Periodicity: Physical Properties of Period 3 Elements

Question 1

Atomic Radius Trends Across Period 3

Answer 1

• Atomic radius decreases across a period
• As the number of protons increase, the nuclear charge increases
• This means the nucleus has stronger attraction for the electrons, pulling them in closer and so the atomic radius decreases across the period
• The extra electrons that elements gain across a period are added to the outer energy level, so they don’t provide any extra shielding

Question 2

Melting Point Trends Across Period 3: Na-Al

Answer 2

• The melting and boiling points increase for the three metallic elements from sodium to aluminium

• Strength of metallic bonds increase across a period as the number of delocalised electrons per atom increases (or size of positive charge increases)
• Also across the period the size of the positive ions decreases
• This means there is greater attraction between the positive ions and the sea of delocalised electrons

Question 3

Melting Point Trends Across Period 3: Si

Answer 3

• Silicon has a much higher melting point than the remaining Period 3 elements

• Silicon has a macromolecular structure where each silicon atom is bonded to four neighbouring silicon atoms by strong covalent bonds
• These must be broken in order for silicon to melt
• This requires a lot of energy, so silicon’s melting and boiling points are high
• The melting and boiling points of phosphorus, sulfur and chlorine are much lower than those of silicon
• This is because they all have a simple molecular structure with weak van der Waals forces holding the molecules together

Question 4

Melting Point Trends Across Period 3: P-Ar

Answer 4

• Phosphorus (P4) 60 electrons
• Sulfur (S8) 128 electrons
• Chlorine (Cl2) 34 electrons
• Argon (Ar) 18 electrons
• Strength of van der Waals forces increase as the number of electrons in the molecule increases, leading to increase in melting point
• Argon is monoatomic and only has 18 electrons therefore only very weak van der Waals forces hold the atoms together so it has a low melting point

Question 5

First Ionisation Energy Trends Across Period 3

Answer 5

• There is a general increase in the first ionisation energies across Period 3

• Across Period 3, the proton number and therefore the nuclear charge increases
• But the amount of shielding is almost constant
• Therefore there is a greater attraction between the nucleus and the outermost electrons means that more energy is required to remove an electron

• Decreases at aluminium
- Electron is being removed from a higher energy level (3p instead of 3s) so less energy is needed to remove an electron

• Decreases at sulphur
- Outermost electrons are paired in the same orbital (3p4) so they experience a repulsion, meaning less energy is needed to remove an electron