12. ATOMS

By the nineteenth century, enough evidence had accumulated in favour of atomic hypothesis of matter. In 1897, the experiments on electric discharge through gases carried out by the English physicist J. J. Thomson revealed that atoms of different elements contain negatively charged constituents (electrons) that are identical for all atoms. However, atoms on a whole are electrically neutral. Therefore, an atom must also contain some positive charge to neutralise the negative charge of the electrons. But what is the arrangement of the positive charge and the electrons inside the atom?

In the late 1800s, scientists gathered enough evidence to support the idea that matter is made up of atoms. J. J. Thomson discovered that atoms contain smaller particles called electrons, which exist in all types of atoms. Since atoms are neutral overall, they must also have an equal amount of positive charge to balance the negative charge of the electrons. The big question that arose was how these positive charges and electrons are arranged within the atom.

Think of an atom like a tiny solar system. Just like the solar system has a sun at its center with planets orbiting around it, an atom has a nucleus (which contains positively charged protons and usually neutral neutrons) at its center, with electrons moving around it.

The first model of atom was proposed by J. J. Thomson in 1898. According to this model, the positive charge of the atom is uniformly distributed throughout the volume of the atom and the negatively charged electrons are embedded in it like seeds in a watermelon. This model was picturesquely called the plum pudding model of the atom.

Thomson's model hypothesized that an atom is more like a 'plum pudding', where the positive charge is spread out, and the electrons (the 'plums') are embedded within this positive cloud. This was a significant development at the time as it proposed that atoms contain smaller subatomic particles.

Imagine a watermelon where the green rind represents the positive charge of the atom, and the red fruit represents the electrons. Just as the seeds in the watermelon are distributed throughout the fruit, electrons are thought to be embedded throughout the positive charge of the atom.

However subsequent studies on atoms, as described in this chapter, showed that the distribution of the electrons and positive charges are very different from that proposed in this model.

While Thomson's model was a good start, it didn't accurately describe how electrons and positive charges are actually arranged in an atom. Further experiments revealed that there is more structure to the atom than just a diffuse positive charge with electrons scattered throughout.

If we think of Thomson's model like a cake with frosting where the frosting is spread evenly, modern science suggests that the frosting actually forms a specific pattern on top of the cake, rather than being spread evenly across the whole thing.

This experiment was later performed around 1911 by Hans Geiger and Ernst Marsden. The details are discussed in Section 12.2. The explanation of the results led to the birth of Rutherford’s planetary model of atom. According to this the entire positive charge and most of the mass of the atom is concentrated in a small volume called the nucleus with electrons revolving around the nucleus just as planets revolve around the sun.

Ernst Rutherford's experiments in 1911 demonstrated that most of an atom's mass and positive charge is concentrated in a small region called the nucleus, with electrons moving in orbits around it, similar to how planets orbit the sun. This replaced the previous models and provided a clearer picture of atomic structure.

This model is akin to how the solar system works, where the sun acts as a central mass, and the planets, representing electrons, revolve around it in defined paths.

However, it could not explain why atoms emit light of only discrete wavelengths. How could an atom as simple as hydrogen, consisting of a single electron and a single proton, emit a complex spectrum of specific wavelengths?

Despite its successes, Rutherford's model struggled to explain some phenomena, particularly why atoms emit light only at certain wavelengths rather than a continuous range. This indicated that more refined theories were needed to address these shortcomings.

Imagine a musical instrument that produces specific notes. Just as the instrument can't play random notes, atoms can only emit specific wavelengths of light, suggesting they have certain defined energy levels.

In summary, Thomson's and Rutherford's models laid the groundwork for the atomic theory but revealed the need for further development, which was later addressed by Bohr's model and quantum mechanics.

These early atomic models, while groundbreaking, highlighted the complexity of atomic structure. It became clear that a deeper understanding of electron behavior and energy levels was needed, which was later developed into quantum mechanics by Niels Bohr.

Just like early attempts at flight by humans paved the way for modern aviation technology, early atomic models set the stage for the advanced theories that explain atomic behavior today.

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