8. Electronic Structure

Electrons are arranged in shells (energy levels) around the nucleus. There are specific rules for how these shells fill up:

The First Shell: Closest to the nucleus, it has the lowest energy level and can hold a maximum of 2 electrons.
The Second Shell: Can hold a maximum of 8 electrons.
The Third Shell: Can also hold a maximum of 8 electrons.
Filling Order: It is best to add electrons one at a time (top, right, bottom, left) before pairing them up.

Stable Atoms: An atom is stable when its outermost shell is completely full.
Unstable Atoms: Most atoms have incomplete outer shells. To become stable, they must react with other atoms to either gain or lose electrons.
Noble Gases: Elements in Group 0 (e.g., Neon, Argon) already have full outer shells. Because they are stable, they are unreactive.

You can show the arrangement of electrons in two ways:

Dot and Cross Diagrams: Draw the nucleus and the shells as rings, placing dots or crosses to represent electrons.
Numerical Representation: List the number of electrons in each shell, separated by commas or spaces.
- Example (Argon): Atomic number 18. Structure is 2, 8, 8.
- Example (Calcium): Atomic number 20. Structure is 2, 8, 8, 2.

When atoms gain or lose electrons to become stable, they become ions.

Positive Ions (Cations): Formed when an atom loses outer electrons.
- Example (Calcium): A calcium atom (2, 8, 8, 2) loses 2 electrons to become a Ca²⁺ ion with a structure of 2, 8, 8.
Negative Ions (Anions): Formed when an atom gains electrons to fill its outer shell.
- Example (Fluorine): A fluorine atom (2, 7) gains 1 electron to become a F⁻ ion with a structure of 2, 8.
Drawing Ions: Always place large square brackets around the ion diagram and write the overall charge in the top right corner.