UNIT # 3 Introduction Objectives 3.1 Structure of the Atom 3.2 3.3 Arrangement of Electrons 3.4 3.5 Atomic Mass Problems 1 | 2 | 3

# ATOMIC STRUCTURE 3.3 - Arrangement of Electrons

3.3.3a - Electrons in the Sublevels

Each orbital can contain a maximum of two electrons. Wolfgang Pauli states that if two electrons occupy the same orbital they must have opposite spin. This is known as the Pauli exclusion principle.

Recall, that from the Bohr model (in section 3.3.1), the maximum number of electrons that can occupy a principal energy level n is

A detail look at principal energy level 3, n = 3

1. Bohr model predicts that a maximum of 2(3)2 = 18 electrons can reside in the 3rd principal energy level.
2. Quantum Mechanical Model of the atom predicts that in principal energy level 3, there are 3 sublevels: 3s, 3p, and 3d.
3. In any s sublevel, there is 1 atomic orbital. Therefore, there is 1 atomic orbital in the 3s sublevel.
4. In any p sublevel, there are 3 atomic orbitals. Therefore, there are 3 atomic orbitals in the 3p sublevel.
5. In any d sublevel, there are and 5 atomic orbitals. Therefore, there are 5 atomic orbitals in the 3d sublevel.
6. Since each atomic orbital can accomodate 2 electrons, it follows that:
• 1 atomic orbital in 3s sublevel x 2 electrons/orbital = 2 electrons can reside in the 3s sublevel
• 3 atomic orbitals in 3p sublevel x 2 electrons/orbital = 6 electrons can reside in the 3p sublevel
• 5 atomic orbitals in 5d sublevel x 2 electrons/orbital = 10 electrons can reside in the 3d sublevel
7. Add up the number of electrons in step 6 to get a total of 2 + 6 + 10 = 18 electrons (as predicted in step 1). The Quantum Mechanical Model allows us to see how the 18 electrons are distributed in each sublevel within the 3rd principal energy level.

Click to see a summary of information about the distribution of electrons in each principal energy level.

Section 4.8
Arrangement of the Electrons in Sublevels ..p91

Section 4.9
Orbitals ..p97

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