Topic S2.2 · SL and HL

The covalent model: notes and practice questions

9 min read Jump to 20 practice questions

Summary

This topic covers models of covalent bonding, molecular structures, and their influence on properties.
A covalent bond forms from electrostatic attraction between shared electron pairs and positively charged nuclei.
Lewis formulas represent valence electrons, including bonding and non-bonding pairs, often following the octet rule.
The VSEPR model predicts molecular shapes based on electron domain repulsion for species with up to four electron domains.
Bond polarity results from electronegativity differences; molecular polarity depends on both bond polarity and molecular geometry.
Intermolecular forces include London (dispersion), dipole-dipole, and hydrogen bonding, influencing physical properties.
Chromatography separates mixtures based on differential intermolecular attractions to mobile and stationary phases.

How it is examined

The heaviest bonding subtopic. May 2025 HL Paper 2 TZ1 asked for a Lewis formula of the nitrate ion [1] and its molecular geometry [1] as adjacent parts, which is the standard pairing. Bond angle questions want the number and the reason. Intermolecular force explanations are where marks are lost most: a marker wants the force named correctly and the comparison made, so "hydrogen bonding between water molecules is stronger than the London forces between methane molecules" earns, while "water has stronger bonds" does not, because it says bonds rather than forces. HL hybridization is usually a 1-mark deduction attached to a drawn structure.

Given in the booklet

Electronegativity values. Average bond enthalpies and bond lengths. Bond angles for the standard geometries are recall, as are the names of the shapes, the VSEPR ordering of repulsions, and the RF definition.

Key ideas

2.2.1 A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons. Students deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom.
2.2.2 Single, double and triple bonds involve one, two and three shared pairs of electrons. Students explain the relationship between the number of bonds, bond length and bond strength.
2.2.3 A coordination bond is a covalent bond in which both electrons of the shared pair originate from the same atom. Students identify coordination bonds in compounds.
2.2.4 The valence shell electron pair repulsion (VSEPR) model enables the shapes of molecules to be predicted from the repulsion of electron domains around a central atom. Students predict the electron domain geometry and the molecular geometry for species with up to four electron domains.

Not assessed

Knowledge of the use of locating agents in chromatography is not required. The technical and operational details of a gas chromatograph or high-performance liquid chromatograph will not be assessed.

At HL

2.2.11 Resonance structures occur when there is more than one possible position for a double bond in a molecule. Students deduce resonance structures of molecules and ions.
2.2.12 Benzene, C₆H₆, is an important example of a molecule that has resonance. Students discuss the structure of benzene from physical and chemical evidence.
2.2.13 Some atoms can form molecules in which they have an expanded octet of electrons. Students visually represent Lewis formulas for species with five and six electron domains around the central atom, and deduce the electron domain geometry and molecular geometry for these species using VSEPR.
2.2.14 Formal charge values can be calculated for each atom in a species and used to determine which of several possible Lewis formulas is preferred. Students apply formal charge to determine a preferred Lewis formula.

Guiding questions

What determines the covalent nature and properties of a substance?

Linking questions

Nature of science What are some of the limitations of the octet rule? How useful is the VSEPR model at predicting molecular geometry? How can advances in technology lead to changes in scientific definitions, for example the updated IUPAC definition of the hydrogen bond?
Structure 1.3 Why do noble gases form covalent bonds less readily than other elements?
Structure 2.1 Why do ionic bonds only form between different elements while covalent bonds can form between atoms of the same element? What properties of ionic compounds might be expected in compounds with polar covalent bonding?
Structure 1.5 To what extent can intermolecular forces explain the deviation of real gases from ideal behaviour?
Structure 3.1 Why are silicon-silicon bonds generally weaker than carbon-carbon bonds? How does the ability of some atoms to expand their octet relate to their position in the periodic table?
Structure 3.2 To what extent does a functional group determine the nature of the intermolecular forces? (HL) What features of a molecule make it "infrared (IR) active"?
Reactivity 2.2 How does the presence of double and triple bonds in molecules influence their reactivity?
Reactivity 3.4 (HL) Why do Lewis acid-base reactions lead to the formation of coordination bonds? What are the structural features of benzene that favour it undergoing electrophilic substitution reactions?
Tool 1 How can a mixture be separated using paper chromatography or thin layer chromatography (TLC)?

Practice questions

45 questions · 13 easy · 30 medium · 2 hard

Showing 20 of 20

Question 1

EasyPaper 2 · calculator1 mark

Which of the following ions has the greatest number of valence electrons?

A. $PO_4^{3-}$

B. $SiO_3^{2-}$

C. $ClO_3^{-}$

D. $NH_4^{+}$

Question 2

MediumPaper 1A · calculator1 mark

Which sequence lists the following molecules in order of decreasing bond angle?

A. $PH_3$ , $SiH_4$ , $BF_3$ , $CO_2$

B. $CO_2$ , $BF_3$ , $SiH_4$ , $PH_3$

C. $BF_3$ , $CO_2$ , $PH_3$ , $SiH_4$

D. $CO_2$ , $SiH_4$ , $BF_3$ , $PH_3$

Question 3

HardPaper 2 · calculator24 marks

(a)(i)

Antimony (Sb) and Bismuth (Bi) are elements in group 15 of the periodic table.

(a) Antimony has two stable isotopes. 57.21% of antimony atoms contain 70 neutrons and the remainder contain 72 neutrons.

(i) Deduce the nuclear symbol of the isotope of antimony containing 72 neutrons. Use section 6 of the data booklet.

[1]

(a)(ii)

(ii) Calculate, to two decimal places, the relative atomic mass of antimony.

[2]

(b)(i)

Bismuth(III) nitrate, $Bi(NO_3)_3$ , is a common salt of bismuth.

(b) (i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.

[2]

(b)(ii)

(ii) Distinguish between covalent and ionic bonding in terms of electron distribution.

[2]

(b)(iii)

(iii) State the enthalpy term that characterizes the strength of the bonding between the ions in an ionic solid.

[1]

(b)(iv)

(iv) Write an equation for the formation of aqueous bismuth(III) nitrate from solid bismuth(III) oxide and nitric acid.

[2]

(b)(v)

(v) Calculate the volume, in $cm^3$ , of $1.50 \text{ mol dm}^{-3}$ nitric acid required to react completely with $5.00 \text{ g}$ of solid bismuth(III) oxide.

[3]

(b)(vi)

(vi) Predict, with a reason, whether bismuth(III) oxide is expected to be primarily acidic, basic or amphoteric.

[1]

(b)(vii)

(vii) Discuss how the relative reactivity of zinc and bismuth could be established using the metals and aqueous solutions of their nitrates.

[2]

(b)(viii)

(viii) Discuss the products formed at the electrodes during the electrolysis of aqueous bismuth(III) nitrate. Use the standard electrode potential $E^\ominus(Bi^{3+}/Bi) = +0.32 \text{ V}$ and section 24 of the data booklet.

[2]

(c)(i)

Bismuth compounds are sometimes used in fireworks to produce special effects.

[1]

(c)(ii)

(ii) Calculate the wavelength, in nm, that corresponds to the first ionization energy of bismuth. Use sections 1, 2 and 8 of the data booklet.

[3]

(c)(iii)

(iii) Explain why the first ionization energy of bismuth is lower than that of polonium (Po), in terms of nuclear charge and electron shielding.

[2]

Question 4

EasyPaper 1A · calculator1 mark

Which force is primarily responsible for the significantly higher boiling point of water, $H_2O$ , compared to hydrogen sulfide, $H_2S$ ?

A. Covalent bonds

B. London (dispersion) forces

C. Dipole-dipole forces

D. Hydrogen bonds

Question 5

MediumPaper 1A · calculator1 mark

The properties of four allotropes of carbon are described below.

Description 1: A molecular solid composed of spherical molecules, soluble in some non-polar solvents.

Description 2: A 2D material with exceptionally high tensile strength and electrical conductivity.

Description 3: A hard, transparent crystal that is an electrical insulator, with each atom bonded to four others.

Description 4: A soft, black solid that conducts electricity and is used as a lubricant.

Which row correctly matches the allotropes to their descriptions?

	Description 1	Description 2	Description 3	Description 4
A.	Buckminsterfullerene	Graphene	Diamond	Graphite
B.	Diamond	Graphite	Graphene	Buckminsterfullerene
C.	Graphene	Diamond	Graphite	Buckminsterfullerene
D.	Graphite	Buckminsterfullerene	Diamond	Graphene

Question 6

HardPaper 2 · calculator16 marks

(a)(i)

Strontium is an alkaline earth metal in group 2 of the periodic table, used to produce the red colour in fireworks.

(a) In a sample of strontium, 10.0% of the atoms have a mass number of 86 and the remainder have a mass number of 88.

(i) Deduce the nuclear symbol of the isotope of strontium with mass number 86. Use section 6 of the data booklet.

[1]

(a)(ii)

(ii) Calculate, to two decimal places, the relative atomic mass of this sample of strontium.

[2]

(a)(iii)

(iii) Explain why the first ionization energy of strontium is greater than that of rubidium.

[2]

(b)(i)

(b) Strontium nitrate has the formula $Sr(NO_3)_2$ .

(i) The compound contains both ionic and covalent bonds. State which particles are joined by covalent bonds and which are joined by ionic bonds.

[2]

(b)(ii)

(ii) Contrast covalent and ionic bonds in terms of how the valence electrons are involved.

[1]

(b)(iii)

(iii) Write a balanced chemical equation for the formation of strontium nitrate solution from the reaction of solid strontium hydroxide with nitric acid.

[2]

(b)(iv)

(iv) Calculate the volume, in $cm^3$ , of $1.50 \text{ mol dm}^{-3}$ nitric acid required to react completely with $5.00 \text{ g}$ of solid strontium hydroxide.

[3]

(b)(v)

(v) Predict, with a reason, whether strontium oxide is acidic, basic or amphoteric.

[1]

(b)(vi)

(vi) Describe an experiment to establish the relative reactivity of strontium and zinc using the metals and aqueous solutions of their nitrates.

[2]

Question 7

EasyPaper 1A · calculator1 mark

Which substance has the lowest melting point?

A. Diamond

B. Graphite

C. Fullerene ( $C_{60}$ )

D. Silicon dioxide ( $SiO_2$ )

Question 8

MediumPaper 1A · calculator1 mark

Which property is the same for tetrachloromethane, $CCl_4$ , containing only the $^{35}Cl$ isotope and tetrachloromethane containing only the $^{37}Cl$ isotope?

A. Molar mass

B. Rate of diffusion in the gaseous state

C. Polarity of the carbon-chlorine bond

D. Frequency of the C-Cl bond vibration

Question 9

EasyPaper 2 · calculator1 mark

The following substances all contain a carbon-to-carbon bond: $C_2H_2$ , $C_2H_4$ , and $C_2H_6$ .

Which shows them in increasing order of carbon-to-carbon bond length (shortest first)?

A. $C_2H_6$ , $C_2H_4$ , $C_2H_2$

B. $C_2H_2$ , $C_2H_4$ , $C_2H_6$

C. $C_2H_4$ , $C_2H_2$ , $C_2H_6$

D. $C_2H_6$ , $C_2H_2$ , $C_2H_4$

Question 10

MediumPaper 2 · calculator5 marks

(a)

Boron trifluoride, $BF_3$ , is a versatile industrial catalyst. It reacts with ammonia, $NH_3$ , to form a stable adduct as shown in the equation below.

$BF_3 + NH_3 \rightarrow F_3BNH_3$

(a) Define the term Lewis acid.

[1]

(b)

(b) Explain, with reference to the electronic structures of the reactants, why this reaction is classified as a Lewis acid-base reaction. Identify the Lewis acid and the Lewis base.

[3]

(c)

[1]

Question 11

EasyPaper 2 · calculator1 mark

Which pair of elements is most likely to form a compound with covalent bonding?

A. Potassium and bromine

B. Calcium and sulfur

C. Nitrogen and chlorine

D. Aluminium and oxygen

Question 12

MediumPaper 2 · calculator1 mark

Under conditions of high pressure and low temperature, real gases deviate from ideal behaviour. Which of the following gases would be expected to show the largest deviation from the ideal gas law?

A. Argon, Ar

B. Ethene, $C_2H_4$

C. Fluorine, $F_2$

D. Fluoromethane, $CH_3F$

Question 13

EasyPaper 2 · calculator1 mark

What is the strongest intermolecular force in liquid propanone, $CH_3COCH_3$ ?

A. Hydrogen bonds

B. Permanent dipole-permanent dipole attraction

C. London (dispersion) forces

D. Covalent bonds

Question 14

MediumPaper 2 · calculator1 mark

Predict the shape of the sulfur dioxide molecule ( $SO_2$ ).

A. Linear

B. Bent

C. Trigonal planar

D. Trigonal pyramidal

Question 15

EasyPaper 1A · calculator1 mark

A white crystalline solid is investigated. It is found to be brittle, has a high melting point, and does not conduct electricity in the solid state. However, it conducts electricity when dissolved in water. Which type of bonding best describes this solid?

A. Covalent molecular

B. Covalent network

C. Ionic

D. Metallic

Question 16

MediumPaper 2 · calculator1 mark

Which of the following molecules is polar?

I. $CF_4$

II. $CH_2F_2$

III. $BeF_2$

A. I only

B. II only

C. III only

D. I and III only

Question 17

EasyPaper 1A · calculator1 mark

Which molecule is polar?

A. $BF_3$

B. $NH_3$

C. $CCl_4$

D. $BeCl_2$

Question 18

MediumPaper 2 · calculator1 mark

What is the approximate $OSO$ bond angle in the sulfite ion, $SO_3^{2-}$ ?

A. $107^\circ$

B. $109.5^\circ$

C. $120^\circ$

D. $180^\circ$

Question 19

EasyPaper 1A · calculator1 mark

Which molecule has the longest nitrogen to nitrogen bond?

A. $N_2$

B. $N_2H_2$

C. $N_2H_4$

D. $NH_3$

Question 20

MediumPaper 2 · calculator4 marks

Consider the following three organic compounds with similar molar masses: propane ( $C_3H_8$ ), propanal ( $CH_3CH_2CHO$ ), and propan-1-ol ( $CH_3CH_2CH_2OH$ ).

(a) Arrange these three compounds in order of increasing boiling point (lowest first). Explain your reasoning by referring to the types and relative strengths of the intermolecular forces present in each substance.

25 more The covalent model questions in the app

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Where marks are lost

Reaching for "human error" or "only one trial." A source of error has to be a specific step in the method, not a general apology for the result.
Joining the dots instead of drawing a curve.
Naming a chemical instead of the property that distinguishes it, or vice versa. Answering with the nearest fact that comes to mind rather than the fact the command term and stem jointly ask for is a recurring way to answer a question that was not, quite, the one asked.

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What does The covalent model cover in IB Chemistry?

This topic covers models of covalent bonding, molecular structures, and their influence on properties. A covalent bond forms from electrostatic attraction between shared electron pairs and positively charged nuclei. Lewis formulas represent valence electrons, including bonding and non-bonding pairs, often following the octet rule.

Is The covalent model SL or HL?

Both. SL and HL students study The covalent model, and HL goes further: 2.2.11 Resonance structures occur when there is more than one possible position for a double bond in a molecule. Students deduce resonance structures of molecules and ions.

How do I revise The covalent model for IB Chemistry?

Start from the core idea: this topic covers models of covalent bonding, molecular structures, and their influence on properties. In the exam: the heaviest bonding subtopic. May 2025 HL Paper 2 TZ1 asked for a Lewis formula of the nitrate ion [1] and its molecular geometry [1] as adjacent parts, which is the standard pairing. Then practise exam-style questions, easiest first, writing out every step of your working before you check it.

How does FourtyFive help me practise The covalent model?

FourtyFive has 45 The covalent model questions. Every answer you write is marked mark by mark, IB-style, and you see where each mark was won or lost. Every part has a hint, the AI tutor helps you through the step you are stuck on, and your Study Profile picks what to practise next.

Is FourtyFive free for The covalent model practice?

Yes. A free account gives you 50 marked answers a month, and you do not need a card to sign up.

Can I handwrite The covalent model answers on an iPad?

Yes. In the FourtyFive iPad app you write your working by hand with Apple Pencil, the way you would on paper, and it is marked the same way.

The covalent model: notes and practice questions

How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

25 more The covalent model questions in the app

Where marks are lost

Practise this topic as a session

Questions,
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How it is examined

Guiding questions

Linking questions

Practice questions

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question 9

Question 10

Question 11

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17

Question 18

Question 19

Question 20

25 more The covalent model questions in the app

Where marks are lost

Questions,answered.

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