STRUCTURE OF ATOMS LECTURE 1 | Rutherford's Atomic Model

CHAPTER 2

STRUCTURE OF ATOMS

LECTURE 1

The structure of atom refers to how the sub-atomic particles are arranged. Atoms are so small that they can only be visualized with a scanning tunneling microscope.

Atom

The word ‘atom’ was first introduced by Greek philosopher Democritus. He gave the concept that matter is made up of very small indivisible particles called atoms. The word atom means indivisible, uncuttable (a-not + tomos-cutting). The sub-atomic particles in an atom are protons (positively charged), electrons (negatively charged) and neutrons (neutral – no charge).

Rutherford’s Atomic Model

In 1911, Rutherford performed an experiment in order to know the arrangement of electrons and protons in atoms. Rutherford's atomic model shows the existence of nucleus in the atom, nature of charge on the nucleus and the magnitude of charge on the nucleus.

Rutherford’s Experiment

Apparatus for Experiment

•          Very thin gold foil of about 0.0004cm thickness.
•          α particles obtained from the disintegration of polonium.
•          Zinc sulphide screen (ZnS).

Experiment

Rutherford bombarded a very thin gold foil about 0.0004cm thickness with α-particles. He used α-particles obtained from the disintegration of radioactive element polonium. He used circular screen coated with zinc sulphide in order to record experimental observations.

 Figure 1 Rutherford's Experiment

Properties of α-particles

 α-particles are helium nuclei that are doubly positively charged (He⁺⁺).

Observations

Most of the α-particles are passed straight through the foil. Only few particles were slightly deflected. But one in million (1/100000) was deflected through an angle greater than 90^ο from their straight paths. Rutherford performed a series of experiments using thin foils of other elements. He observed similar results from these experiments.

Conclusions

From his experiments, Rutherford proposed a new model for an atom. He proposed a planetary model (similar to solar system) for an atom. An atom is a neutral particle. The mass of an atom is concentrated in a very small dense positively charged region. He named this region as a nucleus. The electrons are revolving around the nucleus in circles. These circles are called orbits. The centripetal force due to the revolution of electrons balances the electrostatic force of attraction between the nucleus and the electrons.

Rutherford drew the following conclusions:-

1. Since majority of the α-particles passed through the foil un-deflected, most of the space occupied by an atom must be empty.
2. The deflection of a few α-particles through angles greater than 90ᵒ shows that these particles are deflected by electrostatic repulsion between the positively charged α-particles and the positively charged part of atom.
3. Massive α-particles are not deflected by electrons (See Figure 1 part b).

Defects in Rutherford’s Atomic Model

Rutherford’s model of an atom resembles our solar system. It has following defects:-

1. According to classical physics, electron being charged particle will emit energy continuously while revolving around the nucleus. Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus. This would collapse the atomic structure (see Figure 2).
2. If revolving electron emits energy continuously it should form a continuous spectrum for an atom but actually a line spectrum is obtained.

Figure 2 Defect in Rutherford's Experiment

Figure 3 Difference between continuous spectrum and line spectrum

Fundamentals of Chemistry Lecture 1
Fundamentals of Chemistry Lecture 2

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