structure of atom

• Atom is the smallest indivisible particle of matter, made of electron, proton, and neutrons
• Electrons were discovered using the cathode ray discharge tube experiment
• Nucleus was discovered by Rutherford in 1911
• Charge to mass ratio of an electron was determined by Thomson as 1.758820 x 10^11 C kg^-1
• Charge on an electron was determined by Millikan as -1.6 x 10^-19C
• Neutrons were discovered by James Chadwick by bombarding a thin sheet of beryllium with α-particles
• Atomic number (Z) is the number of protons present in the nucleus
• Mass Number (A) is the sum of the number of protons and neutrons present in the nucleus
• Rutherford's model of the atom explained that the atom consists of a nucleus with protons and neutrons, around which electrons revolve in fixed orbits
• Isotopes are atoms of the same element with the same atomic number but different mass numbers
• Isobars are atoms of different elements with the same mass number but different atomic numbers
• Electromagnetic radiations are associated with electrical and magnetic fields
• Planck's Quantum Theory states that radiant energy is emitted or absorbed discontinuously in the form of small discrete packets of energy called 'quanta'
• A black body is an ideal body that emits and absorbs all frequencies
• The photoelectric effect is the ejection of electrons from the surface of a metal when light of suitable frequency strikes it
• The dual behavior of electromagnetic radiation refers to light possessing both particle and wave-like properties
• Dual behavior of electromagnetic radiation:

  • Light possesses both particle and wave-like properties
  • Interacts with matter displaying particle-like properties (e.g., black body radiation and photoelectric effect)
• When white light passes through a prism, it splits into colored bands known as a spectrum
• Spectrum types:

  • Continuous spectrum: consists of all wavelengths
  • Line spectrum: only specific wavelengths are present, with bright lines and dark spaces between them
• Electromagnetic spectrum:

  • Continuous spectrum ranging from radio waves to gamma rays
  • Arranged in order of increasing wavelengths or decreasing frequencies
• Emission spectrum: radiation emitted by a substance that has absorbed energy
• Absorption spectrum: obtained when radiation is passed through a sample of material, showing dark lines where certain wavelengths are absorbed
• Spectroscopy: the study of emission or absorption spectra
• Rydberg equation: R = Rydberg’s constant = 109677 cm^-1
• Bohr’s model for hydrogen atom:

  • Electrons move in fixed orbits around the nucleus
  • Orbits are concentric and have fixed energy levels
  • Transition between orbits emits or absorbs radiation
• Limitations of Bohr’s model:

  • Fails to account for finer details of the hydrogen spectrum
  • Unable to explain the spectrum of atoms with more than one electron
• Dual behavior of matter:

  • Matter exhibits both particle and wave nature
  • Proposed by de Broglie
• Heisenberg’s uncertainty principle:

  • Impossible to determine exact position and momentum of an electron simultaneously
  • Product of uncertainties is always equal to or greater than h/4π
• Quantum mechanics:

  • Deals with motions of microscopic objects with observable wave-like and particle-like properties
  • Based on Schrodinger’s equation
• Schrodinger’s equation:

  • Describes the possible energy levels and corresponding wave functions of electrons in an atom
  • Only certain permitted solutions exist, corresponding to definite energy states
• Orbital:

  • Region around the nucleus where the probability of finding an electron is maximum
• Quantum numbers:

  • Set of four quantum numbers specifying energy, size, shape, and orientation of an orbital
  • Principal quantum number (n), Azimuthal quantum number (l), Magnetic quantum number (ml), Electron spin quantum number (ms)
• Aufbau Principle:

  • Orbitals are filled in order of increasing energies in the ground state of atoms
• Pauli Exclusion Principle:

  • No two electrons in an atom can have the same set of four quantum numbers
  • Only two electrons may exist in the same orbital with opposite spins
• Hund’s rule of maximum multiplicity:

  • Pairing of electrons in the same subshell occurs only after each orbital has one electron
• Electronic configuration of atoms:

  • Arrangement of electrons in different orbitals of an atom
  • Represented in s, p, d, f notation or orbital diagrams
• Electronic configuration of atoms: Arrangement of electrons in different orbitals of an atom can be represented in two ways:

  • spdf notation
  • Orbital diagram: each orbital of the subshell is represented by a box and the electron is represented by an arrow (↑) for a positive spin or an arrow (↓) for a negative spin
• Stability of completely filled and half filled subshells:

  • Symmetrical distribution of electrons in completely filled or half filled sub-shells makes them more stable
  • Exchange energy: when two or more electrons with the same spin are present in the degenerate orbitals of a sub-shell, they can exchange positions, releasing energy called exchange energy. This is maximum when the subshell is either half filled or completely filled, increasing stability
• One mark questions:

  • Neutrons can be found in all atomic nuclei except in the case of the hydrogen atom, which consists of only one proton
  • Wave number of yellow radiations with a wavelength of 5800 Å is 1.72 x 10^6 m^-1
  • The values of n and l for a 2p orbital are n=2 and l=1
  • Not possible orbitals: 1p and 3f
  • Electronic configuration of the element with atomic number 24 is 1s2 2s2 2p6 3s2 3p6 3d5 4s1
• Two marks questions:

  • Complete symbol for Z=17, A=35 is 35Cl
  • Orbital descriptions:
  • n=1, l=0 is 1s
  • n=3, l=1 is 3p
  • n=4, l=2 is 4d
  • n=4, l=3 is 4f