Period 3 Flashcards
Atomic Radius
Across period 3…..
Decreases along the period
- There is an INCREASE in nuclear charge
- SAME amount of shielding
This means the electron shells are held closer to the nucleus.
Electronegativity
Increases along the period
Across period 3…..
- There is an INCREASE in nuclear charge
- SAME amount of shielding
This means the elements are increasingly capable of withdrawing electron density from a covalent bond.
1st Ionisation Energy
Increases along the period
Across period 3…..
- There is an INCREASE in nuclear charge
- SAME amount of shielding
This means the electrons are held increasingly more tightly and so require more energy to be removed.
Al has a lower 1st IE than Mg because:
- Mg has a 3s electron removed
- Al has a 3p electron removed
- 3p is higher in energy than 3s
S has a lower 1st IE than P because:
- P has a 3p electron removed which is unpaired
- S has a 3p electron removed which is paired
- S has a lower ionisation energy due to electron pair repulsion
Melting Point and Boiling Point
The MP increases from Na to Al because:
- The charge on the metal ion increases from Na+ to Al3+
- The size of the metal ion decreases across the row
- Therefor there is an increase in the strength of attraction between the metal ions and delocalised electrons.
Silicon has the highest MP because:
- It has a macromolecular structure
- With lots of covalent bonds
- Which are very strong
For the non-metals:
- P4, S8 and Cl2 all have a simple molecular structure with Van der Waals intermolecular forces.
- Strength of the Van der Waals depends on the size of the molecule and S8 is largest, followed by P4 then Cl2.
- Argon exists as single atoms with the weakest Van der Waals out of all of them.
Aluminium has the highest boiling point because:
- Once silicon has been melted most of its strong covalent bonds have been broken, so boiling it requires only a little extra energy.
- In the liquid state Aluminium still has strong electrostatic attraction between ions and delocalised electrons, and so still requires a large amount of energy to boil.
Reactions of the Period 3 Elements with Water
Sodium:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g). (pH= 13-14)
- vigorous reaction
Magnesium:
Mg(s) + 2H2O(l) → Mg(OH)2(aq) + H2(g). (pH= 9-10)
- very slow reaction at room temp
Mg(s) + H2O(g) → MgO(s) + H2(g)
Chlorine:
Cl2(g) + H2O(l) ⇌ HClO(aq) + HCl(aq). (pH= 2-3)
sunlight:
2Cl2(g) + 2H2O(l) → 4HCl(aq) + O2(g) (pH= 2-3)
Reactions of the Period 3 Elements with Oxygen
Sodium:
2Na(s) + ½O2(g) → Na2O(s)
- yellow flame to form a white solid:
Magnesium:
2Mg(s) + O2(g) → 2MgO(s)
- white flame to form a white solid
Aluminium:
4Al(s) + 3O2(g) → 2Al2O3(s). (pH= 7)
- If you sprinkle aluminium powder you will get bright white flame & White solid
Silicon:
Si(s) + O2(g) → SiO2(s). (pH= 7)
- Orange flame Pure form of SiO2 is a white solid.
Phosphorus:
P4(s) + 5O2(g) → P4O10(s)
- White Phosphorous gives white flame. No ppt White fumes produced
Sulfur:
S(s) + O2(g) → SO2(s)
- Pale blue flame No ppt Colourless gas
Why does MgO have the highest melting point of all the P3 oxides?
- MgO has ionic bonds.
- There are strong electrostatic forces of attraction between oppositely charged ions.
- These require a lot of energy to overcome.
Why does Al2O3 have more covalent character then MgO?
- The Al3+ ion is smaller than the Mg2+ ion.
- This means the Al3+ ion is more polarising than the Mg2+ ion.
- This distorts the electron cloud around the O2- ion more so that there is more electron density shared between the two ions.
For the oxides of Phosphorus and Sulfur, why does P4O10 have the highest melting point, followed by SO3 then SO2.
- These all have simple molecular structures.
- They all have Van der Waals intermolecular forces between molecules.
- P4O10 is the largest molecule so has the strongest VdW, whereas SO2 is the smallest so has the weakest VdW.
Why does SiO2 have a high melting point?
- This has a macromolecular/giant covalent structure.
- It has lots of covalent bonds.
- Which are very strong
Basic Oxides reacting with water
Na2O(s) + H2O(l) → 2Na+(aq) + 2-OH(aq) (pH= 13-14)
MgO(s) + H2O(l) → Mg(OH)2(aq) (pH= 9-10)
Explain why sodium oxide forms an alkaline solution when it reacts with water
Sodium oxide contains O2– ions
These O2– ions react with water forming OH– ions
OR O2– + H2O → 2OH–
The Insoluble Oxides
Aluminium Oxide and Silicon Dioxide are both insoluble in water and so undergo no reaction. pH is 7
The Acidic Oxides reacting with water
Phosphorus(V) Oxide:
P4O10(s) + 6H2O(l) → 4H3PO4(aq). (pH= 1-2)
Sulfur Dioxide:
SO2(g) + H2O(l) → H2SO3(aq) (pH= 2-3)
Sulfur trioxide:
SO3(g) + H2O(l) → H2SO4(aq) (pH= 0-1)
How do the acid/base properties of the P3 oxides change going across the period?
Across the period the oxides change from being basic to neutral to acidic
