Putting it together
Everything in this chapter comes back to one question about a substance: were its electrons transferred, or are they shared?
The two lists
Ionic compounds
- Made of oppositely charged ions.
- Mostly solid crystalline lattices at room temperature.
- High melting and boiling points.
- Do not conduct electricity when solid.
- Conduct when molten or dissolved in water.
- Most dissolve in water.
Covalent compounds
- Usually exist as molecules of a few atoms.
- Mostly gases or liquids at room temperature.
- Low melting and boiling points, except covalent lattices.
- Some dissolve in water.
- Their solutions generally do not conduct electricity.
Deciding from an observation
| If you observe… | Then the bonding is likely… | Because |
|---|---|---|
| A solution that conducts | Ionic | Free ions are present to carry charge |
| A melting point in the hundreds | Ionic | A whole lattice of attractions must be broken |
| A gas at room temperature | Covalent | Only weak forces between molecules hold it |
| A very hard solid that does not conduct | A covalent lattice | Covalent bonds run through the whole solid, but no ions are present |
Worked example
The problem
A white solid melts at 993 °C. It does not conduct electricity as a solid, but its aqueous solution does. Is it ionic or covalent, and how do you know?
Solution
The melting point is in the hundreds, which points to a lattice of strong attractions rather than separate molecules.
It conducts in solution but not as a solid — the signature of ions that are locked in place until the lattice breaks down.
The compound is ionic.
Glossary
| English | Sinhala |
|---|---|
| Chemical bonds | රසායනික බන්ධන |
| Cation | කැටායනය |
| Anion | ඇනායනය |
| Ionic bonds | අයනික බන්ධන |
| Covalent bonds | සහසංයුජ බන්ධන |
| Polarity | ධ්රැවීයතාව |
| Intermolecular bond | අන්තර් අණුක බල |
| Hydrogen bond | හයිඩ්රජන් බන්ධන |
Chapter summary
- Compounds form when two or more atoms of an element bond chemically.
- In forming a compound, the electrons in the valence shells of the atoms are rearranged.
- An atom that releases electrons becomes a positive ion; an atom that gains electrons becomes a negative ion.
- The electrostatic attraction between oppositely charged ions is called an ionic bond.
- A bond formed by a pair of atoms holding electrons jointly is called a covalent bond.
- In an ionic compound the ions are arranged regularly into a solid crystalline ionic lattice.
- An atomic lattice forms when atoms are arranged into a definite pattern in space.
- Ionic and covalent compounds show characteristic properties according to the nature of their bonding.
- A bond made up of small negative and positive charges is a polar covalent bond, and water is a compound containing them.
- The attractions arising between molecules are intermolecular forces, and they give compounds special properties.
- The intermolecular attractions between water molecules are what give water its special properties.