Chapter 11 Modern Atomic Theory

Rutherford’s Atom What are the electrons doing? How are the electrons arranged How do they move?

Neils Bohr Observations When solutions containing metal ions are heated, they give off characteristic colors. What does this mean? Bohr related the color of light to the electrons in an atom. SodiumPotassiumCalciumBarium

Emission of Energy by Atoms Atomic Spectra Atoms that have gained extra energy release that energy in the form of light. Lithium

Spectroscopy

Emission Line Spectrum Each element has its own emission spectrum, which can be used to identify that element. Line spectrum: shows very specific wavelengths of light that atoms give off or gain

Emission Spectrum Emission spectra of hydrogen

Hydrogen Spectra

Explanation of Emission Spectra Photons (particle of light) are emitted when an electron makes a transition from a higher energy level to a lower energy level.

Bohr’s Model Explained spectrum of hydrogen Energy of atom is related to the distance of electron from the nucleus

The Bohr Model of the Atom Electrons orbit around a nucleus like planets around the sun Electrons can only be in "special" orbits. All other orbits were not possible. Electrons could "jump" between these special orbits. Energy was associated with these “jumps” –Energy can be absorbed- electron move to a larger orbit –Energy (in the form of light) can be given off (emitted) electron move to a smaller orbit

Quantum Leap (Jumps) The atom is quantized, i.e. only certain energies are allowed.

The Bohr Atom

Bohr’s Model: Conclusion Energy of the atom is quantized –Atom can only have certain specific energy states called energy levels. –The electron cannot exist between energy levels –When atom gains energy, electron “moves” to a higher energy level –When atom loses energy, electron “moves” to a lower energy level –Lines in spectrum correspond to the difference in energy between levels

The Bohr Atom Ground state: lowest energy state for the electrons within of an atom Excited states: energy levels higher than the ground state Further away from the nucleus means more energy. High energy electrons are unstable and eventually fall to a lower energy level releasing energy (light)

Problems with the Bohr Model Only explains hydrogen atom spectrum Neglects interactions between electrons Assumes circular or elliptical orbits for electrons (which is not true) Instead of electrons being in orbits of a certain energy, electrons are in special energy levels.

Other Developments: Photoelectric Effect Light of a certain wavelength could cause ejection of the electron from the surface of a metal Einstein showed that light was made of packets of energy called photons (a particle of energy) –Light can have particle like properties –These photons can “collide” with other objects –Light has a dual nature (wave and particle)

Other Developments: Matter Waves DeBroglie: If light has properties of waves and particles then maybe MATTER has properties of waves and particles –electrons can move in waves –used in electron microscope λ = h where m = mass, v = velocity mv λ too small to be observed for visible objects 114g baseball, speed of 110mph, has a wavelength of 1.2 x m

Other Developments: Heisenberg Heisenberg Uncertainty Principle: It is impossible to measure both the position and momentum of an electron at the same time. Cannot determine both location of electron and how it is moving

Other Developments: Schrodinger

Wave Mechanical Model of the Atom Treat electrons as waves Energy is quantized Electron clouds: areas of electron probability Orbitals- area where electron is likely to be found. Orbitals have certain geometric shapes

Electron Cloud The size of the orbital is defined by the region that contains 90% of the total electron probability.

Orbitals spdfspdf

s orbital

p Orbitals (a) p x (b) p z ( c)p y p-orbitals have two lobes Each p sublevel has 3 p orbitals –All have equal energy (degenerate)

d orbital