Chap#2

• Atom


  The smallest particle that has all the properties of an element
• Greek atom


  Democritus proposed that everything in the universe is composed of indivisible and indestructible particles called "atoms," from the Greek word "atomon," meaning "uncuttable" or "indivisible"
• Democritus' ideas were influential, but they were largely philosophical and lacked experimental evidence
• It wasn't until much later, in the 19th and 20th centuries, that experimental evidence and scientific theories, particularly those developed by scientists like John Dalton, J.J. Thomson, Ernest Rutherford, and Niels Bohr, provided a more robust understanding of the atom, leading to the development of modern atomic theory
• Dalton atom


  John Dalton, an English chemist and physicist, is known for his pioneering work in the development of modern atomic theory
• Dalton's atomic theory


  1. Elements are composed of atoms

  2. Atoms of the same element are identical

  3. Atoms combine in whole-number ratios

  4. Chemical reactions involve the rearrangement of atoms
• Dalton's model of the atom was a significant advancement in chemistry and laid the groundwork for further scientific inquiry into the nature of matter
• Later discoveries and advancements in atomic physics revealed that atoms are not indivisible, as Dalton initially proposed, but are composed of even smaller subatomic particles, such as protons, neutrons, and electrons
• Thomson atom

  J.J. Thomson's model of the atom, proposed in the late 19th century, is commonly known as the "plum pudding model"
• Thomson's plum pudding model


  1. Positive sphere

  2. Electrons embedded within the positive sphere

  3. Charge neutrality

  4. Homogeneity
• Thomson's plum pudding model was later replaced by more accurate models as experimental evidence, particularly from Ernest Rutherford's gold foil experiment, revealed the presence of a concentrated nucleus within the atom
• Nonetheless, Thomson's plum pudding model was crucial in advancing the understanding of atomic structure and paved the way for subsequent atomic models
• Bohr atom


  The Bohr model of the atom, proposed by Niels Bohr in 1913
• Bohr's atomic model


  1. Electrons orbit the nucleus in fixed, quantized orbits at specific energy levels

  2. Electrons can only occupy certain discrete energy levels, corresponding to specific orbits, and cannot exist between these levels

  3. Electrons do not emit radiation while in these stable orbits

  4. Electrons can move between energy levels by absorbing or emitting photons of specific energies

  5. The nucleus contains protons and neutrons, with electrons orbiting around it
• Our current understanding of the atom largely reflects Niels Bohr's model proposed a century ago
• Advances in high-energy particle accelerators have enabled physicists to map and identify the structure of the atomic nucleus
• Over 100 subatomic particles have been detected and characterized through these experiments
• Nuclear structure is now well-defined, with nucleons—protons and neutrons—composed of quarks held together by gluons
• For radiologic science, only the three primary constituents of an atom—electrons, protons, and neutrons—are considered significant
• Atom


  The atom can be likened to a miniature solar system, with the nucleus acting as the sun and electrons as the planets
• Electrons


  Incredibly small particles carrying a single unit of negative electric charge, with a mass of approximately 9.1 × 10^-31 kg
• Nucleons


  Protons and neutrons, which are significantly more massive than electrons, with nearly 2000 times the mass of an electron
• Proton


  Carries a single unit of positive electric charge
• Neutron


  Electrically neutral, carrying no charge
• Atomic structure

  Possible electron orbits are grouped into different "shells"
• Neutral atom


  Has the same number of electrons in orbit as protons in the nucleus
• Ionization


  The removal of an orbital electron from an atom
• Electron arrangement


  The number of electrons in the outermost shell is equal to its group in the periodic table, which determines the valence of an atom
• Electron arrangement

  The number of the outermost electron shell is equal to its period in the periodic table
• Electron binding energy


  The strength of attachment of an electron to the nucleus
• Centripetal force


  The force that keeps an electron in orbit
• Centrifugal force


  The force created by the electron velocity, which balances the centripetal force in a normal atom
• Isotopes


  Atoms that have the same atomic number but different atomic mass
• Isobars


  Atomic nuclei that have the same atomic mass number but different atomic numbers
• Isotones


  Atoms that have the same number of neutrons but different numbers of protons
• Isomers


  Atoms that have the same atomic number and the same atomic mass number
• Molecule


  Atoms of various elements may combine to form structures called molecules
• Compound


  A chemical compound is any quantity of one type of molecule or molecules
• Radioactivity


  The spontaneous emission of particles and energy in order to become stable
• Many radioisotopes of barium have been discovered, all of which are artificially produced