Principal Energy Level| Simple Examples

The principal energy level of an electron in an atom refers to the main “zone” or “level” in which the electron is found relative to the nucleus of the atom. It is identified by a quantum number called “n.” Each principal energy level can contain one or more sublevels, and electrons occupy these levels in an atom’s electron cloud. As you move across the periodic table, the first element in a row marks the beginning of a new principal energy level.

Examples

Element	Principal Energy Level (n)	Explanation
Hydrogen	1	Hydrogen, the simplest atom with atomic number 1, has only one electron which occupies the first energy level (n=1).
Helium	1	Helium, with atomic number 2, also has electrons in the first level (n=1). The first energy level can hold up to 2 electrons.
Lithium	2	Lithium, with atomic number 3, introduces a new level – electrons in level 2 (n=2). The second energy level can hold up to 8 electrons.
Oxygen	2	Oxygen, with atomic number 8, has electrons in the second energy level (n=2). The electrons in the first energy level are filled, and the second level can hold up to 8 electrons.
Sodium	1, 2, 3	Sodium, with atomic number 11, has electrons in levels 1, 2, and 3 (n=1, 2, 3). The first and second energy levels are filled, and the third level is being populated.

Rules of Principal Energy Levels

• Electrons fill energy levels sequentially, starting from the lowest (n=1) and moving to higher levels (n=2, 3, …).
• Each energy level has a maximum electron capacity determined by 2n².
• Electrons in higher energy levels are farther from the nucleus and possess higher energy.
• Before moving to the next level, electrons must completely fill the lower energy level.
• Energy levels are composed of sublevels (s, p, d, f), defining electron spatial distribution.
• Aufbau Principle: Electrons fill lower energy levels before higher ones.
• Hund’s Rule: Electrons occupy orbitals singly before pairing up.
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers.

Naming Convention for Principal Energy Levels

The naming convention for the different principal energy levels (shells) uses letters to represent the principal quantum numbers (n). The letters assigned to the first few energy levels are as follows:

• n = 1: This is often referred to as the “K shell.”
• n = 2: This is often referred to as the “L shell.”
• n = 3: This is often referred to as the “M shell.”

Following this pattern, subsequent energy levels are named using subsequent letters in the alphabet, such as N, O, P, and so on. Each letter corresponds to a different principal energy level or shell in the electron configuration of an atom.