Periodicity Flashcards
What are the features of sodium, magnesium and aluminium?
Giant metallic structure:
- Metallic bonding
- Shiny, conduct electricity, and react with dilute acids to give hydrogen and salts.
What are the features of silicon?
Macromolecule/giant covalent structure:
- Semi-metal (covalent bonding with ionic character)
- Conducts electricity to some extent (useful for semiconductor devices.
What are the features of phosphorus, sulphur and chlorine?
Molecular structure (P4, S8, Cl2)
- Non-metals, covalent bonding
- Do not conduct electricity, have low melting and boiling points.
What are the features of argon?
Noble gas
- Atomic
- Chemically unreactive, exists as separate atoms.
What is the trend in melting points of the elements of period 3?
- Increases from Na Al, due to increased metallic bond strength (more electrons in sea, with larger charge on smaller ion)
- Large increase in melting point for silicon, due to the giant macromolecular structure.
- Decrease, increase, decrease (P, S, Cl); S8 > P4 > Cl2, larger van der Waals due to more electrons.
- Decrease for argon, due to it being a noble gas and a monatomic species.
What is the trend in atomic radius of the elements of period 3?
Decreases across the period
- Increased charge pulls electrons closer to the nucleus.
- No additional electron shells to provide more shielding
- Nuclear force of attraction is stronger, meaning the radius is smaller.
What is the trend in ionisation energy of the elements of period 3?
First IE increases across the period.
- As you go across, atomic radius decreases, electrons are held more strongly, due to increased charge but no extra shielding.
- Due to the electron being held more strongly, it gets increasingly difficult to remove the electron.
Which elements of period 3 react with water?
- Metals: Sodium and Magnesium
- Non-metals: Chlorine (chlorination)
Sodium with water
-Vigorous reaction, sodium floats on the surface of the water and fizzes rapidly, melting because of the heat given out from the reaction:
2Na(s) + 2H2O(l) –> 2NaOH(aq) + H2(g)
Magnesium with water
-Very slow at room temperature, only a few bubbles of hydrogen.
-A less alkaline solution is formed than with sodium because magnesium hydroxide is sparingly soluble.
Mg(s) + 2H2O(l) –> Mg(OH)2(aq) + H2(g)
Heated magnesium and steam
-The reaction of magnesium and water is much faster if Mg is heated and steam is used
Mg(s) + H2O(g) –> MgO(s) +H2(g)
Which elements of period 3 react with oxygen?
All elements in period 3 (except argon) are relatively reactive. Their oxides can be prepared by direct reaction of the element with oxygen.
-All reactions are exothermic.
Sodium with oxygen
-Vigorous reaction, burns brightly in air with a yellow/orange flame.
2Na(s) + 0.5O2(g) –> 2Na2O(s)
Magnesium with oxygen
-Vigorous reaction, burns in air with a bright white flame, forms white powder.
-More intense flame if lowered into a gas jar of oxygen
2Mg(s) + O2(g) –> 2MgO(s)
Aluminium with oxygen
-Slow reaction (useful for saucepans, garage doors etc). When lowered into a gas jar of oxygen, it burns brightly with white sparks, forms white powder.
-Although aluminium is a reactive metal, it is always coated with a strongly bonded surface layer of oxide – this protects it from further reaction. Even if scratched off, the exposed Al reacts rapidly and seals the surface.
4Al(s) + 3O2(g) –> 2Al2O3(s)
Silicon with oxygen
-Slow reactions, must be heated strongly in oxygen.
Si(s) + O2(g) –> SiO2(s)
Phosphorus with oxygen
-Red phosphorus must be heated before it will react with oxygen.
-White phosphorus spontaneously ignites in air and the white smoke of phosphorus pentoxide is observed (it is kept in water to prevent this).
4P(s) + 5O2(g) –> P4O10(s)
-If oxygen supply is limited, phosphorus trioxide, P2O3, is also formed.
Sulfur with oxygen
-Burns steadily, when lowered into a gas jar of oxygen (powder), burns with a blue flame to form colourless sulphur dioxide.
S(s) + O2(g) –> SO2(g)
What is the trend in melting point of the oxides of elements in period 3?
- Increase between Na2O and MgO.
- Decrease after MgO.
Why do the metal oxides of period 3 have high melting points?
Sodium, magnesium and aluminium oxides all form giant ionic lattices where the bonding extends throughout the compound. This results in high melting points.
Why does MgO have a higher melting point than Na2O?
MgO has a higher melting point than Na2O because magnesium forms 2+ ions which attract O2- ions more strongly than the 1+ sodium ions in Na2O. This means more energy is required to break this stronger attraction.
Why does Al2O3 have a lower melting point than MgO?
Al2O3 has a lower melting point than MgO (unexpectedly). This is because the 3+ ions distort the oxygen’s electron cloud, making the bonds partially covalent, and therefore a lower melting point than pure ionic compounds with very high melting points.
Why does SiO2 have a higher melting point than the rest of the non-metals?
SiO2 has a higher melting point than other non-metal oxides because it has a giant macromolecular structure. Strong covalent bonds hold the structure together, so a lot of energy is needed to break the bonds.
Why do P4O10 and SO2 have low melting points?
P4O10 and SO2 are covalent molecules. They have relatively low melting points because they form simple molecular structures, held together by weak intermolecular forces (dipole-dipole and van der Waals), which take little energy to overcome.
What is the general trend of the reactions of period 3 oxides with water?
- Metal oxides (left of period) form alkaline solutions.
- Non-metal oxides (right of period) form acidic solutions
- Those in the middle of period do not react.
Sodium oxide with water
-Basic oxide
-Reacts to give sodium hydroxide (strongly alkaline)
-pH = 13-14
Na2O(s) + H2O(l) –> 2Na+(aq) + 2OH-(aq) (+2NaOH)
Magnesium oxide with water
-Basic oxide
-reacts to give magnesium hydroxide, sparingly soluble (somewhat alkaline)
-pH = 10
MgO(s) + H2O(l) –> Mg(OH)2(s) –> Mg2+(aq) + 2OH-(aq)
Why do sodium and magnesium oxides form alkaline solutions?
Na and Mg oxides are composed of ions, both of which contain the oxide ion (O2-), a very strong base (strongly attracts protons). It readily reacts with water to produce hydroxide ions, forming an alkaline solution.
-MgO is less soluble than Na2O, so it produces a less alkaline solution.
Which oxides of period 3 elements do not react with water?
Insoluble oxides (pH = 7)
- Aluminium oxide
- Silicon dioxide
Why doesn’t aluminium oxide react with water?
Aluminium oxide is ionic but the bonding is too strong to be separated, due to additional covalent bonding, it is insoluble.
Why doesn’t silicon dioxide react with water?
-It is a giant macromolecule and water will not affect this type of structure, it is insoluble.
Phosphorus pentoxide and water
-Acidic oxide
-Reacts violently to produce an acidic solution of phosphoric(v) acid.
P4O10(s) + 6H2O(l) –> 4H3PO4(aq)
This ionises so the solution is acidic (fairly strong):
H3PO4(aq) –> H+(aq) + H2PO4-(aq)
-pH = 1-2
Sulfur dioxide and water
-Acidic oxide
-Fairly soluble in water, reacts to give an acidic solution of sulfuric(IV) acid (sulfurous acid)
SO2(g) +H2O(l) –> H2SO3(aq)
-This partially dissociates producing H+ ions, causing acidity
H2SO3(aq) –> H+(aq) + HSO3-(aq)
-pH = 2-3
Sulfur trioxide and water
-Acidic oxide
-Reacts violently with water to produce sulfuric(VI) acid.
So3(g) + H2O(l) –> H2SO4(aq) –> H+(aq) + HSO4-(aq)
-pH = 0-1
Why do phosphorus and sulfur oxides react with water?
P4O10 and SO2/SO3 are covalent molecules and react to form acidic solutions.
How do sodium and magnesium oxides react with acids and bases?
Sodium and magnesium oxides are basic oxides and so only react with acids to give salt and water.
Na2O(s) + H2SO4(aq) –> Na2SO4(aq) + H20(l)
MgO(s) + 2HCl(aq) –> MgCl2(aq) + H20(l)
How does aluminium oxide react with acids and bases?
Al2O3 is amphoteric, meaning it can react with acids and bases
Al2O3(s) + 6HCl(aq) –> 2AlCl3(aq) + 3H20(l)
Al2O3(s) + 2NaOH(aq) + 3H2O(l) –> 2NaAl(OH)4(aq) (NaOH must be hot and concentrated).
How does silicon dioxide react with acids and bases?
SiO2 will react with strong bases, as a weak acid.
SiO2(s) + 2NaOH(aq) –> Na2SiO3(aq) + H2O(l) (NaOH must be hot and concentrated).
How does phosphorus pentoxide react with acids and bases?
-Reacts with alkali (basically the reaction of phosphoric(V) acid, as this is formed when P4O10 reacts with water. It has 3 –OH groups, each with an acidic hydrogen atom, so it reacts in 3 stages. Each time a hydrogen is replaced by a sodium atom.
H3PO4(aq) + NaOH(aq) –> NaH2PO4(aq) + H2O(l)
NaH2Po4(aq) + NaOH(aq) –> Na2HPO4(aq) + H2O(l)
Na2HPO4(aq) + NaOH(aq) –> Na3PO4(aq) + H2O(l)
-Overall:
3NaOH(aq) + H3PO4(aq) –> Na3PO4(aq) + 3H2O(l)
How does sulfur dioxide react with acids and bases?
-If we add NaOH to an aqueous solution of sulfur dioxide, we first form sodium hydrogensulfate(IV):
SO2(aq) + NaOH(aq) –>NaHSO3(aq)
-Followed by sodium sulfate(IV):
NaHSO3(aq) + NaOH(aq) –>Na2SO3(aq) + H2O(l)
