BackAtomic Shells and Quantum Numbers: Study Notes for GOB Chemistry
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Atomic Structure and Electron Shells
Electron Shells and Energy Levels
Atoms are composed of electrons that occupy specific energy levels, or shells, surrounding the nucleus. These shells are associated with the potential energy of the electrons.
Shell (n): A grouping of electrons defined by the principal quantum number, n.
As the value of n increases, both the size and energy level of an atomic orbital increase.
The energy levels (shell numbers) of an atom can be related to the period (row) of the Periodic Table.
Limitation: The principal quantum number n must be an integer from 1 to ∞ (in practice, up to 7 for known elements).
Principal Quantum Number (n)
The principal quantum number (n) determines the main energy level or shell of an electron in an atom.
Allowed values: n = 1, 2, 3, ...
Physical meaning: Higher n values correspond to shells that are farther from the nucleus and have higher energy.
Relation to the Periodic Table: The period number for main group elements often matches the highest occupied shell (value of n).
Examples and Practice
Example: Which of the following is a possible value for the shell of an atom?
a) n = 3
b) n = 4
c) n = 0
d) n = 2 (Correct answer: n must be a positive integer, so n = 2 is valid)
e) n = -5
Practice: Which of the following shell number values is a possible value for the element highlighted (Selenium, Se, in period 4)?
a) n = 4 (Correct answer: Selenium is in period 4, so its highest occupied shell is n = 4)
b) n = 6
c) n = 7
Lowest Energy Electron
Electrons in the shell closest to the nucleus (n = 1) have the lowest potential energy. As n increases, electrons are found farther from the nucleus and have higher energy.
Lowest energy electron: The electron in the n = 1 shell (closest to the nucleus).
Summary Table: Principal Quantum Number and Shell Properties
Principal Quantum Number (n) | Shell Name | Relative Energy | Maximum Number of Electrons |
|---|---|---|---|
1 | K | Lowest | 2 |
2 | L | Higher than n=1 | 8 |
3 | M | Higher than n=2 | 18 |
4 | N | Higher than n=3 | 32 |
Key Equations
Maximum electrons in a shell:
Energy of an electron in a hydrogen atom:
Additional info: The above equation gives the energy of an electron in the nth shell for a hydrogen atom. For multi-electron atoms, electron-electron repulsion and shielding effects must also be considered.
