1. Chapter 4-3 Electron Configurations Coach Kelsoe Chemistry Pages 111–122

2. Electron Configurations The quantum model improves on the Bohr model because it describes the arrangements of electrons in atoms other than hydrogen. The quantum model improves on the Bohr model because it describes the arrangements of electrons in atoms other than hydrogen. The arrangement of electrons in an atom is known as the atom’s electron configuration. The arrangement of electrons in an atom is known as the atom’s electron configuration.

3. Electron Configurations Electrons in atoms tend to assume arrangements that have the lowest possible energies. Electrons in atoms tend to assume arrangements that have the lowest possible energies. The lowest-energy arrangement of electrons for each element is known as the element’s ground-state electron configuration. The lowest-energy arrangement of electrons for each element is known as the element’s ground-state electron configuration.

4. Rules Governing Electron Configurations According to the Aufbau principle, an electron occupies the lowest-energy orbital that can receive it. According to the Aufbau principle, an electron occupies the lowest-energy orbital that can receive it. The orbital with the lowest energy is the 1s orbital. The orbital with the lowest energy is the 1s orbital. The 2s orbital is the next lowest, then the 2p. The energies of the sublevels begin to overlap starting with the third main energy level. The 2s orbital is the next lowest, then the 2p. The energies of the sublevels begin to overlap starting with the third main energy level.

5. Rules Governing Electron Configurations

6. The second rule shows the importance of the spin quantum number. The second rule shows the importance of the spin quantum number. According to the Pauli exclusion principle, an orbital can hold two electrons of opposite spin. According to the Pauli exclusion principle, an orbital can hold two electrons of opposite spin. The two values of the spin quantum number allow two electrons of opposite spins to occupy the orbital. The two values of the spin quantum number allow two electrons of opposite spins to occupy the orbital. 1s orbital

7. Rules Governing Electron Configurations The 3 rd rule requires placing as many unpaired electrons as possible in separate orbitals in the same energy level. The 3 rd rule requires placing as many unpaired electrons as possible in separate orbitals in the same energy level. In this way, electron-electron repulsion is minimized so that the electron arrangements have the lowest energy possible. This is Hund’s rule. In this way, electron-electron repulsion is minimized so that the electron arrangements have the lowest energy possible. This is Hund’s rule.

8. Rules Governing Electron Configurations According to 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. According to 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. The p sublevels

9. Representing Electron Configurations There are three methods used to indicate electron configurations. There are three methods used to indicate electron configurations. –Orbital Notation –Electron-Configuration Notation –Noble-Gas Notation

10. Orbital Notation In orbital notation, an unoccupied orbital is represented by a line, _____, with the orbital’s name written beneath the line. In orbital notation, an unoccupied orbital is represented by a line, _____, with the orbital’s name written beneath the line. An orbital containing one electron is represented as ____. An orbital containing one electron is represented as ____. An orbital containing two electrons is represented as ____, showing the electrons paired and with opposite spins. An orbital containing two electrons is represented as ____, showing the electrons paired and with opposite spins.

11. Orbital Notation Examples of orbital notations: Examples of orbital notations: –Hydrogen –Helium 1s1s 1s1s H He

12. Bell-ringer Write orbital notations for the following elements: Write orbital notations for the following elements: Magnesium Magnesium Beryllium Beryllium Oxygen Oxygen Phosphorus Phosphorus Neon Neon Cobalt Cobalt

13. Electron-Configuration Notation Electron-Configuration notation eliminates the lines and arrows of orbital notation. Electron-Configuration notation eliminates the lines and arrows of orbital notation. Instead, the number of electrons in a sublevel is shown by adding a superscript to the sublevel designation. Instead, the number of electrons in a sublevel is shown by adding a superscript to the sublevel designation. More examples: More examples: –Hydrogen: 1s 1 –Helium: 1s 2

14. Elements of the Second Period Remember that periods are horizontal. Remember that periods are horizontal. According to the Aufbau principal, after the 1s orbital is filled, the next electron occupies the 2s sublevel. According to the Aufbau principal, after the 1s orbital is filled, the next electron occupies the 2s sublevel. The highest occupied level is the electron-containing main energy level with the highest principal quantum number. The highest occupied level is the electron-containing main energy level with the highest principal quantum number.

15. Elements of the Second Period Inner-shell electrons are electrons that are not in the highest occupied energy level. Inner-shell electrons are electrons that are not in the highest occupied energy level. For example, if Lithium is 1s 2 2s 1, the highest occupied level is 2s, and the inner-shell electrons are the 1s 2. For example, if Lithium is 1s 2 2s 1, the highest occupied level is 2s, and the inner-shell electrons are the 1s 2. Electrons will pair up before jumping to the next energy level. Electrons will pair up before jumping to the next energy level.

16. Elements of the Third Period After the outer octet is filled in neon, the next electron jumps to the 3 rd main energy level. Sodium is 1s 2 2s 2 2p 6 3s 1. After the outer octet is filled in neon, the next electron jumps to the 3 rd main energy level. Sodium is 1s 2 2s 2 2p 6 3s 1. Notice that everything in the third period will start with the series 1s 2 2s 2 2p 6. Notice that everything in the third period will start with the series 1s 2 2s 2 2p 6. This allows us to use a new form of notation that saves us time – Noble-Gas Notation. This allows us to use a new form of notation that saves us time – Noble-Gas Notation.

17. Noble Gas Notation The Group 18 elements (He, Ne, Ar, Kr, Xe, Rn) are called the noble gases. The Group 18 elements (He, Ne, Ar, Kr, Xe, Rn) are called the noble gases. To simplify the notation, we use the previous noble gas’ symbol and then add the remaining electron-configuration notation. To simplify the notation, we use the previous noble gas’ symbol and then add the remaining electron-configuration notation. Example: Example: –Sodium – [Ne]3s 1 –Argon – [Ne]3s 2 3p 6

18. Noble Gas Notation A noble-gas configuration is an outer main energy level fully occupied, in most cases, by eight electrons. A noble-gas configuration is an outer main energy level fully occupied, in most cases, by eight electrons.

19. Elements of the Fourth Period Here’s where the electron-configuration gets tricky. Here’s where the electron-configuration gets tricky. At the fourth period, the 4s sublevels are filled first, 3d sublevels next, then the 4p sublevels. Even though the 4s fill up before the 3d, if 3d are occupied, we write them first. At the fourth period, the 4s sublevels are filled first, 3d sublevels next, then the 4p sublevels. Even though the 4s fill up before the 3d, if 3d are occupied, we write them first. Examples: Examples: –Potassium – [Ar]4s 1 –Iron – [Ar]3d 6 4s 2 –Bromine – [Ar]3d 10 4s 2 4p 5

20. An Exception to the Rule Chromium (Cr) has the configuration [Ar]3d 5 4s 1. Chromium (Cr) has the configuration [Ar]3d 5 4s 1. It turns out that this arrangement requires less energy for Chromium, and therefore is more stable. It turns out that this arrangement requires less energy for Chromium, and therefore is more stable. There is no simple explanation, but you do need to know why this happens (because it requires less energy). There is no simple explanation, but you do need to know why this happens (because it requires less energy). Chromium isn’t the only exception. Chromium isn’t the only exception.

21. Elements of the Fifth Period The progression for filling sublevels along the fifth period is 5s, then 4d, then 5p. The progression for filling sublevels along the fifth period is 5s, then 4d, then 5p. Just like with the fourth period, there are exceptions that keep the 5s from being totally filled. Just like with the fourth period, there are exceptions that keep the 5s from being totally filled. I WILL NOT GIVE YOU AN EXCEPTION TO SOLVE ON A TEST! I WILL NOT GIVE YOU AN EXCEPTION TO SOLVE ON A TEST!

22. Elements of the Sixth and Seventh Period The progression for filling sublevels along the sixth period is the 6s, then the 5d, then 4f, then 6p, but the 4f will fill up before the 5d. The progression for filling sublevels along the sixth period is the 6s, then the 5d, then 4f, then 6p, but the 4f will fill up before the 5d. The energies between the 5d and 4f are very close, that is why there are many deviations from what would be expected. The energies between the 5d and 4f are very close, that is why there are many deviations from what would be expected. The seventh period is incomplete and is mostly made up of manmade elements. The seventh period is incomplete and is mostly made up of manmade elements.