Electronegativity Trend
Electronegativity is a measure of the tendency of an atom to attract a shared pair of electrons towards itself in a covalent bond, and its periodic trend describes how this property changes systematically across the periodic table. Electronegativity increases across a period (left to right) because increasing nuclear charge pulls bonding electrons more strongly, and decreases down a group because the bonding electrons are further from the nucleus and shielded by additional inner electron shells. On the Pauling scale, fluorine is assigned the highest electronegativity value of 3.98, making it the most electronegative element, while caesium and francium have the lowest values near 0.79.
Key Formula
χ = 0.359 × (Z_eff / r²) + 0.744 [Allred-Rochow scale]
LaTeX: \chi = 0.359\left(\frac{Z_{\text{eff}}}{r^2}\right) + 0.744
| Symbol | Meaning | Unit |
|---|---|---|
| χ | Electronegativity (dimensionless, Pauling scale) | — |
| Z_eff | Effective nuclear charge felt by valence electrons | — |
| r | Covalent radius of the atom | pm (picometres) |
Worked Example
Problem
The Pauling electronegativities of H, F, and O are 2.20, 3.98, and 3.44 respectively. Determine the polarity and direction of the dipole in HF and H2O, and rank the bonds from most to least polar.
Solution
Step 1 – Electronegativity difference (Δχ) for H–F: Δχ = 3.98 − 2.20 = 1.78 → highly polar covalent bond, dipole points H→F Step 2 – Electronegativity difference for O–H: Δχ = 3.44 − 2.20 = 1.24 → polar covalent bond, dipole points H→O Step 3 – Bond polarity rule: Δχ > 1.7 is considered ionic-like; 0.4–1.7 is polar covalent; < 0.4 is non-polar covalent. Step 4 – Ranking: H–F (Δχ = 1.78) > O–H (Δχ = 1.24)
Answer
H–F is more polar than O–H; both are polar covalent; dipoles point from H toward the more electronegative atom
Pauling electronegativity values of selected elements
| Element | Symbol | Group | Period | Electronegativity (Pauling) |
|---|---|---|---|---|
| Fluorine | F | 17 | 2 | 3.98 |
| Oxygen | O | 16 | 2 | 3.44 |
| Nitrogen | N | 15 | 2 | 3.04 |
| Chlorine | Cl | 17 | 3 | 3.16 |
| Carbon | C | 14 | 2 | 2.55 |
| Caesium | Cs | 1 | 6 | 0.79 |
Interactive Tools
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Related Terms
Periodic Trend
Periodic trends are systematic patterns in elemental properties that arise from the regular variation in nuclear charge and electron configuration across periods and down groups of the periodic table. Key periodic trends include atomic radius, ionisation energy, electron affinity, electronegativity, and metallic character, all of which change predictably as atomic number increases. Understanding periodic trends allows chemists to predict chemical reactivity, bond types, and physical properties of elements and their compounds without needing to memorise individual data for every element.
Halogen
Halogens are the five nonmetallic elements of Group 17 — fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At) — each with seven valence electrons, making them one electron short of a full outer shell and therefore highly reactive oxidising agents. They readily gain one electron to form stable 1− anions (halides) and react with metals to form ionic salts, and with hydrogen to form hydrogen halides such as HCl and HF. Halogens have important industrial and biological applications: chlorine disinfects water supplies, iodine is essential for thyroid hormone synthesis, and fluorine is used in making Teflon and fluoride toothpaste.
Noble Gas
Noble gases are the six elements of Group 18 — helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) — characterised by completely filled outer electron shells, which make them extremely stable and almost entirely unreactive under normal conditions. Their full valence shells give them very high ionisation energies and near-zero electronegativity, meaning they do not readily form chemical bonds. Noble gases have important applications in lighting (neon signs, argon-filled incandescent bulbs), inert atmospheres for welding and chemical synthesis, and medical imaging (xenon anaesthesia, krypton in lung ventilation scans).
From Greek "elektron" (amber, referring to static electricity) + Latin "negativus" (denying) + "trahere" (to draw). The concept was formalised and quantified by Linus Pauling in 1932 as part of his work on the nature of the chemical bond, for which he received the Nobel Prize in Chemistry in 1954.