5.3.1 Ionic Bonds

This topic explains ionic bonds in detail — how they form, what happens to electrons, and how to represent their formation using electron dot and cross structures.

Definition

Ionic bonds are formed between two atoms when one atom loses electrons to form a cation and the other atom gains those electrons to form an anion.

Simple explanation: An ionic bond is the attraction between a positive ion and a negative ion.

More complete definition (from the SLOs): An ionic bond is a strong electrostatic attraction between positively charged metal ions and negatively charged non-metal ions.

Main Idea

Ionic bonding happens between a metal and a non-metal:

• The metal loses electrons → forms a cation (positive ion).
• The non-metal gains electrons → forms an anion (negative ion).
• The opposite charges attract → ionic bond.

Simple explanation: One atom gives, the other takes. The resulting opposite charges stick together.

Core Concept

Formation of Cations (Metals)

Example 5.1: Describing the formation of Na and Mg cations

• Sodium belongs to Group IA. It has one electron in the valence shell. The sodium atom loses its valence electron and is left with an octet.
• Magnesium belongs to Group IIA. It has two valence electrons. A magnesium atom loses these electrons to achieve noble gas configuration.

Solution:

(a) Formation of Na⁺ ion:

Na(1s² 2s² 2p⁶ 3s¹) -e^-→ Na⁺(1s² 2s² 2p⁶)

(b) Formation of Mg²⁺ ion:

Mg(1s² 2s² 2p⁶ 3s²) -2e^-→ Mg²⁺(1s² 2s² 2p⁶)

You can also represent these by electron dot structures.

Formation of Anions (Non-Metals)

Example 5.2: Describing the formation of anions

Describe the formation of anions for oxygen (atomic no. 8) and fluorine (atomic no. 9).

• Write electronic configuration or dot structure.
• Find the number of electrons needed to acquire eight-electron configuration.
• Represent addition of electrons.

Solution:

(a) Formation of anion by oxygen atom:

Oxygen belongs to Group VIA. It has six electrons in its valence shell. It needs two electrons to achieve noble gas configuration.

O(1s² 2s² 2p⁴) + 2e^- → O^2-(1s² 2s² 2p⁶)

(b) Formation of anion by fluorine atom:

Fluorine belongs to Group VIIA. It has seven electrons in the valence shell. It requires only one electron to complete octet.

F(1s² 2s² 2p⁵) + e^- → F^-(1s² 2s² 2p⁶)

Detailed Explanation

What Is an Ionic Bond?

Anions and cations have opposite charges. They attract one another by strong electrostatic forces.

Compounds that consist of ions joined by electrostatic forces are called ionic compounds.

The total positive charge of the cations must be equal to the total negative charge of the anions. This is because ionic compounds are electrically neutral as a whole.

Simple explanation: In an ionic compound, the positive and negative charges must balance out.

Example 5.3: Representing Ionic Bond Formation

For each of the following pairs of atoms, use electron dot & electron cross structures to write the equation for the formation of ionic compound.

(a) Na and Cl (b) Mg and F

• The metal atoms form cations and non-metal atoms form anions.
• The number of electrons lost by metal atoms of group IA, IIA, and IIIA equals the group number.
• To write the final form of the equation, you need to know the simplest ratio of cations to anions required for the neutral compound.
• Write equation using electron dot and electron cross structures.

Solution:

(a) Na and Cl:

Na is a metal and Cl is a non-metal.

• A Na atom has one electron in the outermost shell. It loses one electron to form Na⁺.
• A Cl atom has seven electrons in the outermost shell. It needs one electron to complete octet. So it gains one electron to form Cl⁻.
• For every Na⁺ ion, you need one Cl⁻ ion.

2Na + Cl₂ → 2NaCl (Sodium Chloride)

(b) Mg and F:

Mg is a metal and F is a non-metal.

• A Mg atom has two electrons in the outermost shell. It loses two electrons to form Mg²⁺.
• A F atom has seven electrons in the outermost shell. It gains one electron to form F⁻.
• For every Mg²⁺ ion, you need two F⁻ ions.

Mg + F₂ → MgF₂ (Magnesium Fluoride)

Example 5.4: Recognizing a Compound as Having Ionic Bonds

Recognize the following compounds as having ionic bonds: (a) MgO (b) NaF

• The metal atom loses electrons to form cations and non-metal atom gains electrons to form anions.
• The number of electrons lost by metal atoms of group IA, IIA, and IIIA equals the group number. The number of electrons gained by the non-metal atoms is equal to 8 minus group number.
• Find the simplest ratio of cations to anions to identify the compound.

Solution:

(a) MgO:

• Mg is a metal. It has two electrons in the outermost shell. It loses two electrons to form Mg²⁺.
• O is a non-metal. It has six electrons in the outermost shell. It gains two electrons to form O²⁻.
• For every Mg²⁺ ion, you need one O²⁻ ion.
• Chemical formula: MgO.
• Therefore, MgO is an ionic compound.

(b) NaF:

• Na is a metal. It has one electron in the outermost shell. It loses one electron to form Na⁺.
• F is a non-metal. It has seven electrons in the outermost shell. It gains one electron to form F⁻.
• For every Na⁺ ion, you need one F⁻ ion.
• Chemical formula: NaF.
• Therefore, NaF is an ionic compound.

Intermediate Level

Why Ionic Bonds Form

• Metals have low ionization energy (easy to lose electrons).
• Non-metals have high electronegativity (strong tendency to gain electrons).
• Both achieve noble gas configuration by transferring electrons.

Electron Dot and Cross Structures

• Dots (·) represent electrons from one atom.
• Crosses (×) represent electrons from the other atom.
• Arrows show the transfer of electrons.

Example: NaCl

Na · + · Cl : → Na⁺ [:Cl:]^-

Charge Balance in Ionic Compounds

The total positive charge must equal the total negative charge.

Properties of Ionic Compounds

Important Points

• - Ionic bonds: formed by transfer of electrons from a metal to a non-metal.
• - Metal loses electrons → forms cation.
• - Non-metal gains electrons → forms anion.
• - Ionic bond = strong electrostatic attraction between oppositely charged ions.
• - Ionic compounds are electrically neutral — total positive charge = total negative charge.
• - Examples:
• - Na + Cl → NaCl
• - Mg + O → MgO
• - Mg + F → MgF₂
• - Al + O → Al₂O₃
• - Electron dot and cross structures are used to represent ionic bond formation.
• - Ionic compounds have high melting/boiling points and conduct electricity when molten or dissolved.

Remember

• - Ionic bond = transfer of electrons (metal → non-metal).
• - Metal loses electrons → cation.
• - Non-metal gains electrons → anion.
• - Opposite charges attract → ionic bond.
• - Ionic compounds are neutral overall.
• - Examples: NaCl, MgO, MgF₂, Al₂O₃.
• - Na → Na⁺ (loses 1 e⁻).
• - Mg → Mg²⁺ (loses 2 e⁻).
• - Cl → Cl⁻ (gains 1 e⁻).
• - O → O²⁻ (gains 2 e⁻).

Common Mistakes

• 1. Thinking ionic bonds involve sharing. They involve transfer of electrons.
• 2. Thinking ionic bonds form between non-metals. They form between metals and non-metals.
• 3. Forgetting charge balance. The total positive charge must equal the total negative charge.
• 4. Thinking ionic compounds are neutral because they have no charge. They are neutral because the charges balance out.
• 5. Confusing cations and anions. Cations are positive; anions are negative.
• 6. Thinking ionic compounds conduct electricity in solid state. They conduct only when molten or dissolved (ions free to move).

Quick Revision

• Ionic bonds: formed between two atoms when one atom loses electrons to form a cation and the other gains electrons to form an anion.
• Metal loses electrons → forms cation (positive ion).
• Non-metal gains electrons → forms anion (negative ion).
• Ionic bond = strong electrostatic attraction between oppositely charged ions.
• Ionic compounds are electrically neutral — total positive charge equals total negative charge.
• Examples:
• Na + Cl → NaCl
• Mg + O → MgO
• Mg + F → MgF₂
• Al + O → Al₂O₃
• Electron dot and cross structures are used to represent ionic bond formation.
• Ionic compounds have high melting/boiling points, conduct electricity when molten or dissolved, and are often soluble in water.

60-Second Revision

• Ionic bond = transfer of electrons (metal → non-metal).
• Metal → cation; non-metal → anion.
• Opposite charges attract → ionic bond.
• Ionic compounds are neutral.
• Examples: NaCl, MgO, MgF₂.
• Warning: ionic bonds involve transfer, not sharing.