Substance X has a melting point of 1083 °C and substance Y has a melting point of −39 °C. Using ideas about forces between particles, explain why these two substances have such different melting points.

Substance X is a metal, so its atoms are held together by metallic bonds, which are strong and involve a delocalised sea of electrons.

Substance Y is a liquid at room temperature, so it must be a non‑metal or a weakly bonded compound; its particles are held together by much weaker van der Waals forces.

Because the melting point is the temperature at which the energy supplied is enough to overcome the forces between particles, the stronger metallic bonds in X require more energy to break, giving X a high melting point of 1083 °C.

In contrast, the weaker van der Waals forces in Y need far less energy to separate, so Y melts at only –39 °C.