The Modern Atomic Model

After Thomson: Bohr, Placnk, Einstein, Heisenberg, and

Schrödinger

Electromagnetic Radiation

• All forms move at a constant speed, 3.0 108 m/s.

• 3.0 108 m/s is considered the speed of light through air.

• It is represented by the variable c.

Waves• Waves are repetitive in nature

• Wavelength variable:

Waves

• The number of waves that pass through a specific point over a period of time is called the frequency (variable: ) of the wave, measured in Hertz (Hz)

• The relationship between and c =

Electromagnetic Spectrum• The

electromagnetic spectrum is a chart showing the frequencies and wavelengths of all forms of electromagnetic radiation.

• A closeup of the visible light portion:

Photoelectric Effect• The photoelectric effect refers to emission

of e- from a metal when light shines on the metal

Photoelectric Effect• There is a minimum energy, called the

work function (f), below which no e- are emitted, regardless of how long light is shined on the metal.

• Classical theory suggested that a metal that is continuously bombarded by waves should eventually eject an electron.

• Max Planck’s suggestion: Objects emit energy in small, specific amounts called quanta. A quantum is the minimum amount of energy that can be gained/lost in an atom.

Light as a Particle

• Relationship between quantum and the frequency of radiation:

E = h• Where h = 6.626 1034 J s (Planck’s

constant)

Light as a Particle

• In 1905, Einstein described the dual wave/particle nature for electromagnetic radiation.

• A massless particle carrying a quantum of energy – photon

• Therefore, Ephoton = h• If the energy of a photon is less than the

work function for a metal, electrons will not be emitted.

Electron Energy States

• Ground State – lowest energy state of an atom

• Excited State – state to which electron moves when it has greater energy than the ground state

• An electron moving from the excited state to the ground state gives off energy in the form of a photon

Photons and ElectronsEphoton = Eexcited - Eground

•The diagram to the right shows different energy values for various energy levels

•Note: the electronvolt (eV) is the unit used here 1 eV = 1.602 10-19 J

Hydrogen Atom Line Emission Spectra

• When a beam of colored light is passed through a prism, the beam is separated into several frequencies of visible light.

• Collectively, these frequencies are called the emission spectrum.

Electron Energy States

• Eexcited and Eground are fixed values for every identical atom of a given element.

Silicon

Line Emission Spectra for Selected Elements

Line Emission Spectra for Hydrogen

Bohr Model of the Atom

• The Bohr Model explained the spectral lines for hydrogen

• It did NOT explain:– other atoms’ spectral lines– the chemical behavior of atoms

Electrons as Waves

• 1924 – Louis de Broglie suggests electrons are waves confined to space around the nucleus

• Therefore, electrons can only exist at certain frequencies, which correspond to specific energies, those of Bohr’s orbits.

Heisenberg Uncertainty Principle

• How does one find the electron “waves” in an atom – strike it with a photon!

• However, once the photon strikes the electron, its energy is changed, and therefore its path is changed.

• You can never know both the velocity (speed and direction) and position of an electron simultaneously.

• In fact, the more you know about position, the less is known about velocity (and vice versa).

Schrödinger Equation

• Solutions to the equation, called wavefunctions (), can only be specific values.

• Treats quantization as a natural consequence, not an assumption

• Electrons (in all probability) exist in 3-D regions around the nucleus called orbitals