12.1. INTRODUCTION

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 (1856 – 1940) 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.

By the 19th century, scientists had gathered sufficient evidence to support the idea that matter is made up of individual units called atoms. J.J. Thomson conducted groundbreaking experiments in 1897 that showed all atoms contain electrons, which are negatively charged particles. Importantly, while atoms have these negatively charged electrons, they are overall neutral, meaning they also contain a positive charge that balances out the negative charges.

Think of an atom like a tiny battery. Just as a battery has both a positive and a negative terminal that allow it to function, an atom has both positive and negative charges. Without the positive charge, the atom wouldn’t be neutral and would not be able to exist as it does.

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 plum pudding model of the atom.

In 1898, J.J. Thomson introduced the 'plum pudding model' of the atom. In this model, he suggested that the atom consists of a positively charged medium or 'pudding', with negatively charged electrons scattered within it, similar to how seeds are embedded in a watermelon. This was one of the earliest attempts to describe the internal structure of atoms.

Imagine a chocolate chip cookie, where the cookie dough represents the positive charge of the atom and the chocolate chips are the electrons. Just as the chocolate chips are evenly distributed throughout the dough, in Thomson's model, electrons are spread out in the positive material 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 plum pudding model was a significant step in atomic theory, later experiments demonstrated that the structure and arrangement of particles within an atom were more complex. As research progressed, scientists found that electrons were not embedded in a uniform positive charge but were instead orbiting a dense positive nucleus.

If the plum pudding model is like a chocolate chip cookie, later discoveries showed that atoms are more like a solar system, where planets (electrons) orbit around a sun (the nucleus) rather than being embedded within a uniform mass.

We know that condensed matter (solids and liquids) and dense gases at all temperatures emit electromagnetic radiation in which a continuous distribution of several wavelengths is present, though with different intensities.

Condensed matter, which includes solids and liquids, and gases emit electromagnetic radiation across a spectrum of wavelengths. This radiation doesn’t just come in fixed sizes; rather, it varies continuously in wavelength and intensity based on the material's temperature and properties.

Think of this like a concert hall where different instruments play. Different instruments (types of matter) contribute sounds (wavelengths) that mix together to create a continuous sound which can change in volume (intensity) based on how loudly each instrument plays.

In contrast, light emitted from rarefied gases heated in a flame, or excited electrically in a glow tube such as the familiar neon sign or mercury vapour light has only certain discrete wavelengths. The spectrum appears as a series of bright lines. In such gases, the average spacing between atoms is large.

Gases under certain conditions, like those in neon signs, emit light only at specific wavelengths instead of a continuous spectrum. This results in a series of distinct lines in the spectrum, which correspond to the specific energies associated with electronic transitions within the gas atoms.

You can compare this to a keyboard where each key represents a specific note. When you press the keys, some produce music (light emissions), but they only produce certain notes depending on which keys are pressed (reflecting specific energies), similar to how gases emit specific wavelengths.

In the early nineteenth century it was also established that each element is associated with a characteristic spectrum of radiation, for example, hydrogen always gives a set of lines with fixed relative position between the lines. This fact suggested an intimate relationship between the internal structure of an atom and the spectrum of radiation emitted by it.

Every element has a unique 'fingerprint' in the form of its emission spectrum. For hydrogen, the emission spectrum consists of specific lines that appear consistently, indicating a clear correlation between its atomic structure and the wavelengths of light it emits.

Think of it like a bar code that uniquely identifies products. Just as each product has its unique barcode that reveals its specific details, each element's unique spectra indicate its internal atomic structure.

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