An atom is the smallest unit of an element, derived from the Greek word atomos, meaning indivisible. It consists of three main particles: protons and neutrons, which are found in the nucleus, and electrons, which orbit around the nucleus. In a neutral atom, the number of protons equals the number of electrons, and the number of neutrons is usually similar to the number of protons. The protons and neutrons are held tightly in the nucleus by a powerful force called the strong nuclear force. Our understanding of atomic structure has evolved over time. Several atomic models were proposed and later discarded as new discoveries were made. In this section, we’ll explore the key models that helped shape the modern view of the atom.

Dalton's Model of Atoms

John Dalton was one of the first scientists to introduce a clear theory about the nature of matter. According to his model, all matter is made up of tiny, indivisible particles called atoms, which cannot be created, destroyed, or divided through chemical reactions. He also proposed that all atoms of a given element are identical in mass and chemical properties, while atoms of different elements have different masses and properties. Dalton’s model was a major milestone in understanding atomic structure and laid the foundation for modern chemistry. However, later discoveries- such as the existence of subatomic particles like electrons, protons, and neutrons– revealed that atoms are actually divisible, highlighting some limitations of his theory.

Drawbacks of Dalton’s Theory:

• Dalton said that the atoms of the same element are similar in all respects and the atoms of different elements are different in all respects.
• Various other scientists discovered that atoms were made of electron protons and neutrons.

JJ Thomson Model of Atom

J.J. Thomson proposed a model of the atom in which he compared it to a christmas pudding. According to him, the atom is made up of a positively charged sphere, and the negatively charged electrons are scattered throughout it, like dry fruits in a pudding. He also explained that the positive and negative charges are equal in amount, which makes the atom electrically neutral overall. This model was one of the first attempts to describe the internal structure of the atom.

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Drawbacks of J.J Thomson’s Model of Atom

• It failed to explain how the positive charge holds on the electron inside the atom.
• It also failed to explain the reasons for the stability of an atom.

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Rutherford’s Atomic Model

Rutherford’s Atomic Model was a major advancement in understanding the structure of the atom. He proposed that the atom consist of a small, dense, positively charged nucleus at its centre, where most of the atom’s mass is concentrated. Electrons revolve around this nucleus in the empty space, and the atom as a whole remains electrically neutral.

The model was based on his gold foil experiment, where alpha particles were directed at a thin sheet of gold foil inside a vacuum chamber. Most particles passed straight through, some were deflected slightly, and a few bounced back at large angles. These observations showed that atoms are mostly empty space, and the positive charge is concentrated in a small central region of the nucleus. This experiment disproved earlier atomic models and laid the foundation for the modern understanding of atomic structure.

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Neil Bohr introduced a new atomic model by using the concept of quantisation. He explained that electrons revolve around the nucleus in a fixed circular path or orbits, without losing energy as they move. According to Bohr, electrons can jump from one energy level to another when an electron moves from a lower energy level to a higher one, it absorbs energy, and when it returns to a lower level, it releases energy. The lowest energy level that an electron can occupy is called the ground state. Bohr also stated that these energy levels are represented by numbers called quantum numbers, which start from n= 1, 2, 3, and so on, and are named as K, L, M, N shells respectively. This model helped explain the stability of atoms and how they emit light in specific patterns.

Drawbacks Neil Bohr’s Theory: 

• Bohr’s model was not able to define the effect of magnetic field and electric field on the spectra of atoms.
• The Bohr atomic model made correct predictions for smaller-sized atoms like hydrogen, but poor spectral predictions were obtained when larger atoms are considered.

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