Arrangement of electrons Chapter 4 – Modern Chemistry

4.1 Development of new model Rutherford’s model did not address the arrangement of electrons around the nucleus A model was developed after looking into the properties of light

Properties of Light Light behaves in a dual nature Wave Light belongs to the electromagnetic spectrum which allows light to exhibit properties of waves as it moves through space Parts Wavelength (meters) Frequency (Hertz) Speed Inversely proportional to wavelength and frequency (3.0 x 108 m/s)

Particle Nature of Light Photoelectric Effect Natural phenomena in which a metal will emit an electron after absorbing light From this experiment, light is thought to have a certain quanta of energy which is the minimum energy that can be gained or lost by an atom Max Planck proposed this energy is equal to the frequency times Planck’s constant (6.626 x 10-34 Jxs)

Photon Einstein expanded Planck’s view introduced the dual wave- particle nature of light He stated that all light has zero mass and carries a quantum of energy This particle is known as the photon

Hydrogen Atom Emission Line Spectrum When current is passed through a tube of hydrogen at low pressure, it causes the gas to emit a light This is a result of ground state electrons becoming excited and changing energy levels If this light is passed through a prism, it is separated into its own unique spectrum Attempts to explain why only certain frequencies were present was explain in a new model of the atom

Bohr’s model Niels Bohr was the first to come up with a mathematical representation of why hydrogen has its own unique spectrum (1913) Planetary Model Electrons essentially orbit around the nucleus in fixed energy levels

Bohr’s Model

4.2 Quantum Model What changed from Bohr’s model to the Quantum model? Louis De Broglie suggested that electrons would also have a dual nature Therefore, they acted like waves as well as particles Werner Heisenberg stated his uncertainty principle Due to speed of electrons, it is impossible to know but the velocity and the location simultaneously Erwin Schrodinger created his wave equation and quantum theory This equation gave mathematical representation of areas of probability where electrons could be found

Quantum Numbers Principle Quantum numbers (n) Main energy levels Labeled 1 to 7 Angular Momentum number (l) These are the sublevels (s, p, d, f) Values are 0 for s, 1 for p, 2 for d, and 3 for f

Quantum Numbers Magnetic Quantum number (m) Represent the orbitals in each sublevel Number of orbitals is represented by the sublevel S has 1 P has 3 D has 5 F has 7 Labeled from –l to l (that is L not one, angular momentum number)

Quantum Numbers Spin Quantum Number Either -1/2 or ½ All electrons in the same orbital must have opposite spins No more than 2 electrons per orbital

4.3 Electron Configuration Aufbau’s Principle Electrons fill in the lowest energy levels first Follow the diagonal diagram Pauli Exclusion Principle No one electron can have the same four quantum numbers Hund’s Rule When there are multiple orbitals, each orbital must have one electron before they pair up. Those electrons will have the same spin

Diagonal Diagram

Types of Notations Orbital Notation Electron Configuration Notation Illustrates orbitals and electron spin Electron Configuration Notation Simplifies the notation by placing number of electrons in the superscript of the sublevel Noble Gas Notation Replaces the last full s and p sublevel with corresponding noble gas