1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

Here's how to understand it:

* Principal Quantum Numbers (n): The numbers (1, 2, 3, 4) represent the energy levels of the electrons.

* Sublevels (s, p, d, f): These letters represent the different shapes of electron orbitals within each energy level.

* Superscripts: The superscripts (2, 6) indicate the number of electrons occupying each sublevel.

Explanation:

* 1s²: The first energy level (n=1) has one sublevel, the 's' sublevel, which can hold up to 2 electrons.

* 2s² 2p⁶: The second energy level (n=2) has two sublevels: the 's' sublevel (2 electrons) and the 'p' sublevel (6 electrons).

* 3s² 3p⁶: The third energy level (n=3) also has two sublevels: the 's' sublevel (2 electrons) and the 'p' sublevel (6 electrons).

* 4s²: The fourth energy level (n=4) begins with the 's' sublevel, which contains the last two electrons of calcium.

Key Points:

* Aufbau Principle: Electrons fill orbitals in order of increasing energy.

* Hund's Rule: Within a sublevel, electrons fill orbitals individually before pairing up.

* Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers.

Let me know if you'd like to see the orbital diagram for calcium!