Chemistry Unit 4 Quantum Mechanics & Light

Topic Essential Knowledge

Waves

Wavelength () measures the distance between corresponding points on adjacent waves. The units of wavelength are meters (m). Frequency () measures the number of waves passing a given point per unit of time. The unit of frequency is inverted

seconds (1/s or s-1) or simply Hertz (Hz).Wavelength and frequency are inversely related.

Electromagnetic

Spectrum

The electromagnetic spectrum is arranged by energy. We can only see a very small section of visible light.

Grandpa Xavier Uses Very Intelligent Mini RatsGamma X-Ray Ultraviolet Visible Infrared Microwave

Radio

Wave-Particle Duality

Max Planck explained how an object can glow (emit light) when heated because energy comes in discrete packets he called quanta. Albert Einstein explained the photoelectric effect because electromagetic radiation acts as a stream of particles he called photons.The Dual Nature of Light is the theory that light has both wave and particle-like properties.Louis De Broglie thought that if light can be particle-like then matter can be wave-like. He proved that all matter in motion has an associated wavelength. This is the theory of the Dual Nature of Matter.Wave-Particle Duality combines the dual nature of light and the dual nature of matter to state that all matter and energy exhibits properties of both particles and waves.

Atomic Spectrums

An emission spectrum is a set of bright lines produced by an excited substance returning to the ground state, and an absorption spectrum is those same lines missing from the full spectrum as it is absorbed by a substance. By adding energy, an electron can move energy levels to enter an excited state. That same electron will emit energy when it returns back to its normal ground state.

Niels Bohr used the emission spectrum of hydrogen to prove that energy levels occur only at specific energies since we see the same lines each time.

Quantum Mechanics

The quantum mechanical model (or electron cloud model) gives the locaqtion of an electron as a probability instead of showing it at a certain position.The Heisenberg Uncertainty Principle states that the more accurately we know a particle’s position, the less accurately we can describe its motion. In other words, we cannot know where the electron is and where it is going at the same time.Erwin Schrodinger used wave functions (), a set of three-dimensional coordinates, to find the probability of finding electrons within a particular volume of space around the nucleus. Orbitals are specific regions of space where an electron might be found.

Orbitals

An orbital is not a Bohr orbit, but an area of probability. Each orbital contains a pair of two electrons with opposite spins. There are four subshells for orbitals:

Subshell Shape #

orbitals#

electrons

S Spherical 1 2

P Dumbbell 3 6

D Cloverleaf 5 10

F Various 7 14On the periodic table:

Electron Configuratio

ns

Electron configurations describe the arrangement of an atom’s electrons in the various possible energy levels and subshells.

We can use noble gas configuration as a shorthand by writing the noble gas that came before our element in square brackets. Ex: [Kr]5s24d15p3

Orbital Diagrams

Orbital Diagrams show the spin of the electrons in each orbital.

Three rules govern how to write electron configurations and orbital diagrams:1. Aufbau Principle – electrons fill the lowest energy orbitals first2. Pauli Exclusion Principle – each orbital can only hold two

electrons3. Hund’s Rule – within each sublevel, place one electron per orbital

before pairing them

The Periodic

Table

John Newlands (1865) discovered that there are repeating chemical properties every eighth element. He called this periodic law or periodicity. Dmitri Mendeleev (1865) is the “Father of the Periodic Table” because he was the first to arrange the elements into a table by increasing atomic mass. He correctly left gaps for unknown elements, but there were some inconsistencies with periodic law.Henry Moseley (1914) arranged the periodic table by increasing atomic number, giving us the table we use today.

Periodic Trends

Shielding effect occurs when core electrons “shield” or “screen” valence electrons from feeling the full attraction of the positive nucleus. Effective nuclear charge is the pull of the nucleus on the valence electrons after accounting for shielding effect.Atomic radius is the distance from the nucleus to the outermost energy level.

Cations < Neutral < AnionIonization energy is the amount of energy required to remove an electron.Electronegativity is the ability of an element to gain an electron.