1. Unit 3 Part 2: Electrons

2. Electrons in an Atom
Bohr’s Model: Electrons resided in an “allowed” orbit.
Quantum Mechanics Model: Probability of finding an electron in an area around the nucleus.
This area around the nucleus is called an orbital.

3. Energy of Orbitals
Orbitals can be described using a few important numbers.
1. Principal quantum number: indicates relative size and energy of atomic orbitals.
Bigger n = bigger orbital.
The principal quantum number specifies the atom’s major energy levels, which are called principal energy levels.
The lowest principal energy level is assigned a n of 1.

4. Energy of Orbitals 2. Sublevels
Principal energy levels contain energy sublevels.
Each principal energy level contains a set number of sublevels.
The sublevels are labeled as s, p, d or f.
Each sublevel is related to a specific number of orbitals.

5. Energy of Orbitals n=1 1 n=2 2 n=3 3 n=4 4 2. Sublevels 1s 2s, 2p
Principal energy level
Number of sublevels
Sublevel name
Number of orbitals in each sublevel
n=1 1 1s
1s: one orbital
n=2 2
2s, 2p
2s: one orbital
2p: three orbitals
n=3 3
3s, 3p, 3d
3s: one orbital
3p: three orbitals
3d: five orbitals
n=4 4
4s, 4p, 4d, 4f
4s: one orbital
4p: three orbitals
4d: five orbitals
4f: seven orbitals

6. Energy of Orbitals 2. Sublevels
For a given principal energy level (eg. n=3), the energy of the orbitals increases in this pattern:
s(lowest energy) < p < d < f (highest energy)
Each orbital in the same subshell has the same energy.

7. Orbitals
The set of orbitals with the same principal quantum number is called an electron shell.
Eg.
The set of orbitals with the same principal quantum number AND the same shape of orbital is called a subshell.

8. Orbitals

9. Orbitals Electrons “occupy” a certain orbital at any given instant.
When an electron occupies the lowest energy orbitals, the electron is said to be in its ground state.
Electrons can be excited to a higher-energy orbital by absorption of a photon of appropriate energy.

10. Shapes of Orbitals
s orbital

11. Shapes of Orbitals
p orbitals

12. Shapes of Orbitals
d orbitals

13. Shapes of Orbitals
f orbitals

14. Electron Configurations
The arrangement of electrons around a nucleus for a particular atom is called the atom’s electron configuration.
There are three rules you must follow when you write an electron configuration for any atom.
1. Aufbau Principle
2. Pauli Exclusion Principle
3. Hund’s Rule

15. Electron Configurations
1. Aufbau Principle: Electrons occupy the lowest energy orbital available first.
2. Pauli Exclusion Principle: Only two electrons can occupy any orbital at a time.
3. Hund’s rule: Electrons occupy all the unoccupied orbitals of the same energy subshell first, before pairing up and filling the orbitals.
Eg. Carbon, Nitrogen, Oxygen

16. Electron Configurations
There are two ways you can write the electron configuration for any atom.
1. Orbital Diagram eg.
2. Electron Configuration Notation eg.

17. Electron Configurations
Noble gas configurations/Condensed electron configuration:
Shortened form. Noble gas of the closest lower atomic number given in square brackets:
Eg. Neon Sodium Magnesium Argon Potassium

18. Electron Configurations
Textbook pg. 160 Q 21-25
p.167 Q 77, 85

19. Valence Electrons
Recall that electron shells are all orbitals of the same principal quantum number.
Valence electrons, also called outer-shell electrons, are all electrons in the atom’s outermost shell.
Eg. Sulfur Silicon Fluorine

20. Valence Electrons
Because valence electrons are involved in chemical reactions, electron-dot structures are drawn, which only show the atom’s valence electrons.
Dots representing valence electrons are drawn around the chemical symbol for the atom.
Eg. Draw the dot structures for period 1. Textbook p.162 Q 26-28