Periodicity of physical properties of the elements in Period 3 — common mistakes

When asked to explain trends in melting point or boiling point, always state the structure (e.g., giant metallic, simple molecular) and the specific type of bonding or force being broken or overcome (e.g., strong metallic bonds, weak van der Waals' forces).

This is due to a fundamental change in structure and bonding. Silicon has a giant covalent structure, where melting requires breaking thousands of strong covalent bonds, needing immense energy. Phosphorus, on the other hand, exists as small, discrete P₄ molecules. To melt solid phosphorus, you only need to provide enough energy to overcome the weak van der Waals' forces between these molecules, which requires far less energy.

Not always. For metals that form positive ions (cations) like Na⁺ and Mg²⁺, the ionic radius is smaller than the atomic radius because the atom has lost its entire outer shell of electrons and the remaining electrons are pulled in more tightly by the unchanged nucleus. For non-metals that form negative ions (anions) like P³⁻ and Cl⁻, the ionic radius is larger than the atomic radius because electrons are added to the outer shell, increasing electron-electron repulsion and causing the electron cloud to expand.