Classification of Elements and Periodicity in Properties · Lesson 7 of 10
Electron Gain Enthalpy and Electronegativity
“Distinguish electron attachment to an isolated atom from electron attraction inside a bond.”
• Define electron gain enthalpy and interpret positive and negative values. • Explain general trends and the oxygen–sulphur and fluorine–chlorine exceptions. • Distinguish first and second electron attachment and the electron-affinity sign convention. • Define electronegativity and interpret relative Pauling-scale values. • Connect electronegativity with size and metallic or non-metallic character.
Adding an electron to an isolated atom
Ionization asks how much energy is needed to take an electron away. A different experiment asks what happens when an extra electron approaches a neutral gaseous atom. Attraction to the nucleus can favour attachment, but repulsion from electrons already present can oppose it. The balance depends on size and configuration, so electron attachment is not simply ionization run backwards.
The enthalpy change when an electron is added to an isolated neutral gaseous atom to form a gaseous negative ion, expressed per mole of such additions. It is represented by .
A value of means that attachment releases 349 kJ per mole of atoms under the stated convention. It is more negative than , so more energy is released. The word “higher” is ambiguous with negative quantities; say “more negative”, “less negative”, or “larger magnitude”.
| Element or family member | Electron gain enthalpy / kJ mol−1 | Element or family member | Electron gain enthalpy / kJ mol−1 |
|---|---|---|---|
| H | −73 | He | +48 |
| Li | −60 | Ne | +116 |
| Na | −53 | Ar | +96 |
| K | −48 | Kr | +96 |
| Rb | −47 | Xe | +77 |
| Cs | −46 | Rn | +68 |
| O | −141 | F | −328 |
| S | −200 | Cl | −349 |
| Se | −195 | Br | −325 |
| Te | −190 | I | −295 |
| Po | −174 | At | −270 |
Halogens need one additional electron to reach a noble-gas-like outer arrangement, so their first attachment has a strongly negative enthalpy. For noble gases, an added electron must occupy an energetically unfavourable arrangement beyond the filled shell; the tabulated positive values indicate unfavourable attachment, not an assurance that a stable free noble-gas anion exists.
The trend is real, but not smooth
Across a main-group period the increasing effective nuclear attraction generally makes first electron gain enthalpy more negative. Down a family, a larger cloud places the added electron farther from the nucleus, tending to make attachment less exothermic. Electron pairing and closed or half-filled arrangements interrupt these simple expectations, so this trend is less regular than the first-ionization trend.
Oxygen and fluorine are particularly important exceptions to the expected downward trend. Their added electron must fit into the compact 2p region, where it experiences substantial electron repulsion. Sulphur and chlorine have more spacious 3p regions. That reduction in repulsion makes the first electron gain enthalpy of S more negative than O, and that of Cl more negative than F.
Problem
Among P, S, Cl, and F, which has the most negative and which the least negative first electron gain enthalpy?
- 1.Across period 3, the general attraction trend favours more exothermic attachment towards chlorine. Phosphorus also has a half-filled 3p subshell, so adding an electron requires pairing.
- 2.The F–Cl comparison needs the compact-2p exception: Cl has the more negative value.
- 3.Thus Cl has the most negative value among these four, while P has the least negative.
- 4.This is not the same as saying Cl is more electronegative than F; the two properties describe different situations.
Problem
Why is the second electron gain enthalpy of oxygen positive even though its first is negative?
- 1.The first process adds an electron to neutral , producing .
- 2.The second adds an electron to an already negative ion: .
- 3.The incoming electron is repelled by the negatively charged species, so energy must be supplied for this isolated-ion step.
- 4.The presence of oxide ions in solids does not contradict this. A whole compound can be stabilised by other energy changes, including attraction between oppositely charged ions.
A neutral Group 16 atom being two electrons short of an octet does not mean that both isolated electron additions release energy. Keep the first and second electron-attachment processes distinct.
Electron affinity is often defined as energy released on attachment, or equivalently the energy needed to detach the electron from the resulting anion. Under that convention, a favourable attachment has positive affinity but negative electron gain enthalpy. Check the definition before comparing signs; the two names cannot be interchanged without attention to convention and temperature.
For the ideal-gas approximation discussed in the source, a zero-temperature molar electron affinity is related to electron gain enthalpy at temperature by the thermal correction shown below. It is not needed for qualitative trend comparisons, and it assumes the relevant thermal contributions are approximated consistently.
Attracting shared electrons in a compound
Inside a bond, an atom is not a free gaseous atom receiving a separate electron. It competes with another bonded atom for the electrons they share. Electronegativity expresses this relative attraction and helps explain unequal sharing and the chemical behaviour of compounds.
The relative tendency of an atom in a chemical compound to attract shared electron density towards itself. Pauling-scale values are dimensionless relative numbers, not enthalpies.
Electronegativity is inferred through a model or scale rather than measured as one unique isolated-atom observable. Pauling, Mulliken–Jaffé, and Allred–Rochow scales use different approaches. Approximate Pauling values are useful for comparisons; fluorine is assigned a value near 4.0, the highest in the familiar table. A quoted value is a useful guide, not a claim that an atom’s electron-attracting behaviour is identical in every compound.
| Period 2 | Li | Be | B | C | N | O | F |
|---|---|---|---|---|---|---|---|
| Approximate Pauling value | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 |
| Period 3 | Na | Mg | Al | Si | P | S | Cl |
|---|---|---|---|---|---|---|---|
| Approximate Pauling value | 0.9 | 1.2 | 1.5 | 1.8 | 2.1 | 2.5 | 3.0 |
| Group 1 | Approximate value | Group 17 | Approximate value |
|---|---|---|---|
| Li | 1.0 | F | 4.0 |
| Na | 0.9 | Cl | 3.0 |
| K | 0.8 | Br | 2.8 |
| Rb | 0.8 | I | 2.5 |
| Cs | 0.7 | At | 2.2 |
Across a period, increasing effective attraction and decreasing size generally increase electronegativity. Down a group, greater distance and shielding generally decrease it. The upper-right region, apart from the special treatment of noble gases on simple scales, therefore contains strongly electronegative non-metals. The lower-left region contains electropositive metals that more readily lose outer electrons.
Problem
Fluorine has electronegativity about 4.0 and chlorine about 3.0, but Cl has the more negative electron gain enthalpy. Is this contradictory?
- 1.Electronegativity compares attraction for shared electron density in a bonded environment.
- 2.Electron gain enthalpy concerns adding an electron to an isolated gaseous atom.
- 3.Fluorine attracts shared electrons very strongly, but an additional free electron encounters substantial repulsion in its compact 2p shell.
- 4.The two measurements refer to different physical questions, so the two rankings can differ.
Problem
Compare the statements “Cl has electron gain enthalpy −349 kJ mol−1” and “O is more electronegative than Na”.
- 1.The chlorine statement describes an energy change for ; its negative sign means release of energy.
- 2.The oxygen–sodium statement compares attraction for electrons in a compound, without specifying a separate gaseous attachment reaction.
- 3.In a sodium–oxygen compound, oxygen receives electron ownership in oxidation-state bookkeeping because it is more electronegative.
- 4.Do not attach energy units to the relative electronegativity values.
Quiz
What does a negative electron gain enthalpy indicate?
Which has the more negative first electron gain enthalpy?
Why is isolated second electron attachment to endothermic?
Which property refers specifically to shared electrons in a compound?
Which statement about a Pauling value is appropriate?
If attachment releases energy, how do the common affinity and enthalpy sign conventions compare?
Practice Problems
- Write the gaseous first-electron-attachment equation for chlorine and interpret a value of .
- Explain why first attachment to a noble gas is generally unfavourable.
- Compare O with S and F with Cl, explaining the role of compact orbitals.
- Distinguish electron gain enthalpy, electron affinity, and electronegativity using the physical process and units or sign convention.
- Explain why saying “nitrogen has electronegativity exactly 3.0 in all compounds” is too strong.
- Use the Pauling tables to compare Na with Cl and F with I. Connect each comparison to position and size.
- An atom has a more negative first electron gain enthalpy than another. Why is that fact alone insufficient to decide all their chemical reactivities?
Key Takeaways
Electron gain enthalpy describes isolated gaseous attachment and can be negative or positive. The compact 2p shell makes O and F less exothermic in first attachment than S and Cl respectively. A second attachment to an already negative atomic ion requires energy. Electronegativity concerns shared electron density, is scale-dependent, and has no energy units. Higher electronegativity generally accompanies stronger non-metallic character; trend exceptions require physical explanations.