Compare transfer with sharing
Ionic compounds contain positive and negative ions held together by the electrical attraction between opposite charges, called electrostatic attraction. In an introductory model, a metal atom often loses one or more electrons and a nonmetal atom gains them, producing ions whose total charges balance.
Molecular compounds consist of neutral molecules whose atoms are joined by covalent bonds. A covalent bond forms when atoms share electrons that are attracted to both nuclei; this pattern is common when two nonmetals combine.
Use the periodic table as a first clue
Metals occupy most of the left and center of the periodic table. Nonmetals cluster on the upper right, with hydrogen also classified as a nonmetal. A metal combined with a nonmetal is usually predicted to form an ionic compound, while two nonmetals usually form a molecular compound with covalent bonds.
The pattern is a starting point, not a perfect boundary. Some compounds contain polyatomic ions with covalent bonds inside each ion, and bonding can have a mixture of ionic and covalent character. Use the substance’s actual structure and evidence when those details are available.
Remember these ideas
Key ideas
- Ionic compounds contain ions with opposite charges that attract one another.
- Covalent bonds involve electrons attracted to both nuclei and commonly join nonmetal atoms.
- Metal plus nonmetal usually suggests ionic; nonmetal plus nonmetal usually suggests molecular.
- Periodic-table patterns support a prediction, but real bonding includes exceptions and mixed cases.
Make the reasoning visible
Classify two simple compounds
- 1
For sodium chloride, identify sodium as a metal and chlorine as a nonmetal.
- 2
Predict an ionic compound: sodium forms a positive ion and chlorine forms a negative ion, so opposite charges attract in a neutral compound.
- 3
For carbon dioxide, identify both carbon and oxygen as nonmetals.
- 4
Predict a molecular compound: carbon and oxygen atoms share electrons in covalent bonds within each carbon-dioxide molecule.
Try the next step
Make and explain two predictions
Predict whether calcium chloride, CaCl₂, and sulfur dioxide, SO₂, are more likely ionic or molecular. Explain the element pattern you used.
Why should you describe this periodic-table method as a prediction rather than a rule with no exceptions?