Physical Properties and Trends

Physical Properties

• Melting points in degrees Celsius are Na-98, Mg-649, Al-660, Si-1410, P-44, S-119, Cl--101, Ar--189
• Electrical conductivity is high for Na, Mg, and Al, moderate for Si, and low for P, S, Cl, and Ar
• Structure goes from metallic lattice for Na, Mg, Al to macromolecular for Si to simple covalent molecules for P, S, Cl, Ar
• Melting point goes from low for Na, Mg, Cl, and Ar to high for Al and Si

Melting Points

• As the number of electrons an atom contributes from its outermost shell to the shared delocalized electrons increases, the cation's charge density increases
• Stronger bonding and a higher melting point is correlated with increased cation charge density
• Silicon has an abnormally high melting point because of its giant covalent structure

Van der Waals Forces

• P4, S8, Cl2, and Ar have simple molecular structures held by weak van der Waals' forces
• Weak Van der Waals Forces mean they have low melting points
• S8 has the highest melting point of the four due to the strongest induced van der Waals' forces because it has the most electrons

Electrical Conductivity

• Sodium, Magnesium, and Aluminium have increasing conductivity due to the increasing delocalized electrons
• Silicon is a semi-conductor because of delocalized electrons present throughout the structure
• Phosphorus (P4), Sulfur (S8), Chlorine (Cl2), and Argon (Ar) have simple molecular structures with no delocalized electrons
• Because they have no delocalized electrons, Phosphorus, Sulfur, Chlorine and Argon do not conduct electricity

Reactions with Oxygen

• Oxygen is highly electronegative and brings out the highest oxidation number of the elements
• The oxidation number of an element in its oxides is always positive
• The maximum oxidation number in the oxide is the same as the Group number, corresponding to the number of electrons in the outermost shell.
• All the elements in Period 3 except chlorine and argon react directly with oxygen to form oxides

Reactions of elements with oxygen

• Sodium burns vigorously in air with a yellow flame to form sodium oxide: 2Na(s) + 1/2 O2(g) → Na2O(s)
• Magnesium reacts vigorously when heated in oxygen with a bright white flame forming magnesium oxide: Mg(s) + 1/2 O2(g) → MgO(s)
• Aluminium metal is protected by a layer of aluminium oxide, but powdered aluminium does react well with oxygen and burns with a bright white flame: 4Al(s) + 3O2(g) → 2Al2O3(s)
• Silicon reacts slowly with oxygen when heated to form silicon dioxide: Si(s) + O2(g) → SiO2(s)
• Phosphorus on heating burns with a yellow flame giving out white smokes of phosphorus(V) oxide: P4(s) + 5 O2(g) → P4O10(s)
• Sulphur on heating burns with a blue flame forming sulphur dioxide: S(s) + O2(g) → SO2(g)
• Sulphur dioxide may be converted to sulphur trioxide by reacting the gas with air in the presence of a catalyst: SO2(g) + 1/2 O2(g) → SO3(g)

Reactions with Chlorine

• Sodium, magnesium, and aluminium react vigorously when heated with chlorine to form the respective solid white chlorides
• Aluminium chloride forms the dimer (Al2Cl6)
• Silicon and phosphorus react slowly, forming liquid products
• Silicon forms silicon (IV) chloride, and phosphorus forms phosphorus pentachloride

Reactions with Water

• Sodium reacts vigorously with cold water, giving off hydrogen gas
• Sodium quickly dissolves, leaving a strongly alkaline solution of sodium hydroxide behind: 2Na(s) + 2H2O (l) → 2NaOH(aq) + H2(g)
• Magnesium reacts very slowly with cold water, to produce hydrogen gas and a very weakly alkaline solution: Mg(s) + 2H2O (l) → Mg(OH)2(aq) + H2(g)
• Magnesium reacts vigorously with steam to make MgO and hydrogen gas: Mg(s) + H2O(g) → MgO(s) + H2(g)

Period 3 Oxides

• Na2O is basic, MgO is basic, Al2O3 is amphoteric, SiO2 is acidic, P4O10 is acidic, SO2/SO3 is acidic
• Na2O and MgO have ionic bonding type, SiO2, P4O10 and SO2/SO3 have covalent. AL2O3 has both
• The structures of Na2O, MgO, and Al2O3 are giant ionic, while SiO2, P4O10, and SO2/SO3 have a simple covalent structures
• Na2O, MgO, and Al2O3 have high melting points and SiO2, P4O10, and SO2/SO3 have low melting points

Period 3 Oxide Reactions

• Sodium, Magnesium, & Aluminium oxide are metal oxides
• Phosphorus, Sulfur, & Chlorine oxides are non-metal
• Sodium & Magnesium oxides create ionic bonds
• Phosphorus, Sulfur, & Chlorine oxides create covalent molecular bonds
• Sodium & Magnesium oxides are basic
• Aluminium oxide is amphoteric and Silicon oxide is acidic
• Sodium oxide is soluble and reacts with water, forming an alkaline solution
• Magnesium oxide reacts sparingly & is only somewhat soluble in water, forming a slightly alkaline solution
• Aluminum oxide is insoluble in water
• Silicon dioxide is insoluble in water
• Phosphorus, Sulfur, & Chlorine react with water and the solution formed is acidic

Basic Oxides

• Sodium and magnesium oxides produce alkaline solutions with water
  • Na2O(s) + H2O (l) → 2 NaOH(aq)
  • MgO(s) + H2O (l) → Mg(OH)2(s)
• Mg(OH)2 is not very soluble in water, resulting in a less alkaline solution
• Both basic oxides dissolve in acids to form salt and water
  • Na2O(s) + 2 HCl(aq) → 2 NaCl(aq) + H2O(l)
  • MgO(s) + 2 HCl(aq) → MgCl2(aq) + H2O(l)

Aluminium Oxide

• Aluminium oxide's insoluble oxide shows its amphoteric nature by reacting and dissolving in both acidic and alkaline solutions
  • Al2O3(s) + 6 H+ → 2 Al3+ + 3 H2O
  • Al2O3(s) + 6 HCl → 2 AlCl3 + 3 H2O
  • Al2O3(s) + 2 OH- + 3 H2O → 2 Al(OH)4-
  • Al2O3(s) + 2 NaOH + 3 H2O → 2 NaAl(OH)4

Non-Metal Oxides

• Covalently bonded non-metal oxides of phosphorus and sulfur dissolve and react in water to form acidic solutions.
  • P4O10(s) + 6H2O → 4H3PO4
  • SO2(g) + H2O → H2SO3
  • SO3(g) + H2O → H2SO4
• Non-metal oxides also neutralise alkalis.
  • P4O10(s) + 12NaOH → 4Na3PO4 + 6H2O
  • SO2(g) + 2NaOH → Na2SO3 + H2O
  • SO3(g) + 2NaOH → Na2SO4 + H2O

Period 3 Chlorides

• The Formula is NaCl for Sodium, MgCl2 for Magnesium, Al2Cl6 for Aluminum, SiCl4 for Silicon, PCl5 for Phosphorus
• Sodium, Magnesium, & Aluminum Structure is Giant ionic
• Silicon & Phosphorus Structure is Simple covalent
• Oxidation numbers for the Elements are Sodium +1, Magnesium +2, Aluminum +3, Silicon +4, Phosporus +5
• State at RTP Sodium is Solid, Magnesium is Solid, Aluminum is Solid, Silicon is Liquid, Phosporus is Solid
• pH value is 7 for Sodium, 6.5 for Magnesium, 3 for Aluminum, 2 for Silicon, 2 for Phosphorus
• Sodium, Magnesium, & Aluminum are white solids that dissolve in water to form colorless solutions
• Silicon & Phosphorus both create white fumes of HCl gas when mixed with water

Formation of Period 3 Chlorides

• When sodium metal is heated with chlorine gas there is a vigorous reaction and a yellow flame: 2 Na(s) + Cl2(g) → 2 NaCl(s)
• Magnesium and aluminium react vigorously with chlorine gas:
  • Mg(s) + Cl2(g) → MgCl2(s)
  • 2 Al(s) + 3 Cl2(g) → Al2Cl6(s)
• Silicon reacts slowly with chlorine giving silicon(IV) chloride: Si(s) + 2 Cl2(g) → SiCl4(1)
• Phosphorus also reacts slowly with excess chlorine gas: P4(s) + 10 Cl2(g) → 4 PCl5(1)

Period 3 Chlorides with Water

• Sodium chloride dissolves in water with no reaction, neutral PH
• Magnesium chloride dissolves in water with a slight reaction, slightly acidic PH
• Aluminum chloride produces a fairly vigorous reaction; fumes in moist air, acidic PH
• Silicon and Phosphorus chloride produce a vigorous reaction, acidic PH
• Chlorine dissolves and reacts slightly releasing acidic PH