Period 3 Elements: Physical and Chemical Properties Study Guide

Periodicity and Trends in Period 3

• Periodicity is defined as the repeating pattern of physical and chemical properties shown by different periods in the Periodic Table.

• While elements in the same group tend to exhibit similar chemical and physical properties, there is a distinct change in these properties across a period.

• Periodic trends are observable in several specific characteristics: atomic radii, ionic radii, first ionization energies, electronegativities, densities, melting points, and boiling points.

• Data Overview for Period 3 (Sodium to Argon):     - Sodium (Na): Electronic Configuration: 2.8.1; 1st Ionization Energy: $494\,kJ/mol$; Atomic Radius: $186\,pm$; Ionic Radius: $0.098\,nm$; Melting Point: $371\,K$.     - Magnesium (Mg): Electronic Configuration: 2.8.2; 1st Ionization Energy: $736\,kJ/mol$; Atomic Radius: $160\,pm$; Ionic Radius: $0.065\,nm$; Melting Point: $923\,K$.     - Aluminium (Al): Electronic Configuration: 2.8.3; 1st Ionization Energy: $577\,kJ/mol$; Atomic Radius: $143\,pm$; Ionic Radius: $0.045\,nm$; Melting Point: $932\,K$.     - Silicon (Si): Electronic Configuration: 2.8.4; 1st Ionization Energy: $786\,kJ/mol$; Atomic Radius: $117\,pm$; Ionic Radius: $0.038\,nm$; Melting Point: $1683\,K$.     - Phosphorus (P): Electronic Configuration: 2.8.5; 1st Ionization Energy: $1060\,kJ/mol$; Atomic Radius: $110\,pm$; Ionic Radius: $0.212\,nm$; Melting Point: $317\,K$.     - Sulfur (S): Electronic Configuration: 2.8.6; 1st Ionization Energy: $1000\,kJ/mol$; Atomic Radius: $104\,pm$; Ionic Radius: $0.190\,nm$; Melting Point: $392\,K$.     - Chlorine (Cl): Electronic Configuration: 2.8.7; 1st Ionization Energy: $1260\,kJ/mol$; Atomic Radius: $99\,pm$; Ionic Radius: $0.181\,nm$; Melting Point: $172\,K$.     - Argon (Ar): Electronic Configuration: 2.8.8; 1st Ionization Energy: $1520\,kJ/mol$; Atomic Radius: $192\,pm$; Ionic Radius: N/A; Melting Point: $84\,K$.

Atomic and Ionic Radii Trends

• Atomic Radius:     - The atomic radius decreases across a period (from Sodium to Chlorine). While electrons are being added to the same main energy level, the number of protons in the nucleus increases. This increased nuclear charge attracts the energy level closer to the nucleus.     - Specific Atomic Sizes ($pm$): $Na(186)$, $Mg(160)$, $Al(143)$, $Si(117)$, $P(110)$, $S(104)$, $Cl(99)$.     - Argon shows an increase to $192\,pm$ because it is measured as a Van der Waals radius rather than a covalent radius.

• Ionic Radii:     - Cations: Ionic radii decrease for $Na^+\ (0.098\,nm)$, $Mg^{2+}\ (0.065\,nm)$, $Al^{3+}\ (0.045\,nm)$, and $Si^{4+}\ (0.038\,nm)$. This is because the loss of valence electrons results in the loss of an entire electron shell compared to the parent atom.     - Anions: Anions like $P^{3-}\ (0.212\,nm)$, $S^{2-}\ (0.190\,nm)$, and $Cl^-\ (0.181\,nm)$ contain more electrons than their parent atoms, leading to increased inter-electron repulsion. Across the period from Phosphorus to Chlorine, anion size decreases because the number of electron shells remains constant while the proton number (nuclear charge) increases.

First Ionization Energy and Electronegativity

• First Ionization Energy (1st I.E.):     - Generally, 1st I.E. values increase across Period 3. This is attributed to the increasing nuclear charge while electrons occupy the same energy level, which increases the effective nuclear charge ($Z_{eff}$) and makes electron removal more difficult.     - Specific Values ($kJ\,mol^{-1}$): $Na(494)$, $Mg(736)$, $Al(577)$, $Si(786)$, $P(1060)$, $S(1000)$, $Cl(1260)$, $Ar(1520)$.

• Electronegativity:     - Definition: A measure of the attraction an atom has for a shared pair of electrons when covalently bonded to another atom.     - Trend: Electronegativity increases across the period from left to right as atomic size decreases. Values: $Na(0.9)$, $Mg(1.2)$, $Al(1.5)$, $Si(1.8)$, $P(2.1)$, $S(2.5)$, $Cl(3.0)$, $Ar(0)$.

Conductivity, Density, and Phase Properties

• Electrical Conductivity:     - Measured in siemens per metre ($Sm^{-1}$), where Siemens is the reciprocal of ohms ($S = \Omega^{-1}$).     - Metallic elements (Na, Mg, Al) have high conductivity due to mobile delocalized electrons. Molecular elements have extremely low conductivity.     - Values ($\times 10^8\,Sm^{-1}$.): $Na(0.218)$, $Mg(0.224)$, $Al(0.382)$. Silicon is a semi-conductor ($2 \times 10^{-10}$), while P, S, Cl, and Ar are extremely poor conductors ($10^{-17}$ to $10^{-23}$).

• Density:     - Density increases from sodium to aluminium ($0.97$ to $2.70\,g\,cm^{-3}$) because metallic bond strength, the number of valence electrons, and the number of protons all increase.     - Silicon ($2.3296\,g\,cm^{-3}$) and phosphorus ($1.823\,g\,cm^{-3}$ for white P) have lower densities than Aluminium because they utilize covalent bonds rather than metallic bonds, leading to less closely packed structures.     - Sulfur ($2.07\,g\,cm^{-3}$) is denser than phosphorus due to atomic packing differences ($P_4$ vs $S_8$).     - Chlorine ($0.002898\,g\,cm^{-3}$) and Argon ($0.001633\,g\,cm^{-3}$) have very low densities as they are naturally occurring gases.

Melting and Boiling Points

• Melting and boiling points depend on the internal structure and the attractive forces between particles. The trend is generally consistent for both points.

• Trend Across Period 3: Increases from Na to Si, then decreases from Si to Ar.

• Bonding Types:     - Na, Mg, Al: Metallic bonding. MP increases with the number of valence electrons ($Na: 97^\circ C$, $Mg: 649^\circ C$, $Al: 660^\circ C$).     - Si: Macromolecular structure with very strong covalent bonds, resulting in a very high melting point ($1410^\circ C$).     - $P_4$, $S_8$, $Cl_2$: Simple molecular structures held by weak Van der Waals forces. Sulfur ($123^\circ C$) has a higher MP than Phosphorus ($44^\circ C$) because the larger $S_8$ molecules have stronger Van der Waals forces than $P_4$. Chlorine is $-113^\circ C$.     - Ar: Monatomic molecules with extremely weak attractions between atoms ($-189^\circ C$).

• Boiling Point Data ($^\circ C$): $Na(882)$, $Mg(1090)$, $Al(2467)$, $Si(2355)$, $P(280)$, $S(444)$, $Cl(-35)$, $Ar(-186)$.

Chemical Reactivity with Water

• Sodium: Vigorous/violent reaction. Na floats and fizzes. Equation: $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$.

• Magnesium: Reacts slowly with cold water but faster with hot water or steam.     - Cold: $Mg(s) + 2H_2O(l) \rightarrow Mg(OH)_2(aq) + H_2(g)$.     - Steam: $Mg(s) + H_2O(g) \rightarrow MgO(s) + H_2(g)$.

• Aluminium: Reacts with steam to produce alumina. Equation: $2Al(s) + 3H_2O(g) \rightarrow Al_2O_3(s) + 3H_2(g)$.

• Silicon, Phosphorus, Sulfur: No reaction with water.

• Chlorine: Reacts to form a mixture of acids. Equation: $Cl_2(g) + H_2O(l) \rightarrow HClO(aq) + HCl(aq)$.

• Argon: No reaction.

Chemical Reactivity with Oxygen

• Sodium: Burns vigorously; forms basic oxide. Equation: $4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$.

• Magnesium: Burns vigorously once started; forms basic oxide. Equation: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$.

• Aluminium: Fresh metal tarnishes; only powdered Al burns. Oxide is amphoteric. Equation: $4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)$.

• Silicon: Burns if heated strongly; forms acidic oxide. Equation: $Si(s) + O_2(g) \rightarrow SiO_2(s)$.

• Phosphorus: Red/white allotropes burn readily to form acidic oxides.     - $P_4(s) + 5O_2(g) \rightarrow P_4O_{10}(s)$.     - $P_4(s) + 3O_2(g) \rightarrow P_4O_6(s)$.

• Sulfur: Burns easily to form $SO_2$; can further react to $SO_3$ over a $V_2O_5$ catalyst.     - $S(s) + O_2(g) \rightarrow SO_2(g)$.     - $2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)$.

• Chlorine: Forms various covalent acidic oxides. Examples: $2Cl_2(s) + 7O_2(g) \rightarrow 2Cl_2O_7(l)$ and $Cl_2(s) + 1/2O_2 \rightarrow Cl_2O(g)$.

Acid-Base Behavior of Oxides and Hydroxides

• Na: $Na_2O$ reacts vigorously with water to form $NaOH$. It reacts with acids: $Na_2O(aq) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l)$.

• Mg: $MgO$ is slightly soluble. Reacts with acids: $MgO + 2HCl \rightarrow MgCl_2(aq) + H_2O(l)$.

• Al: $Al_2O_3$ is amphoteric.     - Basic action: $Al_2O_3(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2O(l)$.     - Acidic action: $Al_2O_3(s) + 2NaOH(aq) + 3H_2O(l) \rightarrow 2NaAl(OH)_4(aq)$.

• Si: $SiO_2$ is insoluble in water but reacts with strong bases: $SiO_2(s) + 2NaOH(aq) \rightarrow Na_2SiO_3(aq) + H_2O(l)$. Also reacts with calcium oxide: $SiO_2(s) + CaO(aq) \rightarrow CaSiO_3$.

• P: $P_4O_6$ dissolves slowly in cold water to $H_3PO_3$. $P_4O_{10}$ reacts vigorously to $H_3PO_4$. They react with bases, e.g., $P_4O_{10}(s) + 12NaOH(l) \rightarrow 4Na_3PO_4(aq) + 6H_2O(l)$.

• S: $SO_2$ is very soluble; $SO_3$ reacts violently to form $H_2SO_4$. They react with bases to form sulfite or sulfate salts.

• Cl: Oxides dissolve readily to form acids such as $HClO_4$ and $HClO$.

Reactions with Chlorine and Behavior of Chlorides

• Reactions with Chlorine Gas:     - Na & Mg: Burn to form ionic chlorides: $2NaCl(s)$ and $MgCl_2(s)$.     - Al: Forms intermediate chloride $2AlCl_3(s)$ when heated in a chlorine stream.     - Si: Heated to form volatile covalent liquid $SiCl_4(l)$.     - P: Reacts without heat to form covalent $PCl_3(l)$ and $PCl_5(s)$.     - S: Chlorine passed over molten sulfur yields disulphur dichloride $S_2Cl_2(l)$.

• Reaction of Chlorides with Water:     - $NaCl$ & $MgCl_2$: Dissolve to give neutral solutions.     - $AlCl_3$: Dissolves to give an acidic solution: $AlCl_3(s) + H_2O(l) \rightarrow [Al(H_2O)_6]^{3+}(aq) + 3Cl^-(aq)$.     - $SiCl_4$, $PCl_3$, $PCl_5$: React to give acidic solutions containing $HCl$.     - $S_2Cl_2$, $SCl_2$, $SCl_4$: React to give acidic solutions and complex mixtures including $S(s)$, $H_2S(g)$, and various sulfur oxo-ions.

Oxidation States and Applications

• Oxidation States: The maximum oxidation state is less likely to be exhibited when the electronegativity of the element is close to that of the bonding partner (Oxygen electronegativity = 3.5, Chlorine = 3.0).     - Max States in Oxides: $Na(+1)$, $Mg(+2)$, $Al(+3)$, $Si(+4)$, $P(+5)$, $S(+6)$, $Cl(+7)$.     - Max States in Chlorides: $Na(+1)$, $Mg(+2)$, $Al(+3)$, $Si(+4)$, $P(+5)$, $S(+4)$.

• Uses of Aluminium: $Al(OH)_3$ is an antacid used for heartburn and acid indigestion; it neutralizes existing acid but does not inhibit production.

• Uses of Phosphorus: White P is used for flares/incendiary devices. Red P is used in matchbox striking surfaces. Compounds are used in fertilizers, detergents, glass, and chinaware.

• Uses of Argon: Creates inert atmospheres for titanium production and welding. Used in incandescent bulbs (prevents filament corrosion) and fluorescent tubes (generating UV light with mercury). Used in double-glazed windows and luxury car tires (reduces noise and protects rubber).