Chapter 5 Electronic Structure and Periodic Trends 5.5 Electron Configurations and the Periodic Table Learning Goal Write the electron configuration for an atom using the sublevel blocks on the periodic table.

Periodic Table Blocks Different sections or blocks within the periodic table correspond to the s, p, d, and f sublevels. We can “build” the electron configurations of atoms by reading the periodic table in order of increasing atomic number.

Periodic Table Blocks

The s Block The s block includes hydrogen and helium elements in Group 1A (1) elements in Group 2A (2) Elements in the s block have their final one or two electrons in an s orbital.

The p Block 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 up to six electrons.

The d Block The d block contains transition elements. The d sublevels are one less (n – 1) than the period number. There are 10 elements in each d block, because five d orbitals can hold up to 10 electrons.

The f Block The f block contains inner transition elements, two rows at the bottom of the periodic table. The f sublevel is two less than the period number. There are 14 elements in each f block, because there are seven f orbitals that can hold up to 14 electrons.

Electron Configurations, Using the Periodic Table Using the periodic table, write the electron configuration for silicon. Solution Period 1 1s block 1s2 Period 2 2s → 2p blocks 2s2 2p6 Period 3 3s → 3p blocks 3s2 3p2 (Si) Writing all the sublevel blocks in order gives 1s2 2s2 2p6 3s2 3p2

Guide to Writing Electron Configurations

Sample Problem Use the sublevel blocks on the periodic table to write the electron configuration for chlorine. Step 1 Locate the element on the periodic table. Chlorine (atomic number 17) is in Group 7A (17) and Period 3.

Sample Problem Use the sublevel blocks on the periodic table to write the electron configuration for chlorine. Step 2 Write the filled sublevels in order, going across each period.

Sample Problem Use the sublevel blocks on the periodic table to write the electron configuration for chlorine. Step 3 Complete the configuration by counting the electrons in the last occupied sublevel block. The electron configuration for chlorine (Cl) is 1s2 2s2 2p6 3s2 3p5

Period 4 The 4s sublevel fills before the 3d sublevel because the electrons in the 4s sublevel are slightly lower in energy than those in the 3d sublevel. This order occurs again in Period 5, when the 5s fills before the 4d sublevel, and in Period 6, when the 6s fills before the 5d sublevel.

Period 4

Period 4

Some Exceptions in Sublevel Block Order In filling the 3d sublevel, exceptions occur for chromium and copper. In Cr the 3d sublevel is close to being half-filled, which is more stable; thus one of the 4s electrons moves to the 3d sublevel.

Some Exceptions in Sublevel Block Order In Cu the 3d sublevel is close to being filled, which is more stable; thus one of the 4s electrons moves to the 3d sublevel.

Learning Check Use the periodic table to give the symbol of the element with each of the following electron configurations. A. 1s2 2s2 2p6 3s2 3p6 4s2 3d7 B. [Ar] 4s2 3d10 4p5

Solution Use the periodic table to give the symbol of the element with each of the following electron configurations. A. 1s2 2s2 2p6 3s2 3p6 4s2 3d7 Element is cobalt (Co). B. [Ar] 4s2 3d10 4p5 Element is bromine (Br).