Ionic compounds are formed by the transfer of electrons from one atom to another. The electrostatic force of attraction between the positive and negative ions holds the compound together. Ionic compounds are typically hard, brittle, and have high melting points. Examples include sodium chloride (NaCl), potassium iodide (KI), and calcium fluoride (CaF2).

Covalent compounds are formed by the sharing of electrons between atoms. The covalent bond is a strong bond that results from the attraction between the positively charged nuclei and the negatively charged electrons. Covalent compounds are typically soft, have low melting points, and are soluble in organic solvents. Examples include methane (CH4), water (H2O), and carbon dioxide (CO2).

Metallic compounds are formed by the attraction between metal atoms. The metallic bond is a weak bond that results from the attraction between the positively charged metal ions and the negatively charged electrons. Metallic compounds are typically hard, shiny, and have high melting points. Examples include copper (Cu), aluminum (Al), and iron (Fe).