2.1. Atomic Structure: Atomic Models

Atomic structure is one of the fundamental concepts in chemistry and is crucial to understanding many other topics. This chapter of our e-book will focus on atomic models, which are theoretical representations of how atoms are organized and how they work.

The first atomic model was proposed by John Dalton in the early 19th century. According to Dalton's model, atoms are indivisible particles that make up matter. Each element is made up of identical atoms, and chemical reactions occur when atoms are rearranged. This model, also known as the 'billiard ball', was the basis for the development of modern atomic theory.

However, Dalton's model was challenged by subsequent experiments. J.J. Thomson, through his experiments with cathode rays, discovered the existence of electrons, subatomic particles with a negative charge. This led to the development of the plum pudding model, where electrons are embedded in a sphere of positive matter.

Despite its popularity, the plum pudding model was quickly replaced by the Rutherford model. Ernest Rutherford, through his famous alpha scattering experiment, proposed that most of the atom is empty space, with a small, dense central nucleus containing all the positive charges (protons) and most of the mass of the atom. Electrons, on the other hand, move in orbits around this nucleus.

While Rutherford's model was a great advance, it had its flaws. He could not explain why electrons did not fall into the nucleus due to electrostatic attraction, nor could he explain the spectral lines seen in atomic spectra. These questions were resolved by Niels Bohr, who proposed that electrons occupy specific energy levels, or shells, around the nucleus. When an electron jumps from one energy level to another, it emits or absorbs energy in the form of light, resulting in spectral lines.

Finally, the modern atomic model, or electron cloud model, was developed. This model, based on quantum theory, describes electrons as a probability cloud around the nucleus, rather than particles in specific orbits. The exact position of an electron at a given time cannot be determined, only the probability of finding the electron in a given region of space.

Atomic models are a perfect example of how science evolves with new evidence and discoveries. Each model was an improvement over the last, adding detail and precision to our understanding of atoms. For the National High School Examination (ENEM), it is important to understand each of these models, their characteristics and limitations, as well as the historical context in which they were developed.

In summary, atomic structure is a fascinating and complex topic that lies at the heart of chemistry. We hope that this chapter of our e-book will help you to better understand this topic and to prepare for the ENEM.

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4Atomic Structure: Subatomic Particles

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