Ions & Ionic Bonds
The Big Idea: Atoms gain or lose electrons to achieve a stable outer shell, forming charged ions that attract each other through powerful electrostatic forces, creating ionic compounds.
Chapter Summary
Ion Formation
Atoms lose or gain electrons to fill their outer shell like noble gases, becoming positively or negatively charged.
Cations & Anions
Cations: Metals lose electrons (positive). Anions: Non-metals gain electrons (negative).
Ionic Bonding
Opposite ions attract through electrostatic forces, holding ionic compounds together in giant lattice structures.
Properties
Solid ionic compounds have high melting points but don't conduct electricity. Molten or dissolved compounds do conduct.
1. How Are Ions Formed?
What Is an Ion?
An ion is an electrically charged atom or group of atoms. Ions form when atoms lose or gain electrons. This happens because atoms are "driven" to achieve a full outer shell of electrons — the same electron configuration as the nearest noble gas.
Think of it like this: Noble gases (like neon and argon) are super stable because their outer shells are completely full. Other atoms want to copy this stability, so they either dump electrons they don't need or grab extras to fill up.
The Two Types of Ions
Cations (Positive Ions)
Cations form when atoms lose electrons. If an atom loses electrons, it has more protons than electrons, so it becomes positively charged. All metals form cations — this is their natural behaviour.
Example: Sodium Atom (Na) → Sodium Ion (Na⁺)
Neutral sodium atom: 11 protons, 11 electrons = no charge
Loses 1 electron: 11 protons, 10 electrons = 1+ charge
Why? Sodium is in Group 1, so it has 1 electron in its outer shell. It's easier to lose that one electron than to gain 7 more to fill up. After losing it, sodium has the same electron configuration as neon (a noble gas) — stability achieved!
Anions (Negative Ions)
Anions form when atoms gain electrons. If an atom gains electrons, it has more electrons than protons, so it becomes negatively charged. All non-metals form anions — this is their natural behaviour.
Example: Chlorine Atom (Cl) → Chloride Ion (Cl⁻)
Neutral chlorine atom: 17 protons, 17 electrons = no charge
Gains 1 electron: 17 protons, 18 electrons = 1– charge
Why? Chlorine is in Group 7, so it has 7 electrons in its outer shell. It needs just 1 more to have 8 (a full outer shell like argon). Gaining 1 electron is much easier than losing 7. After gaining it, chlorine has the same electron configuration as argon — stability achieved!
The Ion Charge Rule
The periodic table tells you exactly how many electrons an atom will lose or gain. Here's the golden rule:
⚡ Ion Charges from Periodic Table Groups
- Group 1 elements: Form ions with a 1+ charge (lose 1 electron)
- Group 2 elements: Form ions with a 2+ charge (lose 2 electrons)
- Group 6 elements: Form ions with a 2– charge (gain 2 electrons)
- Group 7 elements: Form ions with a 1– charge (gain 1 electron)
Memory trick: