explain pauli's exclusion principle

Pauli Exclusion Principle

• No two electrons in an atom can have the same set of all the four quantum numbers.

• Two electrons can have the same value of three quantum numbers n, l, and m_e, but must have the opposite spin quantum number (s).

• The maximum number of electrons in the shell with the principal quantum number n is equal to 2n².

For example

In an orbital diagram, an electron is represented by an up arrow (↑) indicating a positive spin, or a down arrow (↓) indicating a negative spin.

And it can not be represented by both the arrows on the same side.