Topic 2 · page 15 of 15

Melting and boiling temperatures of Periods 2 and 3

Melting temperature rises, peaks at the giant covalent elements, then falls sharply at the simple molecules. The pattern is explained by the structure of each element and the forces that must be overcome.

15 min read2.18(i)

Read first: First ionisation energy of elements 1 to 36, Blocks of the Periodic Table, and why configuration matters

What has to be overcome 2.18(i)

When a solid melts or a liquid boils, the particles move apart. Energy is needed to overcome whatever holds them together. So a high melting or boiling temperature means strong forces or many bonds to overcome.

The key step is to name the structure of the element, because that tells you which forces are involved.

Structure Particles Held together by Overcome on melting Temperature
giant metallic positive ions in a sea of delocalised electrons metallic bonding, the attraction between the ions and the electrons metallic bonding high
giant covalent atoms in a lattice many strong covalent bonds many strong covalent bonds very high
simple molecular small molecules weak London forces between molecules London forces only low
monatomic single atoms very weak London forces London forces only lowest

Period 3 2.18(i)

Element Structure Melting / K Boiling / K
Na giant metallic 371 1156
Mg giant metallic 923 1363
Al giant metallic 933 2792
Si giant covalent 1687 3538
P₄ (white) simple molecular 317 554
S₈ simple molecular 388 718
Cl₂ simple molecular 172 239
Ar monatomic 84 87
Bar chart of melting and boiling temperatures in kelvin for Period 3. Sodium, magnesium and aluminium rise, silicon is highest, then phosphorus, sulfur, chlorine and argon fall to very low values.
Period 3. Dark bars are melting temperatures and pale bars are boiling temperatures.

Sodium, magnesium, aluminium (giant metallic). The temperatures rise from Na to Al. Each atom contributes more delocalised electrons to the sea: 1, 2 and 3. The ions also have a greater charge (Na⁺, Mg²⁺, Al³⁺) and are smaller. So the attraction between the ions and the delocalised electrons gets stronger, and more energy is needed to overcome the metallic bonding.

Silicon (giant covalent). Every atom is bonded to four others in a lattice. Melting means breaking a very large number of strong covalent bonds, so silicon has the highest melting temperature in the period.

Phosphorus, sulfur, chlorine (simple molecular). The elements exist as P₄, S₈ and Cl₂ molecules. Melting overcomes only the weak London forces between them, so the temperatures are low. London forces get stronger as the number of electrons in the molecule increases. S₈ has 128 electrons, P₄ has 60 and Cl₂ has 34, so S₈ > P₄ > Cl₂.

Argon (monatomic). Argon is a gas of single atoms with only 18 electrons each. London forces between them are the weakest of all, so argon has the lowest temperatures.

Worked exampleWhy is the melting temperature of sulfur much lower than that of silicon?

  1. Sulfur is simple molecular (S₈). Silicon is giant covalent.
  2. Melting sulfur overcomes only the weak London forces between S₈ molecules. The strong covalent bonds inside each S₈ ring stay intact.
  3. Melting silicon needs a huge number of strong covalent bonds to be broken.
  4. Much less energy is needed for sulfur, so its melting temperature is much lower.

Period 2 2.18(i)

Element Structure Melting / K Boiling / K
Li giant metallic 454 1615
Be giant metallic 1560 2742
B giant covalent 2349 4200
C giant covalent sublimes at about 3915 not applicable
N₂ simple molecular 63 77
O₂ simple molecular 54 90
F₂ simple molecular 53 85
Ne monatomic 25 27
Bar chart of melting and boiling temperatures in kelvin for Period 2. Lithium and beryllium are metals, boron and carbon are highest, and nitrogen, oxygen, fluorine and neon are very low. Carbon is shown as one bar because it sublimes.
Period 2. Carbon changes straight from solid to gas, so it has one bar.

The explanation follows exactly the same steps:

The differences among N₂, O₂ and F₂ are small, and you should not try to rank them by the number of electrons in an answer. Say that all three are simple molecular with weak London forces.

Both periods show the same shape: the temperatures rise to a peak in the middle (Group 4, giant covalent), then fall sharply where the elements are simple molecules. Because the pattern repeats in each period, melting temperature is a periodic property.

Check yourself

  1. Explain why silicon has the highest melting temperature in Period 3.
  2. Explain why aluminium has a higher boiling temperature than sodium.
  3. A student writes: "Sulfur melts at a low temperature because its covalent bonds are weak." Correct the statement.
  4. Explain why the melting temperature of sulfur (S₈) is higher than that of chlorine (Cl₂).
  5. Explain why argon has the lowest boiling temperature in Period 3.
  6. Explain why carbon has a much higher melting temperature than nitrogen.

Answers cover these until you have tried the questions

  1. Silicon has a giant covalent structure. Melting it requires breaking a very large number of strong covalent bonds, which needs a lot of energy.
  2. Both are giant metallic. Aluminium has three delocalised electrons per atom and a smaller Al³⁺ ion with a greater charge, while sodium has one delocalised electron and a larger Na⁺ ion. So the attraction between ions and electrons is stronger in aluminium, and more energy is needed to overcome the metallic bonding.
  3. The S–S covalent bonds are strong and are not broken when sulfur melts. Sulfur is simple molecular (S₈): melting only overcomes the weak London forces between the molecules.
  4. Both are simple molecular, so melting overcomes London forces between molecules. S₈ has more electrons (128, against 34) and is a larger molecule, so its London forces are stronger and more energy is needed.
  5. Argon is monatomic, with single atoms and only 18 electrons each. The London forces between its atoms are the weakest in the period, so very little energy is needed to separate them.
  6. Carbon is giant covalent: many strong covalent bonds must be broken (carbon sublimes at about 3915 K). Nitrogen is simple molecular (N₂), and only weak London forces between molecules are overcome (melting at 63 K).