Quantum Physics, Atomic Models, and Chemical Bonds

Posted on Feb 6, 2025 in Chemistry

Quantum Physics and Atomic Models

Max Planck stated that bodies emit and absorb energy in discrete packets called energy quanta: E = hv.

Hertz observed that when metal electrodes are illuminated with ultraviolet light, they discharge. The emission of electrons from certain metals when exposed to light of a specific wavelength is called the photoelectric effect.

Bohr Model

  1. Electrons revolve around the nucleus in specific circular orbits without losing energy. These orbits are called stationary orbits.
  2. Only certain orbits are allowed, corresponding to discrete energy levels. These are called energy levels.
  3. The farther the energy level from the nucleus, the greater its energy.
  4. An electron can jump to a higher energy level by absorbing energy and return to a lower level by emitting radiation.

Problems: This model could not explain the spectrum of hydrogen when spectroscopes with high resolving power revealed more complex spectra, especially for larger compounds.

Arnold Sommerfeld: Proposed that electron orbits could be elliptical.

Pieter Zeeman: Observed that spectral lines split when the spectrum is subjected to a magnetic field, suggesting that orbits can adopt different orientations in space.

Heisenberg Uncertainty Principle: It is impossible to simultaneously know the exact position and velocity of an electron. Any attempt to measure an electron’s position would alter its momentum, and vice versa.

Orbital: The region of space where there is the highest probability of finding an electron.

Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. Therefore, each atomic orbital can hold a maximum of two electrons with opposite spins.

Electronic Configurations

  • Aufbau Principle (Rule of Lowest Energy): Electrons first fill the lowest energy orbitals, following the Aufbau principle or using a Moeller diagram.
  • Pauli Exclusion Principle: No two electrons in a single orbital can have the same spin.
  • Hund’s Rule of Maximum Multiplicity: When filling degenerate orbitals (orbitals of equal energy), electrons will individually occupy each orbital before any orbital is doubly occupied. Electrons in singly occupied orbitals will have the same spin.

Periodic Table and Chemical Bonding

Johann Döbereiner: Grouped elements into triads based on similar properties, where the atomic mass of the central element was approximately the average of the other two.

John Newlands: Arranged elements in vertical columns of seven based on increasing atomic weight.

Dmitri Mendeleev: Arranged elements in order of increasing atomic mass, leaving gaps for undiscovered elements and predicting their properties. He broke the order in a few cases (e.g., tellurium and iodine) to maintain similar chemical properties within groups.

Mendeleev’s Periodic Law: The physical and chemical properties of elements, as well as their compounds, are periodic functions of their atomic masses.

Periodic Trends

Ionization Energy: The energy required to remove an electron from an isolated atom in its gaseous state. A + IE → A+ + e–. Increases from left to right and from bottom to top on the periodic table.

Electron Affinity: The change in energy when an electron is added to a neutral atom in the gaseous phase to form a negative ion. X + e– → X– + EA. Increases from left to right and from bottom to top on the periodic table.

Electronegativity: The tendency of an atom to attract a bonding pair of electrons. Increases from left to right and from bottom to top on the periodic table.

Chemical Bonds

Ionic Bond

  1. Ionic compounds are not molecules but rather extended lattices of ions.
  2. The chemical formula of an ionic compound is an empirical formula that reflects the ratio of ions in the crystal lattice.

Properties:

  • Crystalline solids at room temperature.
  • High melting and boiling points due to strong electrostatic forces.
  • Hard and brittle. Hardness refers to a material’s resistance to scratching.
  • Many are soluble in polar solvents like water but insoluble in nonpolar solvents.
  • Do not conduct electricity in the solid state but do conduct in solution.

Covalent Bond

Molecular Covalent Compounds Properties:

  1. Exist as gases, liquids (H2O), or solids (iodine, sulfur) at room temperature and pressure.
  2. Low melting and boiling points.
  3. Do not conduct electricity.
  4. Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.

Crystalline Covalent Compounds Properties:

  1. High melting and boiling points.
  2. Hard.
  3. Insoluble.
  4. Do not conduct electricity or heat (except graphite).
  • Diamond: Cubic crystal system, very hard, colorless (color due to impurities).
  • Graphite: Opaque black plates, hexagonal structure, conducts electricity.
  • Silica: High melting point (1710 °C).

Metallic Bond

Metal atoms release their valence electrons into a “sea” of electrons.

Properties:

  1. Most are solid metals (except mercury, which is liquid).
  2. High densities, melting points, and boiling points.
  3. Metallic luster.
  4. Ductile and malleable.
  5. Good conductors of electricity and heat.
  6. Insoluble in ordinary solvents.