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Atoms are the smallest particle of a chemical element that can exist.

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Atoms are tiny particles which make up everything around us. The term atom is derived from the Greek word which means indivisible. The scientific knowledge about the structure of the atom was developed over a long period of time. Following is a brief historical overview of the atomic theory.

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An atom consists of a central nucleus made up of protons and neutrons, with electrons orbiting around it. Protons have a positive charge, neutrons are neutral, and electrons have a negative charge. The number of protons in the nucleus determines the element, while the number of electrons balances the atom's charge in a neutral state.

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1. Ancient Greece (5th Century BCE)

Democritus: Proposed the idea that matter is made up of tiny, indivisible particles called atoms.

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Around 400 B.C., Greek philosophers Leucippus and Democritus suggested that everything is made of tiny, indivisible particles called atoms and empty space. They thought different types of atoms made up different materials, but they had no proof for their ideas. In contrast, Aristotle believed matter was continuous and could be divided forever. He thought everything was made from four elements: water, air, fire, and earth. Aristotle's ideas were more widely accepted and lasted for many years, while the atomic theory was not proven at that time.

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Proposed the first scientific atomic theory:

Matter is made of atoms.

Atoms of the same element are the same.

Atoms combine in simple ratios to form compounds.

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Discovered the electron using cathode ray experiments.

Proposed the plum pudding model, where electrons are spread throughout a positive sphere.

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Discovered the nucleus through the gold foil experiment.

Proposed that atoms have a small, dense nucleus with electrons around it.

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Atoms are commonly represented with a nucleus of protons and neutrons at the center, surrounded by orbiting electrons. In simpler models, like the Bohr model, electrons are shown in circular orbits, while modern models depict electrons in probabilistic "clouds" around the nucleus. These visualizations help us understand atomic structure.

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Suggested that electrons move in fixed orbits (energy levels) around the nucleus.

Explained why atoms give off light in specific colors (spectral lines).

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Schrödinger & Heisenberg: Explained that electrons don't follow fixed orbits but exist in probable regions called orbitals.

Introduced the idea that electron positions can’t be exactly determined (Heisenberg Uncertainty Principle).

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Discovered the neutron, a neutral particle in the nucleus, completing the basic structure of the atom.

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The 2,400-year search for the atom - Theresa Doud