Dalton’s Atomic Theory and the Structure of Matter

Posted on Nov 4, 2024 in Chemistry

Dalton’s Atomic Theory

Dalton’s hypothesis was based on the following premises:

  • Elements are made of atoms, which are independent material particles, unchanging and indivisible.
  • Atoms of the same element are equal in mass and other properties.
  • Atoms of different elements have different masses and properties.
  • Compounds are formed by the joining of atoms of the corresponding elements based on a ratio of simple integers.

Definitions from Dalton’s Atomic Theory

  • An atom is the smallest particle of an element that retains its properties.
  • An element is a substance that consists of equal atoms.
  • A compound is a substance that is formed by different atoms combined in fixed proportions.

Structure of Matter

Pure Substances

A pure substance has a uniform composition and cannot be decomposed into other substances of different classes by physical methods.

  • Elements are pure substances that cannot be decomposed into simpler ones through normal chemical processes.
  • Compounds are pure substances consisting of two or more elements and can be decomposed into them by chemical methods (e.g., sucrose, water).

Discovery of Subatomic Particles

Electrolysis

Electrolysis is a process that uses electric current to produce chemical reactions. The passage of electric current through aqueous electrolyte solutions allows the formation of new substances.

  • The cathode is the negative electrode.
  • The anode is the positive electrode.

Rutherford Model

Rutherford developed a series of conclusions based on his experiments:

  • Matter is mostly empty space, as most particles pass through without deviation.
  • Some particles bounce back due to electrostatic repulsion when passing near positive charges. Since this rarely happens, these positive charges (protons) must occupy a very small space inside the atom called the nucleus. The nucleus is the positive part of the atom and contains most of its mass.
  • He postulated the existence of neutral particles (neutrons) in the nucleus to avoid instability by repulsion between protons.
  • He suggested that electrons must move around the nucleus. Their rotation compensates for the electrostatic force of attraction between charges of opposite signs, preventing them from collapsing into the nucleus.

Limitations of the Rutherford Model

  • It assumed that electrons revolve in orbits around the nucleus, subject to its electrical attraction. According to electromagnetic theory, this implies that electrons must constantly emit electromagnetic waves, losing kinetic energy. Eventually, they would fall into the nucleus, which does not happen.
  • This model cannot explain the absorption or emission bands of atomic spectra, as the energy of the electrons could take any value in the atom.

This model was called the planetary model because electrons orbit the nucleus like planets around the sun.

Definitions

  • Atomic Number (Z) indicates the number of protons in the nucleus of an atom. It coincides with the number of electrons if the atom is neutral.
  • Mass Number (A) indicates the number of protons plus neutrons in the atomic nucleus. It represents almost all the mass of a given atom.
  • Isotopes are atoms that have the same atomic number but different mass numbers.
  • Electromagnetic radiation consists of waves moving at the speed of light (C). This is related to the wavelength (λ) and frequency (ν): C = λν
  • Atomic mass of an element or isotope: atomic mass (element) = [A1 (%) + A2 (%) + A3 (%) + …] / 100
  • Wavelength (λ) is the distance between two successive maxima or two successive minima of a wave. λ = C / ν = C * T
  • Frequency (ν) is the number of oscillations that pass through each point in unit time. ν = 1 / T = C / λ
  • Period (T) is the time it takes the wave to travel one wavelength, i.e., the time needed to produce one oscillation. T = 1 / ν = λ / C
  • Wave Number (k) is the number of oscillations in each unit length. k = 1 / λ = ν / C
  • Planck’s constant (h): Planck assumed that the energy (E) of each quantum of light is related to the frequency (ν) of the radiation absorbed or emitted by E = hν, where h is Planck’s constant (h = 6.62 * 10-34 J s).
  • Electron volt (eV) is the work performed by an electron moving from one energy level to another.
  • Absorption spectra: When electromagnetic radiation passes through a gas, it will absorb part of the light. The resulting pattern is called the absorption spectrum.
  • Emission spectra: If we stimulate a substance in gaseous form using electric shocks, it can emit electromagnetic radiation. This radiation can be captured and analyzed to obtain its emission spectrum.
  • Electrovalence is the number of electrons gained or lost by an atom to form an ion. For example: Formula (e+ or e– represents the electrovalence).
  • Covalency is the number of unpaired electrons. For example: Br: 4s2 4p5 (s p) (covalency = 1)
  • Rydberg constant (R): R = 1.097 * 107 m-1 Formula

Chemical Bonds

  • Ionic bond: Formed between a metal and a nonmetal. The metal atom donates electrons to the nonmetal atom, forming ions that are held together by electrostatic attraction in a crystal lattice.
  • Covalent bond: Formed between nonmetals or between nonmetals and hydrogen. Atoms share electrons to achieve a stable electron configuration. They form molecules or crystal lattices.
  • Metallic bond: Formed between metal atoms. Electrons are delocalized and shared among all atoms in a crystal lattice, creating a”sea of electrons”

Electromagnetic Spectrum

The electromagnetic spectrum is the range of all types of electromagnetic radiation. It includes not only visible light but also microwaves, radio waves, infrared radiation, ultraviolet radiation, X-rays, and gamma rays.