1. Electron Configurations
3.9
The electron configuration describes how the electrons are distributed in the various atomic orbitals.
In a ground state hydrogen atom, the electron is found in the 1s orbital.
Ground state electron configuration of hydrogen
principal (n = 1)
1s1
number of electrons in the orbital or subshell
Energy 2s 2p 2p 2p
angular momentum (l = 0)
The use of an up arrow indicates an electron with ms = + ½ 1s

2. Electron Configurations
If hydrogen’s electron is found in a higher energy orbital, the atom is in an excited state.
A possible excited state electron configuration of hydrogen
2s1
Energy 2s 2p 2p 2p 1s

3. Electron Configurations
The helium emission spectrum is more complex than the hydrogen spectrum.
There are more possible energy transitions in a helium atom because helium has two electrons.

4. Electron Configurations
In a multi-electron atoms, the energies of the atomic orbitals are split.
Splitting of energy levels refers to the splitting of a shell (n=3) into subshells of different energies (3s, 3p, 3d)
Text practice: 3.90

5. Electron Configurations
According to the Pauli exclusion principle, no two electrons in an atom can have the same four quantum numbers.
The ground state electron configuration of helium
Energy 2p 2p 2p
1s2 2s
Quantum number
Principal (n)
Angular moment (l)
Magnetic (ml)
Electron spin (ms) 1 1 1s
describes the 1s orbital
describes the electrons in the 1s orbital
+ ½
‒ ½

6. Electron Configurations
The Aufbau principle states that electrons are added to the lowest energy orbitals first before moving to higher energy orbitals.
Li has a total of 3 electrons
The ground state electron configuration of Li
1s22s1
Energy 2p 2p 2p 2s
The third electron must go in the next available orbital with the lowest possible energy. 1s
The 1s orbital can only accommodate 2 electrons (Pauli exclusion principle)

7. Electron Configurations
The Aufbau principle states that electrons are added to the lowest energy orbitals first before moving to higher energy orbitals.
Be has a total of 4 electrons
The ground state electron configuration of Be
1s22s2
Energy 2p 2p 2p 2s 1s

8. Electron Configurations
The Aufbau principle states that electrons are added to the lowest energy orbitals first before moving to higher energy orbitals.
B has a total of 5 electrons
The ground state electron configuration of B
1s22s22p1
Energy 2p 2p 2p 2s 1s

9. Electron Configurations
According to Hund’s rule, the most stable arrangement of electrons is the one in which the number of electrons with the same spin is maximized.
C has a total of 6 electrons
The ground state electron configuration of C
1s22s22p2
Energy 2p 2p 2p 2s
The 2p orbitals are of equal energy, or degenerate.
Put 1 electron in each before pairing (Hund’s rule). 1s

10. Electron Configurations
According to Hund’s rule, the most stable arrangement of electrons is the one in which the number of electrons with the same spin is maximized.
N has a total of 7 electrons
The ground state electron configuration of N
1s22s22p3
Energy 2p 2p 2p 2s
The 2p orbitals are of equal energy, or degenerate.
Put 1 electron in each before pairing (Hund’s rule). 1s

11. Electron Configurations
According to Hund’s rule, the most stable arrangement of electrons is the one in which the number of electrons with the same spin is maximized.
O has a total of 8 electrons
The ground state electron configuration of O
1s22s22p4
Energy 2p 2p 2p 2s
Once all the 2p orbitals are singly occupied, additional electrons will have to pair with those already in the orbitals. 1s

12. Electron Configurations
According to Hund’s rule, the most stable arrangement of electrons is the one in which the number of electrons with the same spin is maximized.
F has a total of 9 electrons
The ground state electron configuration of F
1s22s22p5
Energy 2p 2p 2p 2s
When there are one or more unpaired electrons, as in the case of oxygen and fluorine, the atom is called paramagnetic. 1s

13. Electron Configurations
According to Hund’s rule, the most stable arrangement of electrons is the one in which the number of electrons with the same spin is maximized.
Ne has a total of 10 electrons
The ground state electron configuration of Ne
1s22s22p6
Energy 2p 2p 2p 2s
When all of the electrons in an atom are paired, as in neon, it is called diamagnetic. 1s

14. Electron Configurations
General rules for writing electron configurations:
Electrons will reside in the available orbitals of the lowest possible energy.
Each orbital can accommodate a maximum of two electrons.
Electrons will not pair in degenerate orbitals if an empty orbital is available.
Orbitals will fill in the order indicated in the figure.

15. Worked Example 3.10
Write the electron configuration and give the orbital diagram of a calcium (Ca) atom (Z = 20).
Setup Because Z = 20, Ca has 20 electrons. They will fill in according to the diagram at right. Each s subshell can contain a maximum of two electrons, whereas each p subshell can contain a maximum of six electrons.

16. Electron Configurations and the Periodic Table
3.10
The electron configurations of all elements except hydrogen and helium can be represented using a noble gas core.
The electron configuration of potassium (Z = 19) is 1s22s22p63s23p64s1.
Because 1s22s22p63s23p6 is the electron configuration of argon, we can simplify potassium’s to [Ar]4s1.
The ground state electron configuration of K:
1s22s22p63s23p64s1
1s22s22p63s23p64s1
[Ar]
[Ar]4s1

17. Electron Configurations and the Periodic Table
Elements in Group 3B through Group 1B are the transition metals.

18. Electron Configurations and the Periodic Table
Following lanthanum (La), there is a gap where the lanthanide (rare earth) series belongs.

19. Electron Configurations and the Periodic Table
After actinum (Ac) comes the actinide series.

20. Electron Configurations and the Periodic Table

21. Electron Configurations and the Periodic Table
There are several notable exceptions to the order of electron filling for some of the transition metals.
Chromium (Z = 24) is [Ar]4s13d5 and not [Ar]4s23d4 as expected.
Copper (Z = 29) is [Ar]4s13d10 and not [Ar]4s23d9 as expected.
The reason for these anomalies is the slightly greater stability of d subshells that are either half-filled (d5) or completely filled (d10). 4s 3d
[Ar] Cr
Greater stability with half-filled 3d subshell

22. Electron Configurations and the Periodic Table
There are several notable exceptions to the order of electron filling for some of the transition metals.
Chromium (Z = 24) is [Ar]4s13d5 and not [Ar]4s23d4 as expected.
Copper (Z = 29) is [Ar]4s13d10 and not [Ar]4s23d9 as expected.
The reason for these anomalies is the slightly greater stability of d subshells that are either half-filled (d5) or completely filled (d10). 4s 3d
[Ar] Cu
Greater stability with filled 3d subshell

23. Worked Example 3.11
Write the electron configuration for an arsenic atom (Z = 33) in the ground state.
Setup The noble gas core for As is [Ar], where Z = 18 for Ar.
The order of filling beyond the noble gas core is 4s, 3d, and 4p. Fifteen electrons go into these subshells because there are 33 – 18 = 15 electrons in As beyond its noble gas core.
Text Practice a, b, c, d

24. Study Guide for sections 3.9-3.10
DAY 7, Terms to know:
Sections electron configuration, Aufbau principle, Hund’s rule, orbital diagram
DAY 7, Specific outcomes and skills that may be tested on exam 1:
Sections
Be able to use the Pauli exclusion principle and Aufbau principle to give a complete or abbreviated electron configuration for an atom in either its ground state or one possible excited state
Given an electron configuration, be able to give a complete elemental symbol for an atom
Be able to use the Pauli exclusion principle, Aufbau principle, and Hund’s rule to give a complete or abbreviated orbital diagram for an atom either its ground state or one possible excited state
Given an orbital diagram, be able to give a complete elemental symbol for an atom or ion
Be able to recognize and explain how Cr and Cu are exceptions to the Aufbau principle

25. Extra Practice Problems for sections 3.9-3.10
Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period

26. On day 8, we will have exam 1 Prep for day 9
Must watch videos:
(ionization energy)
(periodic trends)
Other helpful videos:
(MIT)
(UC-Irvine lectures 7)
Read sections