Why Oxygen Shows Lower Electron Gain Enthalpy Than Sulfur and Trend in Group 16
Oxygen is a colourless and odourless gas used in the respiration process by humans, which is converted into carbon dioxide.
One of the most important sulfur compounds is sulfuric acid, which is made with sulfur, hydrogen, and oxygen. Sulfuric acid has many commercial uses, including removing rust and adding pigment to paints and dyes. Furthermore, sulfuric acid is mixed with phosphate rock to produce phosphates for fertilizers, which helps provide plants with the necessary nutrients.
As a metalloid, selenium can conduct electricity and heat, especially when light is present. Therefore, selenium is used in solar cells, photocopiers, and cameras.
What are Chalcogens?
According to the current periodic table, chalcogens belong to group 16. Five distinct elements are classified as chalcogens: oxygen, sulphur, selenium, tellurium, and polonium. The synthetic element livermorium, abbreviated Lv, is thought to be a member of the chalcogen group. However, when it comes to the chalcogen family, it happens frequently that oxygen is left out. The features of the oxygen family are very distinct from those of the other elements in the groupings.
Physical Properties of Chalcogens
Atomic Radii of the Chalcogens
As an element moves down a group, its atomic radius, also known as its ionic radius, grows. In contrast to polonium, which has the biggest atomic radius, oxygen is the chalcogen with the smallest atomic radius.
Due to the addition of protons and the rising effective nuclear charge, the atomic radii of elements tend to decrease over time. As a result, as compared to lithium, oxygen has a substantially smaller atomic radius.
Ionization Enthalpies of Chalcogens
As the radius or size of the atom rises, the ionisation enthalpy drops. As a result, as one moves down in the group, the ionisation enthalpies of the chalcogens tend to drop. It is well known that oxygen has the highest ionisation enthalpy of all the chalcogen elements.
Since the effective nuclear charge rises as we move through the group and over the period, the ionisation enthalpy also tends to rise. So, compared to lithium, oxygen has a larger ionisation enthalpy.
Electron Gain Enthalpy of Group 16
The electron gain enthalpy tends to decrease as the atom's size grows. As a result, as one moves down the group, the electron gain enthalpies of chalcogens tend to drop. The compressed atomic structure of oxygen, which tends to support the interelectronic repulsion between the valence electrons and other approaching electrons, explains why oxygen has a lower negative electron gain enthalpy than sulphur. The correct order of electron gain enthalpy of chalcogens is S > Se > Te > O.
Electronegativities of Chalcogens
Due to several variables, including a rise in electron-electron repulsion and an increase in atomic radius, the electronegativity tends to decline as one moves down the group. Oxygen is the most electronegative chalcogen element when livermorium is excluded, whereas polonium is the least electronegative.
Metallic Nature of the Group 16 Elements
1. Sulfur and oxygen are non-metals.
2. Metalloids include selenium and tellurium.
3. Under normal circumstances, polonium typically displays metallic properties. But polonium is a radioactive substance.
Chemical Properties of the Chalcogens
Allotropy Exhibited by Group 16 Elements
The majority of chalcogens have many allotropes. Dioxygen and ozone are the two most typical oxygen allotropes. There are nine recognised allotropes of oxygen. Additionally, there are more than 20 known allotropes of sulphur.
There are two allotropes of polonium and at least five different allotropes of selenium. Monoclinic sulphur and rhombic sulphur are the two most stable allotropes of sulphur. Tellurium and selenium frequently occur in crystalline and amorphous forms.
Reactions Between Group 16 Elements and Hydrogen
When chalcogens interact with dihydrogen, they frequently produce hydrides, which have the general formula $${H}_{2}{M}$$, where $${{M}}$$ stands for any one of the chalcogen elements. The chemical reaction has the following general format:
$${M} (chalcogen) + {H}_{2} (dihydrogen) \to {H}_{2}{M} (hydride of chalcogen)$$
What Is Electron Gain Enthalpy?
The energy released when an electron is added to a single gaseous atom is known as the electron gain enthalpy. Energy can either be released or absorbed when an electron is added. It is frequently written as ΔH eg and is measured in electron volts per atom or kilojoules per mole. Electron gain enthalpy of group 16 elements should be in the following order: S>Se>Te>O.
Interesting Facts
Sulfur has been known since ancient times and has been mentioned in the Bible fifteen times. It was known to the ancient Greeks and commonly mined by the ancient Romans. It was also historically used as a component of Greek fire.
Early attempts to separate oxygen from the air were hampered by the fact that air was thought of as a single element up to the 17 and 18 centuries. Robert Hooke, Mikhail Lomonosov, Ole Borch, and Pierre Bayden successfully created oxygen but did not realise it at the time.
Tellurium was first discovered in 1783 by Franz Joseph Muller von Reichenstein.
Selenium was discovered in 1817 by Jöns Jacob Berzelius. Berzelius noticed a reddish-brown sediment at a sulfuric acid manufacturing plant. The sample was thought to contain arsenic. Berzelius initially thought that the sediment contained tellurium but came to realise that it also contained a new element, which he named selenium after the Greek moon goddess Selene.
Important Questions
1. Why do elements in the 15 and 16 groups have lower electron gain enthalpies than one another?
When an element's outer shell is half or filled with electrons, it achieves stability and requires energy to add more electrons. Group 15 elements have a different electronic arrangement of ns2 np3, which receives more stability than group 16 elements because it is half-filled. Since energy is needed to add an electron to group 15 elements, group 15 exhibits a positive electron gain enthalpy value. Therefore, the atoms in group 15 have a higher electron gain enthalpy than the elements in group 16.
2. What do the terms positive and negative electron gain enthalpy mean?
An atom's electron gain enthalpy is said to be positive if it spontaneously tends to gain electrons. In contrast, it is said to be negative if the atom is pushed to take an electron despite having a negative tendency to do so. The electron gain enthalpy generally decreases as the atomic number increases over time. As we move from left to right throughout a period, the atomic size reduces and the effective nuclear charge increases. As a result, adding an electron to a smaller atom will be simpler since the additional electron would typically be closer to the positively charged nucleus.
Conclusion
Moving down the group, the electron gain enthalpy becomes less negative. Going from left to right over some time makes it more detrimental. The electron gain enthalpy changed similarly for chalcogens, becoming less negative from top to bottom. Because it has 16 electrons, sulphur is more stable than the other elements in the group and has a lower negative tendency to gain electrons.
Multiple Choice Questions
1. When the atomic radius increases, electron gain enthalpy
(a) Increase
(b) Decrease
(c ) No change
(d) None of the above
Answer: (b) When the atomic radius increases, electron gain enthalpy decreases.
2. Which group of compounds have the largest electron gain enthalpy?
(a) Alkali metals
(b) Alkaline earth metals
(c ) Halogen family
(d) None of the above
Answer: (c ) Halogen family compounds have the largest electron gain enthalpy.
FAQs on Electron Gain Enthalpy in Chalcogen Group Elements
1. What is electron gain enthalpy of chalcogens?
The electron gain enthalpy of chalcogens is the enthalpy change that occurs when a neutral gaseous chalcogen atom gains an electron to form a negative ion. It is represented as:
X(g) + e− → X−(g)
For Group 16 elements (O, S, Se, Te, Po):
- It generally has a negative value, meaning energy is released.
- It measures the tendency of a chalcogen atom to accept an electron.
- It is closely related to electron affinity and periodic trends.
2. What is the general trend of electron gain enthalpy in Group 16 elements?
The electron gain enthalpy decreases (becomes less negative) down Group 16 from oxygen to polonium. This means sulfur has more negative electron gain enthalpy than selenium, and so on.
- Atomic size increases down the group.
- Nuclear attraction for the incoming electron decreases.
- Therefore, less energy is released.
3. Why does sulfur have more negative electron gain enthalpy than oxygen?
Sulfur has a more negative electron gain enthalpy than oxygen because oxygen’s small size causes greater electron–electron repulsion in its compact 2p orbitals.
- Oxygen configuration: 1s22s22p4
- Sulfur configuration: [Ne]3s23p4
- The added electron in oxygen enters a small 2p orbital with high repulsion.
- In sulfur, the larger 3p orbital reduces repulsion.
4. What is the electron gain enthalpy value of oxygen and sulfur?
The electron gain enthalpy of oxygen is about −141 kJ mol−1, while that of sulfur is about −200 kJ mol−1.
- O(g) + e− → O−(g)
- S(g) + e− → S−(g)
5. How is electron gain enthalpy different from electronegativity in chalcogens?
Electron gain enthalpy is the energy change when an isolated gaseous atom gains an electron, whereas electronegativity is the ability of an atom in a molecule to attract shared electrons.
- Electron gain enthalpy: Measured in kJ mol−1, applies to isolated atoms.
- Electronegativity: Dimensionless value, applies in covalent bonds.
- Oxygen has higher electronegativity than sulfur, even though sulfur has more negative electron gain enthalpy.
6. Why is the electron gain enthalpy of oxygen less negative than sulfur despite being above it in the periodic table?
The electron gain enthalpy of oxygen is less negative than sulfur because the added electron experiences greater inter-electronic repulsion in oxygen’s small 2p orbital.
- Oxygen has a very small atomic radius.
- Electron–electron repulsion is high in compact orbitals.
- This reduces the amount of energy released.
7. How does atomic size affect electron gain enthalpy in chalcogens?
As atomic size increases down the chalcogen group, the electron gain enthalpy becomes less negative because the incoming electron is farther from the nucleus.
- Larger atoms have weaker nuclear attraction.
- Energy released on gaining an electron decreases.
- Order (most negative to least negative): S > Se > Te > Po.
8. What is the second electron gain enthalpy of chalcogens?
The second electron gain enthalpy of chalcogens is always positive because energy is required to add an electron to a negatively charged ion. It is represented as:
X−(g) + e− → X2−(g)
- The incoming electron faces repulsion from the negatively charged ion.
- Therefore, energy must be supplied.
- This explains why O2− and S2− ions are formed mainly in ionic lattices, not in gaseous state.
9. How does electron gain enthalpy influence the chemical reactivity of chalcogens?
More negative electron gain enthalpy increases the tendency of chalcogens to form negative ions and participate in ionic bonding.
- Oxygen and sulfur readily form O2− and S2− in compounds.
- Example: 2Na(s) + S(s) → Na2S(s)
- Higher electron gain enthalpy enhances oxidizing ability.
10. What are the factors affecting electron gain enthalpy of Group 16 elements?
The electron gain enthalpy of chalcogens depends mainly on atomic size, nuclear charge, and electron–electron repulsion.
- Atomic size: Larger size decreases attraction for incoming electron.
- Effective nuclear charge: Higher charge increases attraction.
- Electronic configuration: Half-filled and fully filled stability affects energy change.
- Inter-electronic repulsion: Greater repulsion lowers energy release.
