The P-Block Elements (Group 16 - Oxygen Family)

Group 16 Elements

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Group 16 elements, also known as the chalcogens, include Oxygen (O), Sulfur (S), Selenium (Se), Tellurium (Te), Polonium (Po), and Livermorium (Lv). This group displays a transition from non-metallic to metallic character down the group.

Occurrence

Oxygen (O):

• Most abundant element in the Earth's crust (about 46.6% by mass).
• Exists as dioxygen ($O_2$) in the atmosphere (about 21% by volume).
• Found in water ($H_2O$), rocks, and minerals (e.g., silicates, carbonates, sulfates, phosphates), and in all living organisms.

Sulfur (S):

• Found in free state in some volcanic regions as yellow crystals.
• Abundant in combined state as sulphides (e.g., galena $PbS$, zinc blende $ZnS$, copper pyrites $CuFeS_2$) and sulfates (e.g., gypsum $CaSO_4 \cdot 2H_2O$, epsom salt $MgSO_4 \cdot 7H_2O$, barytes $BaSO_4$).
• It is an essential constituent of many proteins and vitamins.

Selenium (Se), Tellurium (Te), Polonium (Po):

• Occur in small amounts in nature, often associated with sulfide ores of copper and lead.
• Polonium is a radioactive element found in uranium ores.

Livermorium (Lv): A synthetic, highly radioactive element.

Electronic Configuration

General Configuration: The general valence shell electronic configuration of Group 16 elements is $ns^2np^4$.

• O: $[He] 2s^22p^4$
• S: $[Ne] 3s^23p^4$
• Se: $[Ar] 3d^{10} 4s^24p^4$
• Te: $[Kr] 4d^{10} 5s^25p^4$
• Po: $[Xe] 4f^{14} 5d^{10} 6s^26p^4$
• Lv: $[Og] 7s^27p^4$

Significance: The presence of six valence electrons explains their tendency to gain two electrons to achieve a stable noble gas configuration ($np^6$), thus forming $-2$ ions ($O^{2-}$, $S^{2-}$ etc.) or forming covalent bonds by sharing two electrons.

Atomic Radii

Trend: Atomic radii increase down the group from O to Po.

Reasons:

• Increase in the number of electron shells.
• Increased shielding effect of inner electrons.

Comparison to Group 15: Atomic radii are smaller than the corresponding elements in Group 15 of the same period due to the higher effective nuclear charge experienced by the valence electrons.

Ionisation Enthalpies

Trend: First ionization enthalpies generally decrease down the group from O to Po.

Reasons:

• Increase in atomic size.
• Increased shielding effect.

Comparison to Group 15: First ionization enthalpies are generally lower than those of Group 15 elements in the same period. This is because Group 15 elements have a half-filled $p$-subshell ($np^3$), which provides extra stability, making it harder to remove an electron.

Second Ionization Enthalpies: The second ionization enthalpies are significantly higher than the first ionization enthalpies because it is more difficult to remove an electron from a $+1$ ion than from a neutral atom.

Electron Gain Enthalpy

Trend: Electron gain enthalpy generally becomes less negative (less favorable) down the group from O to Po.

Anomalous Behavior of Oxygen: Oxygen has a less negative electron gain enthalpy than Nitrogen. This is because the incoming electron into the $2p$ subshell of oxygen experiences significant electron-electron repulsion due to the small size of the $n=2$ shell.

Significance: The generally negative electron gain enthalpies indicate the tendency of these elements to accept electrons to form $-2$ ions.

Electronegativity

Trend: Electronegativity generally decreases down the group from O to Po.

• O: 3.44
• S: 2.58
• Se: 2.55
• Te: 2.1
• Po: 2.0 (Estimated)

Observations:

• Oxygen is the second most electronegative element (after fluorine).
• The electronegativity of these elements leads them to form covalent compounds and often act as oxidizing agents.

Physical Properties

States of Matter:

• Oxygen and Sulfur (at room temperature) are non-metals.
• Selenium and Tellurium are non-metals with some metalloid properties.
• Polonium is a radioactive metal.
• Livermorium is a synthetic radioactive element.

Allotropes:

• Oxygen exists in two allotropic forms: dioxygen ($O_2$) and ozone ($O_3$).
• Sulfur exhibits extensive allotropy, with common forms like rhombic sulfur and monoclinic sulfur.
• Selenium and Polonium also show allotropy.

Melting and Boiling Points: Melting and boiling points generally increase down the group, reflecting the increase in van der Waals forces due to larger molecular size and mass.

Density: Density generally increases down the group.

Chemical Properties

1. Oxidation States:

• The most common oxidation state is -2.
• Oxygen also exhibits $-1$ (in peroxides, e.g., $H_2O_2$), $-1/2$ (in superoxides, e.g., $KO_2$), and $+1$ (in $OF_2$), $+2$ (in $O_2F_2$) when combined with fluorine.
• Sulfur shows oxidation states from -2 (in $H_2S$, metal sulfides) to +6 (in $SF_6$, $SO_3$, $H_2SO_4$). $+4$ and $-2$ are also common.
• Selenium, Tellurium, and Polonium commonly exhibit -2, +2, +4, and +6 oxidation states, with +4 and +2 becoming more prominent down the group.

2. Formation of Hydrides: They form hydrides like $H_2O$, $H_2S$, $H_2Se$, $H_2Te$, $H_2Po$. Their stability decreases and acidity increases down the group.

• $H_2O$ is neutral.
• $H_2S, H_2Se, H_2Te, H_2Po$ are increasingly acidic.

3. Formation of Halides: They form halides like $OF_2$, $SF_6$, $S_2Cl_2$, $SeCl_4$, $TeCl_4$, $PoCl_3$, etc.

4. Formation of Oxides: They form oxides like $H_2O$, $SO_2$, $SO_3$, $SeO_2$, $TeO_2$, $PoO_2$, $SeO_3$, $TeO_3$, $PoO_3$. Oxides of oxygen and sulfur are acidic. Oxides of Se and Te are amphoteric. Oxides of Po are basic.

5. Oxidizing and Reducing Nature:

• Oxygen is the second most electronegative element and is a strong oxidizing agent. It rarely shows negative oxidation states other than -2 (except in $OF_2$ and peroxides).
• Sulfur and Selenium can act as oxidizing agents (e.g., $S+O_2 \rightarrow SO_2$) and reducing agents (e.g., $H_2S$ reduces $Fe^{3+}$).
• Te and Po are less electronegative and exhibit more metallic character, tending to act as reducing agents.

6. Reaction with Hydrogen: All form hydrides ($H_2X$), which are acidic except for $H_2O$. The bond dissociation enthalpy of $H-X$ bonds decreases down the group, and the reducing character of hydrides increases.

7. Reaction with Metals: Form metal chalcogenides (sulfides, selenides, tellurides, polonides).