What happens to the melting and boiling points of alkali metals as you move down the group?

• A. They stay about the same
• B. They first decrease then increase
• C. They decrease
• D. They increase

Answer: (C)

The key idea here is how metallic bonding changes as you go down a group. In alkali metals, the melting and boiling points reflect the strength of the metallic bond—the attraction between the positive metal ions and the surrounding sea of delocalized electrons.

As you move to heavier alkali metals, the atoms get larger and have more electron shells. The outer electron is further from the nucleus and more shielded by inner electrons, so the attraction between the positively charged ions and the delocalized electrons weakens. With weaker metallic bonding, less energy is needed to overcome the lattice and to separate the particles, so both the melting point and the boiling point decrease. For example, lithium has higher melting and boiling points than cesium, which lie much lower on the group.