Source: General Chemistry, Purdue University – Bonding lecture notes Difficulty: Introductory | Prerequisites: Basic understanding of metals vs non-metals on the periodic table
Chemical bonding is the reason atoms stick together to form the materials around you. This topic sits right at the start of most general chemistry courses because nearly everything that follows (reactions, stoichiometry, molecular geometry) depends on understanding how and why atoms bond.
You need to know where metals and non-metals sit on the periodic table before this will make sense. If you are shaky on that, review your periodic table layout first.
The three bonding types (metallic, ionic, covalent) each produce substances with very different physical properties, and exams love to test whether you can identify a bonding type from a description of those properties.
Metals bonded to metals give metallic bonds. A metal bonded to a non-metal gives an ionic compound. Two non-metals bonded together give a covalent (molecular) compound. Each type has a distinct set of physical properties, and you can identify the bond type from those properties alone.
Metallic bond
The bond formed when two or more metal atoms share their valence electrons across a "sea" of delocalised electrons. Think of it as metals pooling their outer electrons into a communal fund that holds the structure together.
Ionic bond
The electrostatic attraction between a positively charged metal ion (cation) and a negatively charged non-metal ion (anion). In simple terms, one atom hands over electrons and the other takes them, and the opposite charges lock them in place.
Covalent bond (molecular bond)
A bond formed when two non-metal atoms share one or more pairs of electrons. Think of it as two atoms each contributing electrons to a shared pool rather than transferring them outright.
Electrostatic interaction
The attractive or repulsive force between charged particles. Opposite charges attract; like charges repel. This is the glue in ionic compounds.
Molecular (covalent) compound
A discrete unit of atoms held together by covalent bonds, forming a molecule. In simple terms, each molecule is its own self-contained package of atoms.
Metal + metal = metallic bond
Metal + non-metal = ionic bond
Non-metal + non-metal = covalent bond
This is the first thing to check when you see a compound on an exam. Locate the elements on the periodic table, decide if each is a metal or non-metal, and the bond type follows directly.
Crystalline structure, hard and brittle
Very high melting and boiling points
Good electrical conductors when molten or dissolved in water (the ions are free to move)
Often soluble in water
Not soluble in non-polar solvents like carbon tetrachloride (CCl₄)
The high melting points come from the strong electrostatic forces holding the ion lattice together. It takes a lot of energy to pull those charges apart.
Can exist as gases, liquids, or solids at room temperature
Tend to be brittle and weak, or soft and waxy
Low melting and boiling points
Poor electrical conductors (no free ions or delocalised electrons)
Often soluble in non-polar solvents like carbon tetrachloride
Typically not soluble in water
The weak intermolecular forces between individual molecules explain the low melting points. The bonds within each molecule are strong, but the molecules themselves do not cling to each other with much force.
Property | Ionic | Covalent |
|---|---|---|
Melting/boiling point | Very high | Low |
Conductivity (molten/dissolved) | Good | Poor |
Soluble in water | Often yes | Often no |
Soluble in CCl₄ | No | Often yes |
Physical feel | Hard, brittle, crystalline | Soft/waxy or brittle/weak |
No specific formulas for this section. The key reference is the periodic table itself: the metal/non-metal divide (the staircase line) is your decision tool for bond type.
Table salt (NaCl) is the classic ionic compound: high melting point, dissolves in water, conducts electricity when dissolved. Candle wax is a covalent substance: low melting point, soft, does not dissolve in water. Being able to spot the bond type from physical behaviour is useful well beyond the exam, from materials science to cooking.
Students often assume that ionic compounds always dissolve in water. Many do, but not all. Solubility rules (covered later in the course) govern which ionic compounds dissolve and which do not.
Students sometimes think covalent compounds cannot be solids. They can. Sugar (sucrose) is a molecular solid at room temperature.
"Good conductor" for ionic compounds applies only when the compound is molten or dissolved. Solid ionic compounds do not conduct because the ions are locked in a lattice and cannot move.
⚠️ You will almost certainly be asked to identify the bond type from a list of physical properties, or to predict properties given a formula. Know the table above cold.
⚠️ Conductivity is a common trap question. Remember: ionic compounds conduct only when molten or in solution, never as a solid.
⚠️ Solubility in water vs CCl₄ is the quick test for ionic vs covalent. If it dissolves in CCl₄ but not water, it is covalent.
True or false: A compound with a very high melting point and good conductivity when dissolved is likely ionic. (True)
Fill in the blank: A bond between two non-metals is a _______ bond. (Covalent)
True or false: Solid NaCl conducts electricity. (False – only when molten or dissolved)
Fill in the blank: Ionic compounds are typically soluble in _______ but not in carbon tetrachloride. (Water)
Q: A substance is a brittle solid with a high melting point. It does not conduct electricity as a solid, but it does when dissolved in water. What type of bonding does it have?
A: Ionic. The high melting point, brittleness, and conductivity when dissolved are all hallmarks of ionic compounds.
Q: Compound X is a soft, waxy solid with a low melting point. It dissolves in CCl₄ but not in water. Identify the bond type.
A: Covalent (molecular). Low melting point, solubility in a non-polar solvent, and insolubility in water point to a molecular compound.
Q: Why do ionic compounds conduct electricity when molten but not when solid?
A: In the solid state, the ions are fixed in a crystal lattice and cannot move. When melted (or dissolved), the ions become free to move and carry charge.
Q: Metal + non-metal produces which type of bond?
A: Ionic.
This connects directly to molecular geometry and VSEPR theory, which builds on the idea of shared electron pairs in covalent bonds. It also connects to solubility rules later in the course, which tell you exactly which ionic compounds dissolve in water. Understanding bond type is also the foundation for predicting intermolecular forces, which govern boiling points and other bulk properties.
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