Periodic Trends

Electronegativity

The tug-of-war over electrons.

⏱ — min read

Electronegativity, denoted by the symbol χ, quantifies an atom’s ability to attract shared electrons towards itself when it forms a chemical bond. It is not an intrinsic property of an isolated atom — it’s a characteristic of an atom within a molecule.

Electronegativity increases left-to-right across a period (atoms get smaller, hold electrons tighter) and decreases down a group (electrons are further from the nucleus). Noble gases are usually excluded — they rarely bond.

Pauling Scale (most common)

Proposed by Linus Pauling, this is the most widely used scale. Based on the idea that the bond A–B is often stronger than the average of A–A and B–B bonds. The difference is related by:

χ_A − χ_B = √(Δ/eV)

where Δ = E_d(AB) − √(E_d(AA) × E_d(BB)). Pauling assigned fluorine the value 4.0. Values range from ~0.7 (cesium, francium) to 4.0 (fluorine).

Mulliken Scale

Robert Mulliken proposed: χ_M = (IE + E_ea) / 2

where IE is the first ionization energy and E_ea is the electron affinity. This gives a more theoretical basis tied to fundamental atomic properties.

Allred–Rochow Scale

Relates electronegativity to the electrostatic force exerted by the nucleus on valence electrons:

χ_AR = 0.359 (Z_eff / r_cov²) + 0.744

connecting electronegativity to atomic size and effective nuclear charge.

One fascinating application is in the design of organic light-emitting diodes (OLEDs). Researchers strategically manipulate the electronegativity of organic molecules to fine-tune their optical properties.

Tuning HOMO/LUMO energy levels

By selecting molecules with specific electronegativity values, scientists can control the energy levels of the HOMO and LUMO orbitals. Incorporating highly electronegative atoms like fluorine lowers these energy levels, leading to blue light emission. Lower electronegativity tends to result in redder emissions.

Charge injection and transport

The difference in electronegativity between adjacent OLED layers also influences charge injection and transport — directly impacting power efficiency and device lifespan.

Bond polarity

The greater the electronegativity difference between two bonded atoms, the more polar the bond becomes. A difference greater than ~1.7 typically indicates ionic character.

// periodic table dashboard

Trends Across the Table

Groupwise Analysis

Select Group

Trend Analysis

Loading…

Note: Pauling electronegativity is used for trend analysis. Where unavailable, Allred-Rochow electronegativity is shown. Hover over bars to see Pauling, Allred-Rochow, and Mulliken values, plus electronic configuration. Y-axis is logarithmic.

References:

Allred A. L. & Rochow E. G. (1958), J. Inorg. Nucl. Chem., 5(4):264–268
Pauling L. (1932), J. Am. Chem. Soc., 54(9):3570–3582
Mulliken R. S. (1934), J. Chem. Phys., 2(11):782

Periodwise Analysis

Select Period

Trend Analysis

Loading…

Note: Pauling electronegativity is shown by default; Allred-Rochow values are used where Pauling values are unavailable. Y-axis is logarithmic.

References:

Pauling (1932)
Mulliken (1934)

Transition Metals Analysis

Select Series

Trend Analysis

Loading…

Note: Pauling electronegativity used; subtle trends due to d-orbital shielding. Y-axis is logarithmic.

References:

Allred & Rochow (1958)

Lanthanide / Actinide Analysis

Select Series

Trend Analysis

Loading…

Note: Pauling electronegativity is used where available. Allred-Rochow values are substituted for elements without Pauling values.

References:

Allred & Rochow (1958)