Structure decides melting point

By the end of this lesson

By the end you can predict and explain the melting and boiling points of substances from their structure and bonding.

The one question that sorts everything

To melt or boil a substance, you must overcome the forces between its particles.

So ask: what has to be overcome?

structure	what holds the particles together	what melting/boiling must overcome	typical melting point
simple molecular	weak intermolecular forces between molecules	those weak forces	low (iodine $114\ ^\circ\text{C}$ , ice $0\ ^\circ\text{C}$ )
giant ionic	electrostatic attraction between ions	the strong ionic attraction throughout the lattice	high ( $\text{NaCl}$ $801\ ^\circ\text{C}$ )
giant molecular	strong covalent bonds everywhere	covalent bonds throughout the network	very high ( $\text{SiO}_2$ about $1700\ ^\circ\text{C}$ ; diamond sublimes near $3800\ ^\circ\text{C}$ )
giant metallic	metallic bonding (ions + delocalised electrons)	metallic bonding	generally high (copper $1085\ ^\circ\text{C}$ )

Definition

Melting point and structure

The melting point of a substance depends on the strength of the forces between its particles: stronger forces → more energy needed → higher melting (and boiling) point.

Lego vs glued Lego

Simple molecular solids are like Lego bricks clipped lightly together: a gentle pull separates whole bricks.

Ionic and giant molecular solids are bricks welded at every stud: you must destroy the welds, so a blowtorch is needed.

The syllabus data backs it up — iodine melts in a warm hand-held flame; MgO needs a furnace.

Worked example: Ranking four substances

Rank these in order of increasing melting point, and explain: iodine ( $\text{I}_2$ ), sodium chloride ( $\text{NaCl}$ ), diamond, magnesium oxide ( $\text{MgO}$ ).

Have a go first?

Predicting melting points

Multiple choiceQuestion 1 · Apply

Short answerQuestion 2 · Apply

Answer the 2 questions above to continue.