1. Arrangement of Electrons in Atoms
Chapter 4
Arrangement of Electrons in Atoms

2. Electromagnetic Spectrum

3. Photoelectric Effect

4. Quantization of Energy

5. Energy of a Photon

6. Absorption and Emission Spectra

7. Bohr Model of the Atom

8. Electron Cloud

9. Quantum Theory
- describes mathematically the wave properties of electrons.
Definitions:
orbital: three-dimensional region around the nucleus that indicates the probable location of an electron. (Each can hold a maximum of 2 electrons.)
ground state: lowest energy state electrons in an atom have.
excited state: state in which electrons in an atom have a higher energy than ground state.

10. Quantum Numbers
Specify the properties of atomic orbitals and the properties of electrons in orbitals.
Symbol Description
Principal Quantum # n main energy level occupied by an electron
Angular momentum # l shape of orbital in a particular sublevel
Magnetic Quantum # m orientation of orbital
around nucleus
Spin Quantum # s direction of spin of electron

11. Energy Levels and Sublevels
each principal energy level (n) has one or more sublevels.
the number of sublevels is the same as the principal quantum number
First Principal Energy Level (n=1) has 1 sublevel
Second Principal Energy Level (n=2) has 2 sublevels
Third Principal Energy Level (n=3) has 3 sublevels
Each electron in a given sublevel has the same energy

12. Sublevels continued… Sublevels are named using letters:
- the first sublevel is called s
- the second sublevel is called p
- the third sublevel is called d
- the fourth sublevel is called f
and so on (g, h …..)
If n=1, how many sublevels are there and what are they called?
Answer: 1 sublevel called s
If n=2, how many sublevels are there and what are they called?
Answer: 2 sublevels called s and p

13. Orbitals Each sublevel contains 1 or more orbitals
s - sublevel has 1 orbital
p - sublevel has 3 orbitals
d - sublevel has 5 orbitals
f - sublevel has 7 orbitals
Remember each orbital contains a maximum of 2 electrons.
The maximum number of electrons per sublevel:
s → 2
p → 6
d → 10
f → 14

14. More Orbitals
Angular Momentum and Magnetic Quantum Numbers

15. Quantum Numbers and Orbitals

16. Shapes of s, p, and d Orbitals

17. Electrons in Energy Levels and Sublevels
Principal Sublevels # of orbitals # of electrons Total electrons
Energy available in sublevel possible in for energy level
Level sublevel
(n) (n) (n2) (2n2) 1 s 1 2 2 s 1 2 2 8 p 3 6 s 1 2 3 p 3 6 18 d 5 10 s 1 2 p 3 6 4 32 d 5 10 f 7 14

18. Electron Configurations
shows the arrangement of electrons in an atom
there are 3 different ways to show electron configurations: Orbital notation
2. Electron-configuration notation
3. Noble gas notation
- electrons are in the ground state unless otherwise noted.
unfortunately, there is energy overlap beginning at n = 3.
- How can we predict the sublevel order if this occurs?

19. Aufbau Principle 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f
- electrons occupy the lowest energy levels first. 1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f

20. Pauli Exclusion Principle
Hund’s Rule
- Before a second electron can be placed in any orbital, all the orbitals of that sublevel must contain at least one electron.
Pauli Exclusion Principle
- In order for two electrons to occupy the same orbital they must have opposite spin.
Relates to the Spin Quantum Number (s = +1/2 or -1/2)
- Electrons spin clockwise or counterclockwise.

21. Orbital Notation When illustrating orbital notation for an element:
1. Boxes are used to represent orbitals
2. Each box is labeled with principal energy level and sublevel.
3. Arrows are used to represent electrons.
Examples
Hydrogen
Lithium
Aluminum ↑ ↑ ↑↓ 1s ↑↓ ↑ 1s 2s ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ 1s 2s 2p 3s 3p

22. Electron Configuration Notation
When writing electron configurations for an element:
1. Boxes are not used.
2. The principal energy level is written, followed by the sublevel.
3. The total number of electrons are superscripted on each sublevel.
Examples
Helium
Sodium
Bromine
1s2
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

23. Noble Gas Notation
Noble gas notation may be used for elements beginning with period 3:
1. Find the period the element in question is in.
2. Locate the closest noble gas (must have fewer electrons than the element in question).
3. Write the symbol of the noble gas in brackets (This represents ‘x’ number of electrons).
4. Continue the notation with the principal energy level of the period the element is located in.
Examples
Chlorine
Iron
Iodine
[Ne]3s2 3p5
[Ar]4s2 3d6
[Kr]5s2 4d10 5p5

24. Noble-Gas Notation

25. Blocks of the Periodic Table
- The periodic table can be used to predict the order in which electrons enter sublevels.

26. Valence Electrons
- Electrons that are located in the highest principal energy level.
- The maximum number of valence electrons and element can have is eight.
- Usually valence electrons are found only in s and p sublevels.
- Electrons that are not valence electrons are inner-shell electrons.
Examples
Bromine
Chlorine
Iron
[Ne]3s2 3p5
[Ar]4s2 3d6
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5