Quantum Theory and The Atom

Objectives Compare the Bohr and quantum mechanical models of the atom Predict the positions of electrons in an atom using the concepts of quantum numbers and orbitals Identify the relationships among a hydrogen atom’s energy levels, sublevels, and atomic orbitals

Bohr Models

Bohr Model Hydrogen atom has only certain allowable energy states Lowest state = ground state Atom gains energy = excited state Even though H has only 1 electron, it can be in many different excited states When atom gains energy, electrons orbit further from the nucleus….energy levels The higher the energy level, the further from the nucleus the electron orbits

Bohr Model of the Atom Let’s look at hydrogen atom

Electron Configuration Aufbau principle – electrons occupy the lowest energy orbital available Pauli Exclusion principle – max of two electrons may occupy an orbital Hund’s Rule – single electrons with same spin must occupy each orbital before electrons with opposite spin can occupy that orbital.

Explain Bright Line Spectrum You must be able to explain why atoms emit bright line spectrums Read page 128 in book Let’s do stair step model/demo again

Quantum Mechanical Model French scientist Louis de Broglie accounted for fixed energy levels in Bohr’s model Quantized electron orbitals had characteristics similar to a wave

Standing Wave

Atomic Orbitals A 3-dimentional region around the nucleus that describes an electron’s probable location Orbital: The region in space where there is a probablility of finding an electron Predicted by de Broglie’s wave function Λ = h/mv Where λ = wavelength, m = mass of particle, v = velocity of particle, h = Planck’s constant

Subsequent orbitals are just bigger: the electrons are located further from the nucleus because they contain more energy

P Orbital

d orbital

Quantum Numbers If an atom absorbs energy from its ground state, it can be found in the 2nd, 3rd, 4th, etc. energy levels. 1st energy level, n = 1 2nd energy level, n = 2 Etc… “n” is the principle quantum number

1st Energy Level Only one orbital for 1st energy level…. The S orbital

2nd Energy Level Two orbitals…S and P S orbital is larger than 1st energy level

Remember An orbital is a description of an electron wave (remember the standing wave)…describes the probability of an electron being located in a certain space Also describes where we will NOT find an electron (remember the node of a standing wave)

3rd Energy Level Has S, P, and D orbitals 4th Energy Level Contains F orbital (seen only for inner transition elements)

Homework 5.2 assessment P.146 39-56