13.Formation of Ions

1. What is an Ion?

• Definition: An ion is a charged particle. It can be a single atom (e.g., Na+) or a group of atoms (e.g., hydroxide ion, OH-).

• Why they form: Atoms gain or lose electrons to achieve a full outer shell, making them more stable.

2. Ions and the Periodic Table

The group number of an element tells you how many electrons are in its outermost shell. Because gaining or losing electrons requires energy, atoms that only need to move one or two electrons are much more likely to form ions.

• Group 1: Have 1 outer electron. They easily lose it to become 1+ ions.

• Group 2: Have 2 outer electrons. They easily lose them to become 2+ ions.

• Group 6: Have 6 outer electrons. They gain 2 to become 2- ions.

• Group 7: Have 7 outer electrons. They gain 1 to become 1- ions.

• Groups 3, 4, and 5: These would need to move 3 or 4 electrons. This requires a lot of energy, so these elements rarely form ions.

3. Writing Ion Equations

You can write equations to show the gain or loss of electrons.

• Losing Electrons (Positive Ions): The electron is placed on the right side of the arrow to show it has been released.

  • Example (Sodium): Na → Na+ + e-

  • Example (Magnesium): Mg → Mg2+ + 2e-

• Gaining Electrons (Negative Ions): The electron is placed on the left side of the arrow to show it is combining with the atom.

  • Example (Chlorine): Cl + e- → Cl-

  • Example (Oxygen): O + 2e- → O2-