Atoms

Atomic Structure for JEE/NEET

Atoms are the fundamental building blocks of all matter in the universe. They are incredibly small, with a typical size of about 100 picometers (one ten-billionth of a meter). Understanding the structure and behaviour of atoms is crucial to comprehending the nature of the physical world.

Historical Development

  • Ancient Greece: The concept of atoms as indivisible particles was first proposed by Democritus and Leucippus in the 5th century BC.
  • Dalton’s Atomic Theory (1803): John Dalton formulated the first modern atomic theory, stating that elements are composed of atoms, and atoms of different elements have different masses.
  • Discovery of Subatomic Particles: In the late 19th and early 20th centuries, experiments revealed that atoms are not indivisible but are composed of smaller particles: electrons, protons, and neutrons.

Structure of the Atom

  • Nucleus: The dense, positively charged core of the atom, containing protons and neutrons.
    • Protons: Positively charged particles. The number of protons determines the element.
    • Neutrons: Neutral particles with no charge.
  • Electron Cloud: The region surrounding the nucleus where electrons reside.
    • Electrons: Negatively charged particles that orbit the nucleus in shells or energy levels.

Atomic-Structure JEE & NEET Key Concepts

  • Atomic Number (Z): The number of protons in an atom’s nucleus. This defines the element.
  • Mass Number (A): The total number of protons and neutrons in the nucleus.
  • Isotopes: Atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons). 
  • Atomic Mass: The average mass of an atom of an element, taking into account the relative abundance of its isotopes.
  • Electron Configuration: The arrangement of electrons in the electron shells of an atom. This determines the chemical properties of the element.
  • Valence Electrons: The electrons in the outermost shell of an atom, involved in chemical bonding.

Models of the Atom

  • Thomson’s Plum Pudding Model: Proposed that atoms were a sphere of positive charge with embedded electrons.
  • Rutherford’s Nuclear Model: Based on his gold foil experiment, Rutherford proposed that atoms have a small, dense, positively charged nucleus with electrons orbiting around it.
  • Bohr Model: Introduced the concept of quantized energy levels for electrons, explaining the discrete lines in atomic spectra.
  • Quantum Mechanical Model: The current model of the atom, which describes electrons as existing in probability clouds or orbitals.

Atomic Spectra

  • Emission Spectrum: The spectrum of light emitted by an atom when its electrons transition from higher energy levels to lower energy levels.  
  • Absorption Spectrum: The spectrum of light absorbed by an atom when its electrons transition from lower energy levels to higher energy levels.

Applications of Atomic Physics

  • Nuclear Energy: Harnessing the energy released from nuclear reactions.
  • Medical Imaging: Using radioactive isotopes for diagnosis and treatment.
  • Materials Science: Understanding the properties of materials based on their atomic structure.
  • Electronics: Developing electronic devices based on the behavior of electrons in atoms.

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