

The discovery of electrons proved for the first time that atoms themselves were made of fundamental particles, which were the constituents of all atoms and therefore, atoms did have a fine structure that was unique to every different atom.

Therefore, this discovery of electrons proved that some particles were themselves the fundamental building blocks of atoms.

Dalton’s Atomic Theory

In \(1803,\) John Dalton developed a theory of the structure of matter based on the law of chemical combination. The main postulates of this theory are:

1. The matter is composed of a large number of extremely small particles called an atom.
2. Atoms are indivisible and cannot be destroyed in a chemical reaction.
3. All the atoms of an element are alike, i.e., identical in mass, size and every other aspect.
4. Atoms of different elements possess different properties, size, mass, etc.
5. Compounds are formed by the combination of atoms in a simple whole-number ratio.
6. In a given compound, the relative number and kinds of atoms are constant.

Demerits of Dalton’s Atomic Theory

1. Atoms are now known to be made of subatomic particles- electrons, protons and neutrons.
2. Isotopes are atoms of the same element with different mass number. Therefore, the assumption that all atoms have the same mass does not hold true.
3. Allotropic form of the same elements with different properties was discovered.
4. Isobars are atoms of different elements with the same mass number. Therefore, the assumption that masses of atoms of different elements are different does not hold so good.

Thomson’s Atomic Model

J.J Thomson made the first attempt at explaining the fine structure of the atom. He proposed that-The atoms were uniform, positively charged spheres. Along with all the positive charges, most of the mass of the atom was thought to be uniformly distributed in this uniform ‘cloud’ of positive charge.

The tiny negatively charged electrons were believed to be embedded in these positively charged cloud-like seeds in a watermelon or like the plums in a pudding, and hence, it was named the plum pudding model. It explained the existence of positive and negative charges and even explained the electrical neutrality of matter by assuming the total charge of electrons to be equal and opposite to that of the total positive charge cloud.

Limitations

The reason for the failure of the plum pudding model are as follows:

i. He could not offer any experimental support to his atomic model.
ii. He could not explain the results of the experiments carried by Rutherford and other scientists at a later stage.

Therefore, Thomson’s model was rejected. However, his predictions that an atom is electrically neutral and has no net charge is still accepted. This was indeed a big contribution to the structure of an atom.

Rutherford’s Model of an Atom

The major contribution in knowing the relative positions of electrons and protons in an atom was by Ernest Rutherford. He performed the \(\alpha \)-particle scattering experiment in the year \(1911.\) For this, he selected a thin foil of heavy gold metal. It was \( \sim 100\,{\text{nm}}\) thick. He bombarded the same with the high-speed alpha particles. These alpha particles were positively charged helium nuclei \(\left( {{\text{H}}{{\text{e}}^{2 + }}} \right).\) Each such particles carries a mass of \(4\,{\text{amu}}\) and a charge of \( + 2\) units. These were emitted from a radioactive element enclosed in a lead box.

These alpha particles were first passed through a fine slit and then made to strike against the gold foil. As a result, these particles got scattered and produced bright flashes on a circular screen coated with a substance called zinc sulphide.

Observations from the Experiment

From the scattering experiment, Rutherford made the following observations:

• i. Most of the alpha particles were able to pass through the gold foil undeflected.
• ii. Some of the alpha particles were deflected by small angles.
• iii. A very few (one out of approximately \(20000\)) alpha particles suffered major deflections and even came back in the same direction \(\left({{{180}^ \circ. }} \right)\)

Conclusions from the Observation

The conclusions that were drawn from the observations are as follows:

• i. As most of the alpha particles passed through undeflected, this means that they did not come across any obstruction in their path. Thus, most of the space in an atom is expected to be empty.
• ii. As a few alpha particles suffered minor deflections and a very few major deflections, this means that these must have met with some obstructions in their path
• iii. Most of the mass and all the positive charge of the atom were located near the atom’s central core. He called the central core the nucleus.

According to Rutherford Model of an Atom

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
2. The electrons revolve around the nucleus in circular paths.
3. The size of the nucleus is very small as compared to the size of the atom.

Rutherford Model Drawbacks

The drawback of the Rutherford model is:

It couldn’t explain the stability of an atom. According to the model, the orbital revolution of electrons around the nucleus is not stable as it will undergo acceleration and emit energy which will result in the loss of energy of electrons, and they will ultimately collide with the nucleus.

Bohr’s Model of an Atom

In \(1913,\) Neil Bohr explained the stability of the atom based on the observations of his experiments. According to him:

1. An atom consists of a  very small central core called the nucleus. The nucleus carries all the positive charge and most of the mass of the atom.
2. Electrons revolve around the nucleus in circular paths. These circular paths are called orbits. The centripetal force required for the circular motion is provided by the electrostatic force of attraction between the electrons and the nucleus.
3. Electrons can revolve only in certain permissible orbits called shells or energy levels, each of which is associated with a  fixed amount of energy.
4. While an electron is revolving in orbit, it neither gains nor loses energy. Therefore, these orbits are called stationary states or energy levels.
5. An electron may jump from an orbit of higher energy to that of lower energy, thereby emitting energy.

For convenience, these energy levels are labelled \({\text{K,L,M,N}}\) and so on. The orbit closest to the nucleus is the \({\text{K}}\) shell and has the least amount of energy, and the electrons present in it are \({\text{K}}\) electrons, and so on with the successive shells and their electrons.

Summary

In this article, we learned in detail the four Atomic models- John Dalton’s atomic model, J.J Thomson’s atomic model- Plum pudding model, Ernest Rutherford model- Nuclear model of an atom, Neil Bohr’s model of the atom- Planetary model. Also, we learned their limitations, postulates, etc. 

FAQs

We have provided some frequently asked questions about Atomic Models here:

Q.1. What are the drawbacks of Thomson’s model of the atom?
Ans: The reason for the failure of the plum pudding model are as follows-
i. He could not offer any experimental support to his atomic model.
ii. He could not explain the results of the experiments carried by Rutherford and other scientists at a later stage.

Q.2. What are the limitations of Rutherford’s model of the atom?
Ans: The drawback of the Rutherford model is:
It couldn’t explain the stability of an atom. According to the model, the orbital revolution of electrons around the nucleus is not stable as it will undergo acceleration and emit energy which will result in the loss of energy of electrons, and they will ultimately collide with the nucleus.

Q.3. What are the five atomic models?
Ans: The five atom is models are as follows:
John Dalton’s atomic model
J.J Thomson’s atomic model- Plum pudding model
Ernest Rutherford model- Nuclear model of an atom
Neil Bohr’s model of the atom- Planetary model
Erwin Schrödinger’s model-Quantum model.

Q.4. What is the most accurate atomic model?
Ans: The most accurate atomic model was given by Neil Bohr and is called Bohr’s model of atom or Planetary model.

Q.5. What are the postulates of Dalton’s Atomic Theory?
Ans: The main postulates of Dalton’s atomic theory are:
– The matter is composed of a large number of extremely small particles called an atom.
– Atoms are indivisible and cannot be destroyed in a chemical reaction.
– All the atoms of an element are alike, i.e., identical in mass, size and every other aspect.
– Atoms of different elements possess different properties, sizes, mass, etc.
– Compounds are formed by the combination of atoms in a simple whole-number ratio.
– In a given compound, the relative number and kinds of atoms are constant.