Dalton's Atomic Theory and its Limitations

• Atomic theory postulates: all matter consists of indivisible and indestructible atoms.
• Atoms of a particular element will have identical masses and properties.
• Compounds arise from combinations of two or more different kinds of atoms.
• Chemical reactions involve only the rearrangement of atoms.

Limitations of Dalton's Theory

• Atoms are divisible into protons, neutrons, and electrons, contradicting Dalton's initial postulate.
• Isotopes exist, meaning atoms of the same element have the same mass as Dalton stated.
• Isobars exist, meaning atoms of different elements can have the same mass, contrary to Dalton. Tellurium (atomic number 52) and iodine (atomic number 53) both have an atomic mass of 127
• Atoms are the smallest part of elements, molecules are the smallest part of compounds.

J.J. Thomson's Contribution

• J.J. Thomson (1856-1940) discovered the electron.
• Thomson's gifted teaching led to many Nobel Prizes won by his students and research assistants.
• Subatomic particles were discovered at this time.

Discovery of the Electron

• British physicist J.J. Thomson proved atoms aren't indivisible in 1897.
• Thomson used a high-voltage source with electrodes sealed in an evacuated glass tube known to have a cathode (negative electrode).
• The tube emitted a greenish light caused by the interaction of glass with cathode rays.
• Cathode rays consist of beams of negatively charged particles (electrons).
• The electrons are also constituents of all matter.

Plum Pudding Model

• Thomson's plum pudding model proposed that atoms have an overall neutral charge, with negative particles floating in a soup of diffuse positive charge.
• An atom has a positively charged sphere with embedded electrons, and is electrically neutral overall.
• The plum pudding model failed to explain how the positive charge holds electrons or an atom's stability.
• This model did not describe a nucelus.

Rutherford's Gold Foil Experiment and Idea of the Nucleus

• Rutherford projected alpha (α) particles at 180,000 miles per second onto gold foil in 1911.
• The alpha-scattering experiment showed that most alpha particles passed through, a few scattered at large angles, and some bounced back.

Rutherford's Atomic Model

• Most of the atomic space is empty.
• The mass of an atom is concentrated in the center of the nucleus that's positively charged, comprising 99.95% or more of atom mass.
• Electrons move around the nucleus.
• Total electrons = number of positively charged particles in the nucleus; therefore the atom is electrically neutral.
• The inward electrostatic attraction is balanced by the centrifugal force from electron rotation.

Limitations of Rutherford's Model

• Rutherford stated the electrons revolve in orbits. Maxwell stated accelerating charged particles emit electromagnetic radiation.
• An electron would lose energy and spiral into the nucleus in less than 10^-8 seconds.
• Rutherford's model did not account for electron arrangement, making the theory incomplete.
• He did not describe the shape of the orbits.

Max Planck

• Max Planck (1858-1947) rendering of services led to the advancement of physics for his discovery of energy quanta (1918 Nobel Prize in Physics).

Niels Bohr

• Niels Bohr (1885-1962) conducted investigations of the structure of atoms and the radiation emanating from them (1922 Nobel Prize in Physics).

G.P. Thomson

• G.P. Thomson (1885-1962) conducted experimental discovery of the diffraction of electrons by crystals (1937 Nobel Prize in Physics).

Aage Niels Bohr

• Aage Niels Bohr (1922-2009) made a discovery of the connection between particle and collective motion in atomic nuclei (1975 Nobel Prize in Physics).

Bohr's Atomic Model

• Niels Bohr's model contributed understanding to atomic structure and quantum mechanics in 1913.
• It introduced the electron and nucleus.
• When an electron revolves in this orbit, it doesn't irradiate energy and need not lose energy.
• Quantization of angular momentum occurs with each electron.
• Each orbit is called a shell or energy level, containing certain energy.
• An electron absorbs energy when moving from a lower energy level to a higher one, causing an excited state.
• When electrons move from one orbit to another, they emit photons that produce absorption and emission spectra.
• Spectra can determine material composition.
• Orbits closer to the nucleus are more stable, representing the grand state.

Calculation of Radius of Bohr's Orbit

radius of orbit = r = (n²h² / 4π²me²) x (1/Z) = 0.529 x (n²/Z) Å

• n = principal quantum number of orbit
• Z = atomic number

Formula for Velocity

Vn = (2πZe²/h) x (Z/n)

Formula for Energy

En = - (2π²me^4 / n²h²)

Limitations to Bohr's Atomic Model

• Only shows spectra of single-electron species like hydrogen.
• Only predicted one spectral line.

Spectrum

• White light through a prism resolves into color components and is therefore part of the spectrum.
• Line Spectrum: Spectrum displaying specific wavelengths
• Continuous spectrum: Spectrum of all wavelengths
• Emission spectrum: Atoms or molecules heated, emit light (bright)
• Absorption spectrum: White light is passed through substance (dark)

Emission Spectrum of Hydrogen

• Hydrogen spectrum is an important piece of evidence to show the quantized electronic structure.
• Line, or hydrogen, is emitted by energetics.
• Rydberg proposed a math formula.

Explore Dalton's atomic theory postulates, including the indivisibility of atoms and the properties of elements. Review the limitations of Dalton's theory, including isotopes and the existence of subatomic particles. Learn about J.J. Thomson's discovery of the electron.