Atomic Spectra and Electron Orbitals

The Classical Atom Electrons orbited the nucleus. Electrons orbited the nucleus. Problem!! Problem!! Accelerating charges emit radiation (energy) Accelerating charges emit radiation (energy) With less energy the electron orbitals should shrink and the electrons should spiral into the nucleus With less energy the electron orbitals should shrink and the electrons should spiral into the nucleus

The Bohr Atom Electrons orbit in specific energy levels without radiating Electrons orbit in specific energy levels without radiating Each energy level has a specific energy Each energy level has a specific energy To change energy levels, atoms must either absorb or emit energy (photons) To change energy levels, atoms must either absorb or emit energy (photons) Link Link Link

Ground State vs. Excited States Ground state is the lowest energy level in the atom. Ground state is the lowest energy level in the atom. All other energy levels are known as excited states. All other energy levels are known as excited states.

Quantization of Energy A result of the Bohr atom is that energy cannot come in any amount it wants, it must be in specific amounts of energy called quanta A result of the Bohr atom is that energy cannot come in any amount it wants, it must be in specific amounts of energy called quanta Like steps or rungs of a ladder Like steps or rungs of a ladder

Electron Volts (eV) The energies involved in atomic spectra are very small, making it inconvenient to use Joules as our units of energy The energies involved in atomic spectra are very small, making it inconvenient to use Joules as our units of energy 1 eV = 1.6x J 1 eV = 1.6x J eV  J multiply by 1.6x eV  J multiply by 1.6x J  eV divide by 1.6x J  eV divide by 1.6x10 -19

Hydrogen Atom: Energy Levels and Energies of Transition n - energy level of atom n - energy level of atom primary quantum number n = 1, 2, 3, … E = x J n 2

Photon Energy E = hf = hc/ E = hf = hc/ h = x J*s h = x J*s c = 3x10 8 m/s c = 3x10 8 m/s f – frequency f – frequency – wavelength – wavelength E photon =  E atom E photon =  E atom  E atom > 0 if atom absorbs photon  E atom > 0 if atom absorbs photon  E atom < 0 if atom emits photon  E atom < 0 if atom emits photon

Emission Spectra The set of energies emitted by an atom. The set of energies emitted by an atom. Energy determines frequency which determine color Energy determines frequency which determine color

Electromagnetic Spectrum

Absorption Spectrum How we know the gaseous atmosphere surrounding the sun How we know the gaseous atmosphere surrounding the sun

Spectral Series Spectral series are the set of energies produced when electrons from excited states emit photons and transition down to the same final energy level Spectral series are the set of energies produced when electrons from excited states emit photons and transition down to the same final energy level Three important series for Hydrogen: Three important series for Hydrogen: Lyman Series: n f = 1 Lyman Series: n f = 1 Ballmer Series: n f = 2 Ballmer Series: n f = 2 Paschen Series: n f = 3 Paschen Series: n f = 3

Atoms that aren’t Hydrogen The formulae in the previous slides only apply to Hydrogen. The formulae in the previous slides only apply to Hydrogen. For other atoms, the energies cannot be so easily calculated, and will often be given to you directly. For other atoms, the energies cannot be so easily calculated, and will often be given to you directly.

Energy Level Diagram  E = E f - E i  E = E f - E i

Ionization Energy In order to remove an electron from an atom you have to add enough energy make the energy zero. In order to remove an electron from an atom you have to add enough energy make the energy zero. i.e. if you are at ground level and have an energy of eV that means you need 10.38eV if energy to ionize i.e. if you are at ground level and have an energy of eV that means you need 10.38eV if energy to ionize All left over Energy become Kinetic Energy

Example An atom’s only electron is in the fourth energy level (-2.35 eV). An atom’s only electron is in the fourth energy level (-2.35 eV). How many different photon energies can be emitted as this photon returns to the ground state (-15.8 eV)? How many different photon energies can be emitted as this photon returns to the ground state (-15.8 eV)? What is the frequency of the photon that would be emitted if the electron returned to the ground state in a single transition? What is the frequency of the photon that would be emitted if the electron returned to the ground state in a single transition?