Arrangement of Electrons in Atoms Chapter 4

Rules of Governing Electron Configurations Electrons added to orbitals one by one according to three basic rules. Aufbau principle- an electron occupies the lowest energy orbital that can receive it. Pauli’s Exclusion Principle- no two electrons in the same atom can have the same four quantum numbers. Hund’s rule- orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron , and all electrons in singly occupied orbitals must have the same spin state.

Representing electron Configuration Orbital Notation- an unoccupied orbital is represented by a line, ____, with the orbitals name written underneath the line. Orbital containing one electron:___ Orbital containing 2 electrons: _____ An orbital containing two electrons shows the electrons paired with opposite spin states. 1s 1s

Electron Configuration Notation Electron configuration notation eliminates the lines and arrows of orbital notation. The superscript indicates how many electrons are present in the sublevel. Hydrogen Electron Configuration- 1s1 Helium Electron Configuration- 1s2

Aufbau’s Principle

Review: Electrons in each Sublevel P-6 d-10 f-14

Elements of the second period Lithium (Li) atomic number 3, ___ ___, 1s22s1 Oxygen (O): atomic number 8, ___ ___ ___ ___ ___, 1s22s22p4 1s 2s 1s 2s 2p

Nobel Gas Notation Refers to the outer main energy level occupied in most cases by eight electrons. Examples: Sodium (Na): atomic number (11), 1s22s22p63s1, [Ne]3s1 Chlorine (Cl): atomic number (17), 1s22s22p63s23p5, [Ne]3s23p5 *[Ne]=1s22s22p6

Examples using Aufbau’s Rule: Scandium(Sc): atomic number (21) 1s22s22p63s23p63d14s2, [Ar]3d14s2 Zirconium (Zr): atomic number (40), 1s22s22p63s23p63d104s24p64d25s2, [Kr]4d25s2

Exceptions Contrary to what expected by Aufbau’s rule: -Chromium is said to have a configuration of 3d5 4s1 as opposed to 3d4 4s2 -Copper atoms are said to have a configuration of 3d10 4s1 as opposed to 3d9 4s2 as might have been expected from the general trend.