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Electron Configurations Objectives List the total number of electrons needed to fully occupy each main energy level State the Aufbau principle, the Pauli exclusion principle, and Hund’s rule Describe the electron configurations for the atoms of any element using orbital notation, electron-configuration notation, and when appropriate, noble-gas notation Electron Configuration Arrangement of electrons in an atom Atoms occupy the lowest energy arrangement—ground-state electron configuration Rules 1. Aufbau principle: electrons enter orbitals of lowest energy levels first Orbitals in same sublevel (s, p, d, f) are of equal energy s sublevel has lowest energy Sublevels in different main levels can overlap 1s is the lowest energy orbital Notice the 4s sublevel is lower than 3d It has less energy than the 3d (this means that the 4s fills before 3d) http://www.chemguide.co.uk/atoms/properties/atomorbs.html Text p. 105 Figure 4-16 “The order of the atomic orbitals can be somewhat difficult to remember. Fortunately, there is a mnemonic device that can help in this regard. The slide sequence below details this device.” http://www.iun.edu/%7Ecpanhd/C101webnotes/modern-atomic-theory/mnemonicdev.html Rules 2. Pauli exclusion principle: quantum numbers of atomic orbitals may describe only one electron Two electrons in the same orbital must have opposite spins Spins are clockwise & counterclockwise 1s orbital Rules 3. Hund’s Rule: when electrons occupy orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with parallel spins Sublevels add one electron to each orbital first Second electrons added to the orbitals are then paired with an opposite spin a p orbitals filling below b c Orbital Notation Empty orbitals: ____ Name of orbital goes below line One electron: ____ Two electrons: ____ Oxygen 8 electrons 1s 2s 2 p 2 p 2 p Electron-Configuration Notation Eliminate lines and arrows Number of electrons in a sublevel is the superscript Oxygen 8 electrons 1s2 2s2 2p4 http://glencoe.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/007874637x /514701/chem_ch05_t05_4.swf::Electron%20Configurations%20and%20Orbit al%20Diagrams%20for%20Elements%201–10 Practice Write the orbital notation for Al. Write the electron-configuration notation for Mn. Practice The electron configuration of nitrogen is 1s2 2s2 2p3. How many electrons are present in nitrogen? What is the atomic number of nitrogen? Write the orbital notation for nitrogen. Practice The electron configuration of fluorine is 1s2 2s2 2p5. What is the atomic number of fluorine? How many of its p orbitals are filled? How many unpaired electrons does a fluorine atom contain?