1. 4.7 Electron Configurations
Electron configurations follow the order of occupied sublevels on the periodic table.
Learning Goal Draw the orbital diagram and write the electron configuration for an element.

2. Orbital Diagrams Orbital diagrams use boxes to show how electrons
are arranged in the orbitals of an atom.
fill the orbitals and energy levels from lowest to highest energy level.
fill orbitals within the same sublevel one at a time, before pairing the electrons.

3. Electron Configurations
Chemists use a notation called electron configuration to
indicate the placement of electrons in an atom.
show how electrons fill energy levels and sublevels in order of increasing energy.
write an abbreviated form using a noble gas to represent all electrons preceding it.
Electron Configuration for Carbon
Core Chemistry Skill Writing Electron Configurations

4. Period 1: Hydrogen and Helium

5. Period 2: Lithium to Neon

6. Guide to Drawing Orbital Diagrams

7. Drawing Orbital Diagrams
Nitrogen atoms are found in amino acids, proteins, and nucleic acids. Draw the orbital diagram for nitrogen.
STEP 1 Draw boxes to represent the occupied orbitals.
Nitrogen’s atomic number is 7, which means it has seven electrons. For the orbital diagram, we draw boxes to represent the 1s, 2s, and 2p orbitals.

8. Drawing Orbital Diagrams
Nitrogen atoms are found in amino acids, proteins, and nucleic acids. Draw the orbital diagram for nitrogen. STEP 2 Place a pair of electrons with opposite spins in each filled orbital. First we place a pair of electrons with opposite spins in the1s and 2s orbitals.

9. Drawing Orbital Diagrams
Nitrogen atoms are found in amino acids, proteins, and nucleic acids. Draw the orbital diagram for nitrogen. STEP 3 Place the remaining electrons in the last occupied sublevel in separate orbitals. Place three remaining electrons in the three separate 2p orbitals, with arrows drawn in the same direction.

10. Period 3: Sodium to Argon

11. 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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

12. Blocks on the Periodic Table
1. The s block contains elements in Groups 1A (1) and 2A (2). This means the final one or two electrons are in the s sublevel. 2. 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. 3. 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.

13. 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.
Core Chemistry Skill Using the Periodic Table to Write Electron Configurations

14. Guide to Writing Configurations Using Sublevel Blocks
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. STEP 2 Write the filled sublevels in order, going across each period.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

15. Guide to Writing Configurations Using Sublevel Blocks
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. Because chlorine is the fifth element in the 3p block, there are five electrons in the 3p sublevel.
The electron configuration for chlorine (Cl) is
1s22s22p63s23p5.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

16. Electron Configurations: Period 4 and Above
Beginning in Period 4,
the 4s sublevel fills before the 3d sublevel, because the 3d
sublevel is slightly lower in energy than the 4s sublevel.
the 5s sublevel fills before the 4d sublevel.
the 6s sublevel fills before the 5d sublevel.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

17. Electron Configurations: Period 4 and Above
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

18. Electron Configurations: Period 4 and Above
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.

19. Study Check
Use the sublevel blocks on the periodic table to write the electron configuration for selenium.

20. Solution
Use the sublevel blocks on the periodic table to write the electron configuration for selenium. STEP 1 Locate the element on the periodic table. Selenium is in Period 4, Group 6A (16). STEP 2 Write the filled sublevels in order, going across each period.

21. Solution
Use the sublevel blocks on the periodic table to write the electron configuration for selenium. 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 four electrons to place in the 4p sublevel. The electron configuration for selenium (Se) is 1s22s22p63s23p64s23d104p4.

22. 3d Sublevel Exceptions
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 4s13d5.
For copper (Cu), moving one of the 4s electrons to the 3d sublevel adds stability with a filled d subshell, and the resulting configuration is 4s13d10.

23. Study Check
Use the periodic table to give the symbol and name for the element with the electron configuration of 1s22s22p63s23p64s23d7.

24. Solution
Use the periodic table to give the symbol and name for the element with the electron configuration of 1s22s22p63s23p64s23d7. There are seven electrons in the 3d sublevel. Thus, the element is cobalt (Co).

25. Study Check
Write the electron arrangement for the following elements: C Si O

26. Solution
STEP 1 Locate the element on the periodic table. C: atomic number 6, Period 2, Group 4A (14) Si: atomic number 14, Period 3, Group 5A (15) O: atomic number 8, Period 2, Group 6A (16)

27. Solution
STEP 2 Write the filled sublevels in order, going across each period. C 1s22s2 Si 1s22s22p63s2 O 1s22s2

28. Solution
STEP 3 Complete the configuration by counting the electrons in the last occupied sublevel block. C, the second element in the 2p block, has two electrons to place in the 2p sublevel: 1s22s22p2. Si, the second element in the 3p block, has two electrons to place in the 3p sublevel: 1s22s22p63s23p2. O, the fourth element in the 2p block, has four electrons to place in the 2p sublevel: 1s22s22p4.