Periodic Properties: Electron Gain Enthalpy and Electronegativity

TL;DR

Electron gain enthalpy measures the energy change when an atom gains an electron, while electronegativity describes an atom's ability to attract shared electrons in a bond. These properties show predictable trends across the periodic table, helping us understand how elements interact. Generally, non-metals have high electronegativity and more negative electron gain enthalpies, making them good at attracting electrons.

1. The Mental Model

Imagine atoms as having different "personalities" when it comes to electrons. Some atoms are electron "magnets," strongly pulling electrons towards them, while others are less bothered. These two properties tell you how much an atom "wants" an extra electron or "pulls" on shared electrons.

2. The Core Material

Electron Gain Enthalpy (ΔH_eg)

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Electron gain enthalpy is the energy change that occurs when an isolated gaseous atom gains an electron to form a gaseous anion. It's usually measured in kilojoules per mole (kJ/mol).

• Exothermic (negative ΔH_eg): When an atom readily accepts an electron, energy is released, and the value is negative. This means the anion formed is more stable than the neutral atom. Most non-metals have negative electron gain enthalpies.
• Endothermic (positive ΔH_eg): If energy must be supplied to force an atom to accept an electron, the value is positive. This usually happens when adding an electron to an already stable electron configuration (like noble gases) or when adding a second electron to an already negatively charged ion, overcoming repulsion.

Trends in Electron Gain Enthalpy:

• Across a Period (left to right): Generally becomes more negative (energy is released more readily). This is because atomic size decreases, and the effective nuclear charge increases, pulling the incoming electron more strongly. Halogens have the most negative electron gain enthalpies.
• Down a Group (top to bottom): Generally becomes less negative (or more positive). As you go down, atomic size increases, and the added electron is further from the nucleus, experiencing less attraction.

Electronegativity (χ)

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Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons (i.e., electrons in a chemical bond) towards itself. It's a dimensionless quantity, often measured on the Pauling scale, where fluorine is the most electronegative element (4.0).

• It's a concept related to bonded atoms, unlike electron gain enthalpy, which is about isolated atoms.
• Higher electronegativity means a stronger pull on bonding electrons.

Trends in Electronegativity:

• Across a Period (left to right): Generally increases. As you move across, the effective nuclear charge increases, and atomic radius decreases, causing a stronger attraction for bonding electrons.
• Down a Group (top to bottom): Generally decreases. As you go down, the atomic radius increases, and inner electrons shield the valence electrons more effectively, reducing the attraction for bonding electrons.

graph TD
    A["Atomic Property"] --> B["Electron Gain Enthalpy (ΔH_eg)"]
    A --> C["Electronegativity (χ)"]

    B --> B1["Energy change when atom gains e-"]
    B --> B2["Negative: Energy released (stable anion)"]
    B --> B3["Positive: Energy absorbed (unstable anion)"]
    B --> B4["Trends: More negative L→R, Less negative/More positive T↓B"]

    C --> C1["Tendency to attract shared e- in bond"]
    C --> C2["Higher value = stronger pull"]
    C --> C3["No units (Pauling scale)"]
    C --> C4["Trends: Increases L→R, Decreases T↓B"]

    B4 --> D["Non-metals: Generally more negative ΔH_eg"]
    C4 --> D
    D --> E["Non-metals: High Electronegativity"]

3. Worked Example

Let's compare Oxygen (O) and Sulfur (S).

• Electron Gain Enthalpy:
  • Oxygen (Period 2, Group 16): ΔH_eg = -141 kJ/mol. This is quite negative, meaning oxygen readily accepts an electron.
  • Sulfur (Period 3, Group 16): ΔH_eg = -200 kJ/mol. Notice that sulfur's electron gain enthalpy is more negative than oxygen's. This is an exception to the general down-the-group trend. While you'd expect it to be less negative, the smaller size of oxygen means greater electron-electron repulsion when an incoming electron tries to join its small 2p orbitals, making it slightly less favorable than for the larger sulfur atom with its 3p orbitals.
• Electronegativity (Pauling scale):
  • Oxygen: 3.44
  • Sulfur: 2.58
  • Here, the trend holds: Oxygen is above sulfur in Group 16, and it's significantly more electronegative, meaning it pulls bonding electrons much more strongly than sulfur does.

4. Key Takeaways

• Electron gain enthalpy measures the energy change when an atom gains an electron to become an anion.
• A negative electron gain enthalpy means energy is released, indicating a stable anion formation.
• Electronegativity measures an atom's ability to attract shared electrons in a chemical bond.
• Both properties generally increase from left to right across a period.
• Both properties generally decrease from top to bottom down a group (with some exceptions for electron gain enthalpy in Period 2).
• Non-metals typically have high electronegativity and more negative electron gain enthalpies.

Common Mistakes to Avoid:
- Don't confuse electron gain enthalpy with ionization enthalpy; ionization enthalpy is about removing an electron.
- Remember that electronegativity is for bonded atoms, while electron gain enthalpy is for isolated gaseous* atoms.
- Don't assume all second-period elements follow the exact same electron gain enthalpy trend as larger elements in their group.
- Don't assign units to electronegativity; it's a relative scale.

5. Now Try It

Choose two elements: Chlorine (Cl) and Bromine (Br). Based on the periodic trends discussed, predict which element would have a more negative electron gain enthalpy and which would be more electronegative. Explain your reasoning briefly for each prediction. What would success look like? You'd correctly identify the element for each property and provide a concise reason based on atomic size and/or effective nuclear charge.