Atomic Structure and Chemical Reactions

Atomic Structure

Dalton’s Atomic Theory and Its Demise

Dalton’s atomic theory proposed that matter consists of indivisible particles called atoms. However, the discovery of the electron, a subatomic particle, challenged this theory.

Cathode Rays and the Electron

Cathode rays exhibit specific properties:

  • They travel in a straight line from the cathode to the anode, producing fluorescence upon hitting the tube walls.
  • They behave like a negatively charged electric current, deflecting towards the positive plate in an external electric field.
  • Their properties are independent of the gas within the tube.
  • They possess high kinetic energy, capable of spinning a small wheel placed in their path, suggesting they are not light radiation but particles.

The electron is the smallest electrically charged particle found in nature.

Thomson’s Atomic Model

Between 1898 and 1904, Thomson, recognizing the electron as a fundamental component of matter, developed an atomic model. He envisioned the atom as a positively charged material containing small granules of electrons. These electrons are distributed evenly in sufficient numbers to maintain electrical neutrality and stability.

Canal Rays and the Proton

Canal rays are positive ions whose composition depends on the gas in the tube. If the gas is hydrogen, the resulting positive ion (H+) is another elementary particle: the proton, named by Rutherford in 1914.

Rutherford’s Atomic Model

Rutherford’s model describes the atom as having a central nucleus containing almost all the mass and the entire positive charge. Electrons orbit the nucleus at a considerable distance in circular paths, held by electrostatic attraction.

Ions

An atom becomes an ion by gaining or losing electrons, altering its neutral state. Gaining electrons forms an anion, while losing electrons forms a cation.

Electromagnetic Spectrum and Spectra

The electromagnetic spectrum encompasses all existing electromagnetic radiation, arranged by frequency from lowest to highest: radio waves, microwaves, infrared radiation, visible light, ultraviolet, X-rays, and gamma rays.

A spectrum arises from the interaction of electromagnetic radiation with matter. It represents the distribution or dispersion of the intensity of electromagnetic radiation as a function of its wavelength or frequency.

Bohr’s Atomic Model

Bohr’s model postulates that electrons orbit the nucleus in specific circular orbits. Only certain orbits are permissible. When an electron transitions between orbits, energy is exchanged as a photon, described by Planck’s equation: E = hv.

Modern Atomic Model

The currently accepted atomic model describes electrons residing in orbitals, regions around the nucleus where they move without a defined path, occupying different energy levels or layers.

Periodic Table and Periodic Properties

The periodic table organizes chemical elements in order of increasing atomic number.

Ionization energy is the energy required to remove an electron completely from a gaseous atom.

Electron affinity is the energy change accompanying the addition of an electron to a gaseous atom.

Electronegativity is the tendency of an atom to attract electrons when part of a chemical bond.

Metallic character is the tendency of an atom to lose electrons.

Chemical Reactions

Synthesis Reactions

In synthesis reactions, reactants combine to form a single product: A + B → C.

Decomposition Reactions

Decomposition reactions involve two types:

  • Simple decomposition: A substance breaks down into two or more products (A → B + C).
  • Reactive decomposition: A substance AB reacts with C to produce AC and BC (AB + C → AC + BC).

Substitution or Displacement Reactions

In substitution reactions, one or more atoms in a compound are replaced by atoms from another compound: AB + C → AC + B. The more reactive element displaces the less reactive one.

Double Substitution Reactions

Double substitution reactions involve an exchange of atoms or groups of atoms between the reactants: AB + CD → AC + BD.