Ch 1 · Electronegativity and bonding · AS & A Level Chemistry 9701
Topic 1 of 7 in Chemical bonding — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Electronegativity is the idea that lets you predict, before ever seeing a compound, whether its bonding will be ionic, polar covalent or non-polar covalent. Cambridge examiners set questions that ask you to explain trends using nuclear charge, shielding and atomic radius together, and to use electronegativity differences to justify a bond type, so precise wording matters as much as the correct answer.
Key ideas — Defining electronegativity
Electronegativity is the power of an atom to attract the electron density of a shared (bonding) pair of electrons towards itself, in a covalent bond. It is measured on the dimensionless Pauling scale, symbol chi, running from about 0.7 for caesium to 4.0 for fluorine, the most electronegative element. Electronegativity is a property of an atom within a bond, not of an isolated atom, so noble gases are not normally assigned values.
🧑🏫 Teacher's explanation
Imagine a game of tug-of-war where two people are pulling on a rope. In a covalent bond, the 'rope' is the pair of electrons shared between two atoms. Electronegativity is essentially how much 'strength' an atom has to pull that rope toward its side. If one atom is much stronger than the other, the rope (electron density) shifts heavily toward the stronger atom, creating a polar bond.
📖 Definitions
🔍 Worked example — Easy
Calculate the electronegativity difference (Δchi) between Hydrogen (chi = 2.1) and Chlorine (chi = 3.0).
🔍 Worked example — Standard
Predict the bond type between Sodium (chi = 0.9) and Chlorine (chi = 3.0).
🔍 Worked example — Exam-style
Explain why electronegativity increases across Period 3 from Magnesium (chi = 1.2) to Chlorine (chi = 3.0).
⚠️ Common errors
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🎯 Exam question
(a) Calculate the electronegativity difference for the C-O bond. [1] (b) Predict whether the C-O bond is polar or non-polar. Explain your answer. [2] [3 marks]
Key ideas — Factors influencing electronegativity
Three factors determine how strongly a nucleus attracts a bonding pair:
🧑🏫 Teacher's explanation
Imagine you are at a crowded music festival. Electronegativity is like how strongly a celebrity (the nucleus) can pull fans (the electrons) toward them. If the celebrity is very famous (high nuclear charge) and standing right next to you (small atomic radius), you are very likely to be pulled toward them. However, if there is a massive crowd of people standing between you and the celebrity (high shielding), you won't feel their influence as strongly.
📖 Definitions
🔍 Worked example — Easy
Explain why the electronegativity of Fluorine (F) is higher than that of Lithium (Li).
🔍 Worked example — Standard
Predict the bond type between Hydrogen (H) and Chlorine (Cl) using Pauling electronegativity values: H = 2.1 and Cl = 3.0.
🔍 Worked example — Exam-style
An element X has an electronegativity of 0.8 and element Y has an electronegativity of 3.8. Determine the bond type and the nature of the partial charges.
⚠️ Common errors
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🎯 Exam question
Key ideas — Periodic trends in electronegativity
Across a period (left to right), electronegativity increases. Nuclear charge increases by one proton per element while electrons are added to the same outer shell, so shielding stays roughly constant; the increasing nuclear charge pulls the outer shell inward, decreasing atomic radius and increasing the attraction for a bonding pair.
🧑🏫 Teacher's explanation
Imagine a game of tug-of-war where two atoms are pulling on a shared pair of electrons. Electronegativity isn't how hard an atom pulls on its own electrons, but how much 'muscle' it has when it's sharing them with someone else. The stronger the atom, the more it can distort the electron cloud toward itself.
📖 Definitions
🔍 Worked example — Easy
An element X has an electronegativity of 2.1 and element Y has an electronegativity of 3.5. Calculate the electronegativity difference and predict the bond type.
🔍 Worked example — Standard
Explain why the electronegativity of Fluorine (F) is higher than that of Lithium (Li).
🔍 Worked example — Exam-style
Predict the bond type for HCl given EN(H) = 2.2 and EN(Cl) = 3.0.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the type of bond formed in NH_3. [1] (b) Explain why the electronegativity of elements generally increases across a period from left to right. [3] [4 marks]
Key ideas — Predicting bond type from electronegativity difference
The electronegativity difference, Δchi = |chi_A - chi_B|, between two bonded atoms indicates how the shared electron pair is distributed:
🧑🏫 Teacher's explanation
Imagine a game of tug-of-war where two people are pulling on a rope. If both people are equally strong, the rope stays right in the middle; this is like a non-polar covalent bond where electrons are shared equally. If one person is slightly stronger, the rope moves toward them, but they are still sharing it; this is a polar covalent bond. If one person is a professional athlete and the other is a toddler, the athlete simply takes the rope away; this represents an ionic bond where the electron is effectively transferred.
📖 Definitions
🔍 Worked example — Easy
Calculate the electronegativity difference (Δchi) for a bond between Carbon (chi = 2.5) and Hydrogen (chi = 2.1). Predict the bond type.
🔍 Worked example — Standard
A bond exists between an atom with electronegativity 4.0 and an atom with electronegativity 0.9. Determine the bond type.
🔍 Worked example — Exam-style
Predict the bond type for F_2 molecules, given the electronegativity of Fluorine is 4.0.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Worked example
Question. Using the Pauling electronegativity values chi(Na) = 0.9, chi(H) = 2.1, chi(C) = 2.5, chi(Cl) = 3.0 and chi(F) = 4.0, predict and explain the type of bonding present in (a) NaCl, (b) CCl_4, and (c) HF. [4]
🔍 Worked example — Easy
Calculate the electronegativity difference (Δchi) for MgO given chi(Mg) = 1.2 and chi(O) = 3.5. Predict the bond type.
🔍 Worked example — Standard
Predict the bond type for CH_4 using chi(C) = 2.5 and chi(H) = 2.1.
🔍 Worked example — Exam-style
Explain why electronegativity increases across Period 3 (from Na to Cl) using the concept of nuclear charge and shielding.
⚠️ Common errors
Common mistakes
Practice
Answers