Atomic Structure

• The rich diversity of chemical behavior of different elements can be traced to the differences in the internal structure of atoms of these elements
• The discovery of electron, proton, and neutron and their characteristics are key topics in the study of the structure of the atom
• Thomson, Rutherford, and Bohr atomic models are important to understand the structure of the atom
• The quantum mechanical model of the atom and the nature of electromagnetic radiation are essential concepts in atomic structure
• The photoelectric effect, atomic spectra, de Broglie relation, Heisenberg uncertainty principle, atomic orbitals, aufbau principle, Pauli exclusion principle, and Hund's rule of maximum multiplicity are all crucial aspects of atomic structure
• The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who believed that atoms are the fundamental building blocks of matter
• John Dalton's atomic theory, proposed in 1808, regarded the atom as the ultimate particle of matter and successfully explained the law of conservation of mass, law of constant composition, and law of multiple proportion
• Experimental observations made towards the end of the nineteenth and beginning of the twentieth century established that atoms are made of sub-atomic particles: electrons, protons, and neutrons
• The discovery of electrons was made through experiments on electrical discharge through gases, leading to the understanding that electrons are the basic constituents of all atoms
• In 1897, J.J. Thomson measured the ratio of electrical charge to the mass of the electron using a cathode ray tube, determining the charge to mass ratio of the electron
• R.A. Millikan's oil drop experiment determined the charge on the electron to be -1.6 × 10^-19 C
• The discovery of protons and neutrons came from electrical discharge in the modified cathode ray tube, leading to the identification of positively charged particles known as protons and electrically neutral particles called neutrons
• Chadwick's discovery of neutrons in 1932 was made by bombarding a thin sheet of beryllium with α-particles
• Before the discovery of sub-atomic particles, scientists aimed:

  • To account for the stability of the atom
  • To compare the behavior of elements in terms of physical and chemical properties
  • To explain the formation of different kinds of molecules by the combination of different atoms
  • To understand the origin and nature of the characteristics of electromagnetic radiation absorbed or emitted by atoms
• Millikan's Oil Drop Method:

  • Oil droplets in the form of mist were produced by an atomizer
  • Droplets entered through a tiny hole in the upper plate of an electrical condenser
  • The downward motion of droplets was viewed through a telescope with a micrometer eyepiece
  • By measuring the rate of fall of droplets, Millikan measured the mass of oil droplets
  • The air inside the chamber was ionized by passing X-rays through it
  • The electrical charge on oil droplets was acquired by collisions with gaseous ions
  • The fall of charged oil droplets could be retarded, accelerated, or made stationary depending on the charge and voltage applied
• Thomson Model of Atom (Plum Pudding Model):

  • Proposed by J.J. Thomson in 1898
  • Atom possesses a spherical shape with positive charge uniformly distributed
  • Electrons are embedded to give a stable electrostatic arrangement
  • Mass of the atom is assumed to be uniformly distributed
  • Not consistent with later experiments
• Rutherford's Nuclear Model of Atom:

  • Proposed by Ernest Rutherford
  • Most of the mass and positive charge of the atom concentrated in a small nucleus
  • Electrons move around the nucleus in circular paths called orbits
  • Resembles the solar system with the nucleus as the sun and electrons as planets
  • Held together by electrostatic forces of attraction
• Atomic Number and Mass Number:

  • Atomic number (Z) equals the number of protons in the nucleus
  • Number of electrons in a neutral atom is equal to the atomic number
  • Mass number (A) equals the number of protons plus the number of neutrons in the nucleus
• Isobars and Isotopes:

  • Isobars: Atoms with the same mass number but different atomic numbers
  • Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons
• Isobars are atoms with the same mass number but different atomic number
• Example of isobars: 6^14C and 7^14N
• Isotopes are atoms with identical atomic number but different atomic mass number
• The difference between isotopes is due to the presence of a different number of neutrons in the nucleus
• Example of isotopes:

  • Protium (1^1H) with 1 proton
  • Deuterium (1^2D) with 1 proton and 1 neutron
  • Tritium (1^3t) with 1 proton and 2 neutrons
• Chemical properties of atoms are controlled by the number of electrons, determined by the number of protons in the nucleus
• Number of neutrons present in the nucleus has very little effect on the chemical properties of an element
• All isotopes of a given element show the same chemical behavior
• In an isotope notation like A^Z X, determine if the species is a neutral atom, a cation, or an anion before using the notation
• For a neutral atom, the number of protons equals the number of electrons, which equals the atomic number
• For an ion, determine if the number of protons is larger (cation, positive ion) or smaller (anion, negative ion) than the number of electrons
• Number of neutrons is always given by A-Z, whether the species is neutral or an ion
• Electromagnetic radiation consists of different kinds of waves characterized by frequency (ν) and wavelength (λ)
• The SI unit for frequency (ν) is hertz (Hz, s–1), defined as the number of waves passing a given point in one second
• Wavelength is measured in meters (m), and smaller units are used for electromagnetic radiation due to their much smaller wavelengths
• In vacuum, all types of electromagnetic radiations travel at the speed of light, which is 3.0 × 10^8 m/s (2.997925 × 10^8 m/s)
• The frequency (ν), wavelength (λ), and velocity of light (c) are related by the equation c = ν λ
• The wavenumber is defined as the number of wavelengths per unit length and is commonly expressed in cm–1
• The wavelength of electromagnetic radiation can be calculated using the formula λ = c/ν
• The visible spectrum ranges from violet (400 nm) to red (750 nm) in terms of wavelength
• The frequency of violet light is 7.50 × 10^14 Hz, and the frequency of red light is 4.00 × 10^14 Hz