# Chemical Bonds and Electronegativity

Chemistry I · Atoms, Bonds and Reaction Rates · https://tryals.app/learn/chemistry-i/chemical-bonds-and-electronegativity

## One Spectrum, Three Names

A chemical bond forms when an arrangement of nuclei and electrons is lower in energy than the separated atoms. How the electrons are shared out gives the three classic bond types.

An **ionic bond** transfers electrons outright, producing cations and anions held by electrostatic attraction in a giant lattice. Its strength is captured by the Coulomb expression for lattice energy,

$$E \propto \frac{q_1 q_2}{d}$$

so lattice energy rises sharply with ionic charge and falls as the ions get bigger. MgO ($2+/2-$) is bound roughly four times as strongly as NaF ($1+/1-$) at similar spacing, which is why MgO melts at 2852 °C and NaF at 993 °C.

A **covalent bond** shares a pair of electrons between two nuclei. A **metallic bond** pools valence electrons into a delocalised sea across a lattice of cations, which explains conduction, malleability and lustre in one stroke.

**Electronegativity** is the tendency of an atom in a bond to attract the shared electrons. On the Pauling scale it runs from about 0.7 (Cs) to 4.0 (F), rising across a period and falling down a group, the same $Z_{\text{eff}}$ story as before. The electronegativity difference $\Delta$EN predicts the character of the bond:

| $\Delta$EN | Bond character |
|---|---|
| Below 0.4 | Essentially nonpolar covalent |
| 0.4 to 1.7 | Polar covalent |
| Above 1.7 | Largely ionic |

These boundaries are conventions on a continuum, not physical walls. A polar bond has a **dipole moment** $\mu = q \times d$, drawn as an arrow pointing toward the more electronegative atom.

> **Common pitfall:** treating ionic and covalent as a strict either/or. Real bonds sit on a sliding scale of shared-to-transferred; HF at $\Delta$EN $= 1.9$ is conventionally "ionic" yet exists as discrete molecules, and even NaCl retains a little covalent character.

## Practice questions

8 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. Bonding models treat ionic and covalent interactions as discrete categories, yet physical bonds occupy a continuum. What does this distinction imply for a molecule like hydrogen fluoride (ΔEN = 1.9)?

A. It displays molecular behaviour despite a high polarity
B. Its extreme polarity forces it into a continuous lattice
C. It converts entirely to separate ions in the gas phase
D. It exhibits pure ionic character across all temperatures

**Answer:** A. It displays molecular behaviour despite a high polarity

**Why:** Categorising bonds by arbitrary numerical thresholds overlooks their hybrid nature. High difference in electronegativity creates substantial polarity without necessarily stripping a substance of its molecular identity or enforcing a lattice.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/bonding-models-treat-ionic-and-covalent-interactions-as-discrete

### 2. Two bonded atoms have electronegativities 3.0 and 2.1. Set the electronegativity difference of the bond.

**Answer:** 0.9 (within ±0.15)

**Why:** $\Delta$EN $= 3.0 - 2.1 = 0.9$, which lands in the 0.4 to 1.7 band, a **polar covalent** bond, with the shared pair displaced toward the more electronegative atom but not transferred outright.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/two-bonded-atoms-have-electronegativities-3-0-and-2-1-set-the

### 3. An electronegativity difference of exactly 1.7 marks a physical boundary at which a bond stops being covalent and becomes ionic.

**Answer:** False

**Why:** False, bonding varies continuously from equal sharing to complete transfer. The 1.7 figure is a convenient convention, and compounds either side of it behave very similarly.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/an-electronegativity-difference-of-exactly-1-7-marks-a-physical

### 4. Magnesium oxide melts at 2852 °C while sodium fluoride melts at 993 °C, despite similar ion sizes. What accounts for the difference?

A. The greater electronegativity difference in NaF weakens its ionic network
B. The larger nuclear mass of magnesium requires more thermal energy to disrupt
C. MgO ions carry double charges, so lattice energy is about four times larger
D. MgO forms a network of discrete molecules held by strong dipole attractions

**Answer:** C. MgO ions carry double charges, so lattice energy is about four times larger

**Why:** Lattice energy goes as $q_1q_2/d$. Going from $1 \times 1$ to $2 \times 2$ multiplies the charge product by four at comparable spacing, and that dominates the melting point.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/magnesium-oxide-melts-at-2852-c-while-sodium-fluoride-melts-at-993

### 5. Sort each substance by its dominant bonding type.

**Answer:**

- Ionic: Sodium chloride, Magnesium oxide
- Covalent: Carbon dioxide, Diamond
- Metallic: Copper, Sodium metal

**Why:** Metal-plus-nonmetal pairs (NaCl, MgO) transfer electrons; nonmetal-plus-nonmetal (CO2, diamond) share them; pure metals (Cu, Na) pool them into a delocalised sea.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/sort-each-substance-by-its-dominant-bonding-type

### 6. Complete the description of how electronegativity varies and what it is not.

**Answer:** Electronegativity **increases** from left to right across a period, **decreases** going down a group, and peaks at **fluorine**. It must not be confused with **ionisation energy**, which is measured on a free atom in the gas phase.

**Why:** Electronegativity rises across a period and falls down a group, peaking at fluorine (4.0). Ionisation energy is a different quantity, an energy in kJ/mol measured on an isolated atom, not a tendency within a bond. Caesium is the distractor: it is the least electronegative element.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/complete-the-description-of-how-electronegativity-varies-and-what-it

### 7. Match each bonding model to the property it explains most directly.

**Answer:**

- Delocalised electron sea → Electrical conductivity in metals
- Electrostatic lattice of ions → High melting point and brittleness of salts
- Shared electron pair → Discrete molecules with definite shapes
- Unequal sharing of a pair → A permanent bond dipole

**Why:** Each model earns its keep by an observation: mobile electrons give conduction, a rigid charged lattice gives high melting points and brittleness, a localised shared pair gives molecular shape, and unequal sharing gives a dipole.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/match-each-bonding-model-to-the-property-it-explains-most-directly

### 8. A bond between two atoms of the same element has no dipole moment.

**Answer:** True

**Why:** True, identical atoms have identical electronegativity, so $\Delta$EN is zero, the pair is shared equally, and no dipole arises. This is why H2, O2 and N2 are perfectly nonpolar.

Page: https://tryals.app/practice/chemistry-i/chemical-bonds-and-electronegativity/a-bond-between-two-atoms-of-the-same-element-has-no-dipole-moment