Mr Cole Chemistry

Melting Points and Boiling Points of Period 3 Questions

Physical Properties of Period 3

Period 3 Worksheet
1. Draw a diagram of the structure of each element: Sodium, Silicon, Phosphorus, Sulfur, Chlorine.
Answer

The diagrams for the structures are shown below:

Diagrams showing structures of Sodium, Silicon, Phosphorus, Sulfur, and Chlorine
2. State how the atomic radius of a metal is determined. State and explain which atom has the largest atomic radii out of sodium and magnesium.
Answer

The atomic radius of a metal is determined by measuring the distance between two adjacent nuclei when the metal is an element. Half this distance is the atomic radius.

Sodium has a larger atomic radius than magnesium. This is because they both have the same number of shells, whereas Magnesium has one more proton. This increased nuclear charge results in a stronger attraction between the nucleus and the outer shell electrons, resulting in them being pulled slightly closer.

3. State which element has the longer bond length: sulfur or chlorine. Explain why that is the case.
Answer

Sulfur has longer bonds than chlorine. This is because chlorine has a higher electronegativity than sulfur due to having one more proton (while having the same number of shells). This results in a stronger electrostatic force of attraction between the chlorine nuclei and the bonding pair of electrons and therefore shorter bonds.

Note: The bond length of sulfur actually varies significantly as there are many different allotropes. S8 is the most common allotrope and has a bond length of 205pm, compared to chlorine’s 190pm.

4. The graph shows the melting and boiling points of period 3 elements. Graph of melting and boiling points of Period 3 elements
a) Explain why there is an increase in the melting point between sodium and magnesium.

Magnesium is in Group 2, so the lattice of metal ions is made of 2+ ions rather than 1+ ions like in sodium. Therefore, there is a stronger electrostatic attraction between the ions in the lattice and the delocalised electrons. This means that more thermal energy is required to overcome the attraction, increasing the melting point.

b) Explain why there is an increase in the melting point between aluminium and silicon.

Silicon is a giant covalent structure whereas aluminium is metallic. In order to melt silicon, the covalent bonds have to be broken. As covalent bonds are strong—and there are many of them—this requires a lot of energy. More energy is required to break the covalent bonds in silicon than the metallic bonds in aluminium.

c) Explain why there is a decrease in the boiling point between silicon and phosphorus.

Silicon is a giant covalent structure whereas phosphorus is a simple covalent molecule. To boil silicon, you have to break covalent bonds. However, to boil phosphorus you only have to overcome the Van Der Waals forces between the molecules of phosphorus, which takes significantly less energy.

d) Explain the trend in the boiling points from phosphorus to argon.
  • Phosphorus → Sulfur: Increases because S8 is larger than P4. Larger molecules have more electrons and therefore have larger induced dipoles. This results in stronger Van Der Waals forces and a higher boiling point.
  • Sulfur → Chlorine: Decreases because Cl2 is smaller than S8. The rationale is the same as above but reversed.
  • Chlorine → Argon: Decreases because Ar is monatomic (and smaller) compared to Cl2. The rationale is the same as above.
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