This cheat sheet compares ionic and covalent compounds by looking at how their particles are held together and how that affects their properties. Students need this reference because bonding type helps predict melting point, state at room temperature, conductivity, and solubility. It also helps explain why salts behave differently from molecular substances like water or sugar.
Key Facts
- Ionic compounds form when electrons are transferred, usually from a metal to a nonmetal, creating ions such as and .
- Covalent compounds form when nonmetal atoms share electrons, as in , , and .
- An ionic formula gives the lowest whole-number ratio of ions, so and combine as .
- A covalent molecular formula gives the actual number of atoms in one molecule, such as .
- Ionic compounds usually have high melting points because strong attractions hold oppositely charged ions in a crystal lattice.
- Covalent molecular compounds usually have lower melting points because the forces between separate molecules are often weaker than ionic attractions.
- Ionic compounds conduct electricity when molten or dissolved in water because mobile ions can carry charge.
- Most covalent molecular compounds do not conduct electricity because they do not form many charged particles in solution.
Vocabulary
- Ionic bond
- An ionic bond is the attraction between oppositely charged ions formed after electrons are transferred.
- Covalent bond
- A covalent bond is a bond in which atoms share one or more pairs of electrons.
- Ion
- An ion is an atom or group of atoms with a positive or negative charge because it has lost or gained electrons.
- Crystal lattice
- A crystal lattice is a repeating three-dimensional arrangement of positive and negative ions in an ionic solid.
- Molecule
- A molecule is a neutral group of atoms held together by covalent bonds.
- Electrolyte
- An electrolyte is a substance that produces mobile ions in water or when molten, allowing electricity to flow.
Common Mistakes to Avoid
- Calling a molecule is wrong because ionic compounds are made of repeating ion ratios, not separate neutral molecules.
- Assuming all compounds with high melting points are covalent is wrong because ionic crystal lattices often require large amounts of energy to melt.
- Saying solid conducts electricity is wrong because its ions are locked in place and cannot move through the solid.
- Writing instead of is wrong because one ion needs two ions to make a neutral compound.
- Thinking every covalent compound is insoluble in water is wrong because some polar covalent molecules, such as and sugar molecules, can dissolve well.
Practice Questions
- 1 What is the correct ionic formula for a compound made from ions and ions?
- 2 How many ions are needed to balance ion in a neutral ionic compound?
- 3 Classify each compound as mostly ionic or covalent: , , , and .
- 4 A substance is a brittle solid with a high melting point and conducts electricity only when dissolved in water. Explain whether it is more likely ionic or covalent.
Understanding Ionic vs Covalent Compound Properties Comparison
An ionic solid is not made of separate pairs that can move around easily. Each positive ion attracts many nearby negative ions, making a repeating three dimensional crystal pattern. This pattern explains why a salt crystal can be hard but brittle.
When the layers are pushed out of place, ions with the same charge can end up beside each other. Their strong repulsion causes the crystal to split. Covalent substances are often made of individual molecules.
The atoms inside each molecule are strongly connected, but the attraction between neighboring molecules may be much weaker. Heating must overcome these outside attractions before the substance can melt or boil.
Electrical conductivity depends on moving charged particles, not simply on whether a substance contains charged ions. In a solid ionic crystal, the ions are fixed in position, so they cannot carry current through the solid. When the crystal melts, the ions become free to move.
Water can separate ions from many ionic solids, producing a solution that conducts. A molecular substance can dissolve without becoming conductive. Sugar dissolves because water attracts parts of its molecules, yet the dissolved particles remain neutral molecules.
Some covalent substances are important exceptions. Acids such as hydrogen chloride can react with water to form ions, so their water solutions conduct electricity.
Melting point patterns need careful interpretation. Many small molecular substances are gases or liquids near room temperature because their molecules separate fairly easily. Larger molecules usually have stronger attractions between molecules, so they may be solids.
A special group called covalent network solids does not fit the usual molecular pattern. Diamond and silicon dioxide have atoms joined in a large continuous structure. Breaking or melting such a structure requires a great deal of energy.
Their melting points can be very high even though their bonds are covalent. Graphite is another useful exception. It has layers of covalently connected carbon atoms and can conduct along those layers because some electrons can move.
Solubility is influenced by the kind of particles in both the solute and the liquid. Water has uneven electrical charge across each molecule, so it can surround and stabilize many ions. This helps explain why many salts dissolve in water.
It does not mean every ionic compound is soluble. The attraction within some crystals is too strong for water to pull the ions apart effectively. When reading formulas, first decide whether the formula represents a ratio in a crystal or one separate molecule.
Then check charges for ionic formulas and count atoms for molecular formulas. Students often confuse dissolving with melting, or assume every dissolved substance conducts. Keeping particle movement in mind prevents both mistakes.