STRUCTURE OF ATOM
Science
INTRODUCTION
• Atom is the smallest unit of matter that retains the chemical properties of an element.
• The word atom is derived from Greek word ‘atomos’ meaning indivisible.
• Modern science has shown that atoms are divisible and consist of subatomic particles.
SUBATOMIC PARTICLES
• Atom is mainly composed of three subatomic particles:
– Electron
– Proton
– Neutron
ELECTRON
• Discovered by J.J. Thomson in 1897.
• Negatively charged particle.
• Mass is very small (1/1836 times the mass of proton).
• Revolves around the nucleus in definite energy levels or shells.
• Responsible for chemical bonding and electricity.
PROTON
• Discovered by Ernest Rutherford.
• Positively charged particle.
• Present in the nucleus of an atom.
• Mass approximately equal to that of neutron.
• Determines the atomic number of an element.
NEUTRON
• Discovered by James Chadwick in 1932.
• Electrically neutral particle.
• Present in the nucleus.
• Responsible for isotopes and nuclear stability.
ATOMIC NUMBER
• Atomic number (Z) is the number of protons present in the nucleus.
• It uniquely identifies an element.
• Example: Atomic number of Hydrogen = 1.
MASS NUMBER
• Mass number (A) is the sum of protons and neutrons.
• A = Protons + Neutrons.
• Example: Carbon-12 has 6 protons and 6 neutrons.
ISOTOPES
• Atoms of the same element having same atomic number but different mass numbers.
• Example: Hydrogen has three isotopes – Protium, Deuterium, Tritium.
• Used in medicine, archaeology, nuclear power.
ISOBARS
• Atoms of different elements having same mass number but different atomic numbers.
• Example: Argon-40 and Calcium-40.
EARLY ATOMIC MODELS
DALTON’S ATOMIC THEORY
• Proposed by John Dalton.
• Atoms are indivisible particles.
• Atoms of same element are identical.
• Failed to explain isotopes and subatomic particles.
THOMSON’S PLUM PUDDING MODEL
• Proposed by J.J. Thomson.
• Atom is a positively charged sphere with electrons embedded in it.
• Failed to explain scattering experiment.
RUTHERFORD’S NUCLEAR MODEL
• Proposed after alpha particle scattering experiment.
• Atom has a small, dense, positively charged nucleus.
• Electrons revolve around nucleus.
• Failed to explain stability of atom.
BOHR’S ATOMIC MODEL
• Proposed by Niels Bohr.
• Electrons revolve in fixed circular orbits called shells.
• Each orbit has fixed energy.
• Energy is emitted or absorbed when electrons jump between orbits.
• Successfully explained hydrogen spectrum.
LIMITATIONS OF BOHR MODEL
• Could not explain spectra of multi-electron atoms.
• Could not explain Zeeman and Stark effects.
MODERN ATOMIC MODEL (QUANTUM MECHANICAL MODEL)
• Based on wave-particle duality of electrons.
• Exact position of electron cannot be determined.
• Uses probability concept.
• Introduced orbitals instead of orbits.
ELECTRONIC CONFIGURATION
• Distribution of electrons in different shells.
• Maximum electrons in shell = 2n².
• Shells are K, L, M, N etc.
• Valence electrons determine chemical properties.
VALENCY
• Combining capacity of an element.
• Determined by number of electrons lost, gained or shared.
• Noble gases have zero valency.
SIGNIFICANCE OF ATOMIC STRUCTURE
• Helps in understanding chemical bonding.
• Basis of modern chemistry and physics.
• Applications in nuclear energy, medicine and electronics.
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