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Atomic Structure and Isotopes
Atomic Structure
Atoms are the smallest particles of matter that retain the properties of an element. They are made up of three main
particles:
● Protons: Positively charged particles found in the nucleus.
● Neutrons: Neutral particles found in the nucleus.
● Electrons: Negatively charged particles orbiting the nucleus.
The number of protons in an atom determines its atomic number, which identifies the element. The total
number of protons and neutrons in an atom is called its mass number.
Isotopes
Isotopes are atoms of the same element that have different numbers of neutrons. This means They
have the same atomic number but different mass numbers. For example, carbon-12, carbon-13, and
carbon-14 are all isotopes of carbon.
Isotopes of the same element have similar chemical properties but may have different physical properties due to the
difference in mass. Some isotopes are radioactive, meaning they emit radiation as they decay.
History of the Atom
19th Century
● John Dalton (1766-1844): Proposed the atomic theory, suggesting that elements are composed of tiny,
indivisible particles called atoms.
● J.J. Thomson (1856-1940): Discovered the electron, a negatively charged particle found within atoms.
● Ernest Rutherford (1871-1937): Proposed the nuclear model, suggesting that most of the atom's mass is
concentrated in a small, positively charged nucleus.
● Niels Bohr (1885-1962): Developed the Bohr model, which described electrons as orbiting the nucleus in
specific energy levels.
20th Century
● Quantum Mechanics: Revolutionized understanding of atomic structure, describing electrons as wave-like
particles.
● Subatomic Particles: Discovery of protons and neutrons in the nucleus, and other particles like quarks and
leptons
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