How does the electronegativity difference between two atoms decide whether a bond is ionic, polar covalent or nonpolar covalent?
Electronegativity and bond polarity: use electronegativity differences from Table S to classify bonds as ionic, polar covalent or nonpolar covalent.
A focused Regents Chemistry answer on electronegativity difference and bond polarity: how subtracting Table S electronegativities classifies a bond as nonpolar covalent, polar covalent or ionic, and how that difference shapes the unequal sharing of electrons.
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What this topic is asking
The Core Curriculum asks you to use electronegativity to judge bond character. Specifically, you compute the difference in electronegativity between two bonded atoms (using the values on Table S) and classify the bond as nonpolar covalent, polar covalent or ionic. This is a recurring Part A and Part B-2 skill that bridges the periodic-trends page and the molecular-polarity page.
Electronegativity as a pull on electrons
Because electronegativity increases across a period and decreases down a group (see the periodic-trends page), the most electronegative atoms are at the top right of the table. When two atoms with different electronegativities bond, the more electronegative one takes a larger share of the bonding electrons, giving it a partial negative charge.
Classifying a bond by the difference
The exam most often asks you to rank bonds by polarity or to classify a single bond, rather than to apply a precise cut-off. For ranking, the bond with the largest electronegativity difference is the most polar (or most ionic). Identical atoms always give a difference of zero and therefore a nonpolar covalent bond.
Partial charges and the dipole
In a polar covalent bond the unequal sharing produces partial charges: the more electronegative atom carries a partial negative charge and the less electronegative atom a partial positive charge. This separation of charge across the bond is a bond dipole, and it is what makes a bond "polar". Whether the whole molecule is polar then depends on its shape, which is covered on the Lewis-structures and molecular-polarity page.
Try this
Q1. Classify the bond in as ionic, polar covalent or nonpolar covalent. [1 point]
- Cue. Both atoms are fluorine, so the difference is : a nonpolar covalent bond.
Q2. Using Table S, state whether the bond between sodium () and chlorine () is ionic or covalent. [1 point]
- Cue. Difference , a large difference, so the bond is ionic.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part A style)1 marksWhich bond is the most polar? (1) (2) (3) (4) Show worked answer →
A 1-point Part A item using Table S electronegativities. The answer is (4) .
The polarity of a bond increases with the electronegativity difference between the two atoms. Fluorine has the highest electronegativity of any element (about on Table S), so the difference between hydrogen and fluorine is the largest of the four halogen choices. As you go down the halogens from F to I, electronegativity falls, so the bond with hydrogen becomes less polar.
Markers reward selecting the bond with the largest electronegativity difference.
Regents (Part B-2 style)3 marksUsing Table S, classify the bonding between the atoms in each pair as ionic, polar covalent or nonpolar covalent. (a) potassium and fluorine (electronegativities and ) (b) hydrogen and chlorine ( and ) (c) two bromine atoms ( and ).Show worked answer →
A 3-point constructed-response item applying electronegativity differences.
(a) K and F (1 point): difference , a large difference, so the bond is ionic.
(b) H and Cl (1 point): difference , a moderate difference, so the bond is polar covalent.
(c) Br and Br (1 point): difference , so the bond is nonpolar covalent (electrons shared equally).
Markers reward computing each difference and classifying it: a large difference is ionic, a small nonzero difference is polar covalent, and zero difference is nonpolar covalent.
Related dot points
- Types of chemical bonds: distinguish ionic, covalent and metallic bonding in terms of electron transfer or sharing, and relate bond type to the elements involved.
A focused Regents Chemistry answer on ionic, covalent and metallic bonding: how electrons are transferred or shared, why bonds form to reach stability, the role of energy, and how to predict bond type from the elements involved.
- Lewis structures and molecular polarity: draw Lewis electron-dot diagrams for simple atoms, ions and molecules, and decide whether a molecule is polar or nonpolar from its bonds and shape.
A focused Regents Chemistry answer on Lewis electron-dot diagrams and molecular polarity: how to draw dot structures for atoms, ions and small molecules, and how bond polarity together with molecular symmetry decides whether the whole molecule is polar.
- Periodic trends: describe and explain the trends in atomic radius, ionization energy, electronegativity and metallic character across a period and down a group, using Table S where appropriate.
A focused Regents Chemistry answer on periodic trends: atomic radius, ionization energy, electronegativity and metallic character, why each trend runs the way it does, and how to read the numbers from Table S of the Reference Tables.
- Properties of ionic, molecular and metallic substances: relate melting point, electrical conductivity, hardness and solubility to the type of bonding and structure.
A focused Regents Chemistry answer on how bonding type explains properties: why ionic solids have high melting points and conduct only when molten or dissolved, why molecular substances are soft and low-melting, and why metals conduct and are malleable.
- Intermolecular forces: describe hydrogen bonding, dipole-dipole forces and weak dispersion forces, and use them to explain trends in boiling point and the properties of water.
A focused Regents Chemistry answer on intermolecular forces: hydrogen bonding, dipole-dipole attractions and weak dispersion (van der Waals) forces, how they differ from chemical bonds, and how they explain boiling points and water's high boiling point and surface tension.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)