1.3.3—Main energy levels

A main energy level, or shell, is identified by the principal quantum number n = 1, 2, 3, and so on.

$maximum electrons = 2n²$

n	Maximum electrons
1	2
2	8
3	18
4	32

Substitute the stated n value into 2n²; do not confuse the shell number with the capacity.

For n = 3, the theoretical capacity is 2(3²) = 18 electrons. This is a capacity, not a claim that every third shell is full: the actual occupancy depends on the atom and the relative energies of available sublevels.

How it is assessed

Short multiple-choice questions ask learners to calculate the maximum electron capacity for a stated main energy level using the 2n² rule.

Command terms

state

What earns marks

Substitute the stated integer n into 2n² and select or state the resulting maximum electron count.

Watch for

Using n² instead of 2n², or confusing a main energy level's total capacity with the capacity of one subshell or orbital.

Representative question

[Maximum number: 1]

What is the maximum number of electrons that can occupy the n=3 main energy level?

Retrieve the chain: emission lines reveal discrete levels; capacities, sublevels, orbitals, and spin rules build configurations; first and successive ionization energies then reveal how electrons are held and arranged.

When checking an answer, ask: Did I link a line to a transition? Did I use 2n² and the filling rules? Did I explain an ionization trend or count electrons before a successive-IE jump?

1.3.3—Main energy levels

Main Energy-Level Capacity

Calculating Shell Capacity

Assessment in practice

Electron Configurations Summary

Question 1