Ch. 4.7 Electron Configuration and Orbitals

Orbital Diagrams

Orbital Diagrams use boxes to show how electrons:

  • are arranged in the orbitals of an atom

  • fill the orbitals within the same sublevel at one time, before pairing the electrons

Electron Configurations

Chemists use a notation called electron configuration to:

  • Indicate the placement of electrons in an atom

  • Show how electrons fill energy levels in order of increasing energy

  • write an abbreviated form using a noble gas to represent all electrons preceding it

Electron Configurations and the Periodic Table

  • The electron configurations of elements are related to their positions on the periodic table. Different sections or blocks correspond to sublevels s, p, d and f

Blocks on the Periodic Table

  • The f block, the inner transition elements, is the two rows of elements at the bottom of the periodic table

  • There are 14 elements in each f block, because seven f orbitals can hold a maximum of 14 electrons

  • The s block contains elements in Groups 1A (1) and 2A (2) which means the final one or two electrons are in the s sublevel

  • The p block consists of elements in Group 3A (13) to Group 8A (18). There are six p block elements in each period, because three p orbitals can hold a maximum of six electrons

  • The d block, which contains transition elements, first appears after calcium (atomic number 20). There are 10 elements in the d block, because five d orbitals can hold a maximum of 10 electrons

  • The 4s sublevel fills before the 3d sublevel, because the 4s sublevel is slightly lower in energy than the 3rd sublevel

  • The 5s sublevel fills before the 4d sublevel

  • The 6s sublevel fills before the 5d sublevel

Guide to Writing Configurations Using Sublevel Blocks

  • Use sublevel blocks on the periodic table to write the electron configuration of an element

STEP 1

  • Locate the element (selenium in this example) on the periodic table. Selenium (atomic number 34) is in Group 6A (16) and Period 4

STEP 2

  • Write the filled sublevels in order, going across each period

STEP 3

  • Complete the configuration by counting the electrons in the last occupied sublevel block. Because Selenium is the fourth element in the 4p block, there are 4 electrons in the 4p sublevel

  • For Chromium (Cr), moving one of the 4s electrons to the 3d sublevel adds stability with a half-filled d subshell, and the resulting configuration is 4s^13d^5

  • 3d occupancy quick note: in the transition metals, the 3d subshell is filled after the 4s but can be rearranged in observed configurations due to stability considerations.