Simple Molecules & Covalent Bonds

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Covalent Bonds

A covalent bond is formed when pairs of electrons are shared between atoms.
Only non-metal elements participate in covalent bonding.
Each atom gains a full outer shell of electrons (noble gas configuration).
Covalently bonded substances may consist of small molecules or giant molecules.
Dot-and-cross diagrams show the electronic configuration: dots for one atom's electrons, crosses for the other's.
Electron shells overlap and shared electrons are shown in the overlap region.

A single covalent bond in hydrogen

Single Covalent Bonds

A single covalent bond involves one shared pair of electrons.
Examples: H₂ (H–H), Cl₂ (Cl–Cl), H₂O, CH₄, NH₃, HCl.
In H₂O, oxygen shares one electron with each hydrogen atom, forming two single bonds.
In CH₄, carbon shares one electron with each of four hydrogen atoms.
In NH₃, nitrogen shares one electron with each of three hydrogen atoms.
In HCl, hydrogen and chlorine share one pair of electrons.

H₂O dot-and-cross diagram: oxygen forms a single covalent bond with each hydrogen atom.

Double and Triple Covalent Bonds (Extended)

Some atoms share two pairs of electrons to form a double bond.
Some atoms share three pairs of electrons to form a triple bond.
O₂ has a double bond (O=O) – each oxygen shares two electrons.
N₂ has a triple bond (N≡N) – each nitrogen shares three electrons.
Ethene (C₂H₄) has a double bond between the two carbon atoms.
Carbon dioxide (CO₂) has two double bonds (O=C=O).

A double covalent bond in oxygen

Properties of Simple Molecular Compounds

Simple molecular compounds have low melting and boiling points.
They are usually liquids or gases at room temperature.
As molecule size increases, melting and boiling points increase.
They have poor electrical conductivity – do not conduct in solid or liquid state.
They are insulators (e.g., plastic coating on wires).

Explaining Properties (Extended)

Atoms within molecules are held by strong covalent bonds.
Between molecules there are weak intermolecular forces.
Low melting/boiling points are due to weak intermolecular forces requiring little energy to overcome.
Intermolecular forces are about one tenth as strong as covalent bonds.
Larger molecules have more electrons, increasing intermolecular forces and raising melting/boiling points.
Poor conductivity because there are no free ions or electrons to carry charge.

Bonds within and between water molecules

Distinguishing Covalent from Ionic

Covalent compounds contain only non-metals.
Ionic compounds contain metal and non-metal.
In dot-and-cross diagrams, covalent compounds show overlapping shells with shared electrons.
Ionic compounds show separate ions with square brackets and charges.
Covalent compounds have low melting points; ionic compounds have high melting points.

Ionic bonding in sodium chloride

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Practice questions

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1.What type of bonding is found in methane?
Easy
- ACovalent
- BIonic
- CMetallic
- DHydrogen bonding
2.Which of the following substances has a simple molecular structure?
Easy
- ACarbon dioxide
- BCalcium carbonate
- CCopper
- DMagnesium oxide
3.A covalent bond is formed when two atoms:
Easy
- AShare a pair of electrons
- BTransfer electrons
- CShare protons
- DAttract via opposite charges
4.When a simple molecular substance melts, what is broken?
Easy
- AIntermolecular forces
- BCovalent bonds
- CIonic bonds
- DMetallic bonds
5.Which of the following statements explains why ammonia does not conduct electricity?
Easy
- AIt does not contain any free ions or free electrons
- BIts ions are in a fixed position
- CIt has delocalised electrons
- DIt contains free electrons
6.The boiling point of nitrogen is very low even though the bond between atoms in a nitrogen molecule is very strong. Why?
Medium
- AOnly weak intermolecular forces need to be overcome
- BThe covalent bonds are broken during boiling
- CNitrogen molecules are very small
- DNitrogen is a gas at room temperature
7.Which of the following elements can form covalent bonds?
Easy
- ACarbon
- BSodium
- CCalcium
- DIron
8.Which of the following molecules contains a double bond?
Easy
- AOxygen (O2)
- BHydrogen (H2)
- CChlorine (Cl2)
- DMethane (CH4)

More questions

9.A substance has a melting point of -94°C, does not conduct electricity as a solid or liquid. What type of structure does it most likely have?
Medium
- ASimple molecular
- BIonic
- CGiant covalent
- DMetallic
10.Which of the following is a property of simple molecular compounds?
Easy
- ALow melting and boiling points
- BHigh electrical conductivity
- CHard and brittle
- DHigh melting points
11.Substance X has a melting point of -94°C, a boiling point of 63°C, and does not conduct electricity as a solid or liquid. Which type of structure does substance X have?
Medium
- ASimple molecular
- BGiant ionic
- CGiant covalent
- DMetallic

12.

The table shows the melting points and electrical conductivities of five substances. Which substance is likely to be a simple molecular compound?

Substance	Melting point (°C)	Conductivity as solid	Conductivity as liquid
A	-210	does not conduct	does not conduct
B	777	does not conduct	good conductor
C	962	good conductor	good conductor
D	1410	does not conduct	does not conduct
E	1064	good conductor	good conductor

Medium

13.The boiling point of nitrogen (N₂) is -196°C, yet the bond between nitrogen atoms is a very strong triple bond. Why is the boiling point so low?
Medium
- AOnly weak intermolecular forces need to be overcome for boiling, not the covalent bonds
- BThe triple bond breaks easily when heated
- CNitrogen molecules are very small so they boil easily
- DThe covalent bond is weak in nitrogen
14.A molecule of sulfur chloride (SCl₂) contains covalent bonds. Which diagram correctly represents the arrangement of outer electrons? (Use x for sulfur electrons, o for chlorine electrons.)
Hard
- ACl (with 7 o) sharing one pair with S (with 6 x) and another Cl sharing one pair, each Cl has 3 lone pairs, S has 2 lone pairs
- BCl (with 7 o) sharing two pairs with S (with 6 x) and another Cl sharing two pairs, each Cl has 2 lone pairs
- CCl (with 7 o) sharing one pair with S (with 6 x) and another Cl sharing one pair, each Cl has 3 lone pairs, S has 1 lone pair
- DCl (with 7 o) sharing one pair with S (with 6 x) and another Cl sharing one pair, each Cl has 3 lone pairs, S has no lone pairs
15.Which two substances from the list could exist as simple covalent molecules? List: A (melting point -210°C, boiling point -196°C), B (melting point 777°C, boiling point 1627°C), C (melting point 962°C, boiling point 2212°C), D (melting point -94°C, boiling point 63°C), E (melting point 1410°C, boiling point 2355°C), F (melting point 1064°C, boiling point 2807°C)
Medium
- AA and D
- BB and C
- CC and F
- DD and E
16.Why does neon exist as single atoms while fluorine exists as F₂ molecules?
Medium
- ANeon has a full outer shell of electrons and does not need to form bonds; fluorine has seven outer electrons and shares one pair to achieve a full shell
- BNeon is a noble gas and cannot form bonds; fluorine is a halogen and always forms diatomic molecules
- CNeon atoms are too small to bond; fluorine atoms are larger and can share electrons
- DNeon has a higher melting point than fluorine, so it prefers to be monatomic

17.

The table shows the properties of four substances. Which substance is most likely to be a simple molecular substance?

Substance	Melting point (°C)	Boiling point (°C)	Conductivity (solid)	Conductivity (liquid)
P	801	1413	does not conduct	conducts
Q	-95	36	does not conduct	does not conduct
R	3550	4827	does not conduct	does not conduct
S	650	1107	conducts	conducts

Medium

18.A student measures the melting point of a substance and finds it to be -94°C. The substance does not conduct electricity as a solid or liquid. Which type of structure does it most likely have?
Medium
- AGiant ionic
- BSimple molecular
- CGiant metallic
- DGiant covalent
19.Substance X has a melting point of 777°C, a boiling point of 1627°C, and conducts electricity when molten but not as a solid. What type of structure does substance X have?
Medium
- ASimple molecular
- BGiant ionic
- CGiant metallic
- DGiant covalent
20.Which statement explains why ammonia (NH₃) does not conduct electricity?
Easy
- AIt contains free electrons and free ions.
- BIt does not contain free electrons and does not contain free ions.
- CIt contains free electrons but no free ions.
- DIt contains free ions but no free electrons.
21.The boiling point of nitrogen (N₂) is -196°C. Why is the boiling point so low even though the N≡N triple bond is very strong?
Medium
- AThe triple bond is weak.
- BOnly weak intermolecular forces need to be overcome during boiling.
- CThe covalent bonds break during boiling.
- DNitrogen atoms are very small.
22.Which two substances from the list below could exist as simple covalent molecules? List: A: melting point -210°C, boiling point -196°C, does not conduct B: melting point 777°C, boiling point 1627°C, conducts when molten C: melting point 962°C, boiling point 2212°C, conducts as solid D: melting point -94°C, boiling point 63°C, does not conduct E: melting point 1410°C, boiling point 2355°C, does not conduct F: melting point 1064°C, boiling point 2807°C, conducts as solid
Medium
- AA and B
- BA and D
- CC and E
- DD and F
23.A student draws a dot-and-cross diagram for a molecule of carbon dioxide. Which diagram is correct? (Use dots for carbon electrons and crosses for oxygen electrons.)
Medium
- AO (with 6 crosses) double bonded to C (with 4 dots) double bonded to O (with 6 crosses); each O has 2 lone pairs, C has no lone pairs.
- BO (with 6 crosses) single bonded to C (with 4 dots) single bonded to O (with 6 crosses); each O has 3 lone pairs, C has 2 lone pairs.
- CO (with 6 crosses) triple bonded to C (with 4 dots) single bonded to O (with 6 crosses); one O has 1 lone pair, the other has 3 lone pairs, C has 1 lone pair.
- DO (with 6 crosses) double bonded to C (with 4 dots) double bonded to O (with 6 crosses); each O has 4 lone pairs, C has 2 lone pairs.
24.Which two substances from the list could exist as simple covalent molecules? List: A: melting point -210°C, boiling point -196°C, does not conduct electricity B: melting point 777°C, boiling point 1627°C, conducts electricity when molten C: melting point 962°C, boiling point 2212°C, conducts electricity as solid and liquid D: melting point -94°C, boiling point 63°C, does not conduct electricity
Medium
- AA and D
- BB and C
- CA and B
- DC and D
25.Which of the following is the correct dot-and-cross diagram for a molecule of chlorine (Cl2)? (Use dots for electrons from one atom and crosses for the other.)
Medium
- ATwo Cl atoms with overlapping shells, each showing 7 outer electrons, with one shared pair in the overlap
- BTwo Cl atoms with separate shells, each showing 8 outer electrons, no overlap
- CCl+ and Cl- ions with square brackets and charges
- DTwo Cl atoms sharing two pairs of electrons

26.

The table shows the melting points and electrical conductivities of five substances. Which substance is likely to be a simple molecular compound?

Substance	Melting point (°C)	Electrical conductivity (solid)	Electrical conductivity (liquid)
P	-95	does not conduct	does not conduct
Q	801	does not conduct	conducts
R	3550	does not conduct	does not conduct
S	650	conducts	conducts
T	-7	does not conduct	does not conduct

Medium

27.

A student is given four substances with the following properties. Which substance is most likely to be a simple molecular substance?

Substance	Melting point (°C)	Boiling point (°C)	Electrical conductivity (solid)	Electrical conductivity (liquid)
A	-94	63	does not conduct	does not conduct
B	777	1627	does not conduct	good conductor
C	962	2212	good conductor	good conductor
D	1410	2355	does not conduct	does not conduct

Medium

28.Why does the sodium chloride crystal shown have a high melting point?
Easy
- AStrong attractions between oppositely charged ions require much energy to overcome
- BIts molecules are held together by weak forces
- CIts electrons are all free to move
- DIts ions have no electric charge
29.A student measures the boiling point of a liquid and finds it to be -162°C. The liquid does not conduct electricity. Which substance could it be?
Medium
- AMethane
- BWater
- CSodium chloride
- DCopper
30.Covalent bonds are formed by the sharing of electrons between atoms.
Easy
True or false?
31.Only metal elements can form covalent bonds.
Easy
True or false?
32.In a dot-and-cross diagram, electrons from one atom are represented by dots and from the other atom by crosses. What does the overlapping region represent?
Easy
- AThe shared pair of electrons in the covalent bond
- BThe nucleus of the atoms
- CThe inner shell electrons
- DThe ionic bond
33.Which of the following are properties of simple molecular compounds? (Select all that apply)
Medium
- ALow melting and boiling points
- BHigh electrical conductivity
- CPoor electrical conductivity
- DUsually liquids or gases at room temperature
- EStrong intermolecular forces
34.Match each molecule to the number of covalent bonds between its atoms (single, double, or triple bond).
Easy
- Hydrogen (H₂)
- Oxygen (O₂)
- Nitrogen (N₂)
- Single bond
- Double bond
- Triple bond
35.Arrange the following molecules in order of increasing strength of intermolecular forces (weakest first).
Medium
- Methane (CH₄)
- Ethane (C₂H₆)
- Propane (C₃H₈)
36.Simple molecular compounds conduct electricity in the solid state.
Easy
True or false?
37.Which of the following correctly describes the relationship between covalent bonds and intermolecular forces in a simple molecular substance?
Medium
- ACovalent bonds are stronger than intermolecular forces.
- BIntermolecular forces are stronger than covalent bonds.
- CCovalent bonds and intermolecular forces are equal in strength.
- DIntermolecular forces do not exist in simple molecular substances.
38.Match each molecule to the type of covalent bond it contains (single, double, or triple bond).
Easy
- Nitrogen (N₂)
- Oxygen (O₂)
- Chlorine (Cl₂)
- Single bond
- Double bond
- Triple bond
39.Arrange the following substances in order of increasing melting point (lowest first).
Medium
- Water (H₂O)
- Methane (CH₄)
- Ethanol (C₂H₅OH)
40.Complete the sentence.
Easy
A covalent bond is formed when two atoms ____ a pair of electrons.
41.How many electrons are shared in a double covalent bond?
Easy
42.What is the name given to the forces that act between molecules in a simple molecular substance?
Easy
43.Complete the sentence: In a dot-and-cross diagram for a covalent compound, the electron shells of each atom ___ and the shared electrons are shown in the area of overlap.
Medium
In a dot-and-cross diagram, the electron shells of each atom ____.
44.Complete the sentence about covalent bonding.
Easy
Covalent bonds are formed when pairs of electrons are ____ between atoms.
45.How many electrons are shared in a double covalent bond?
Easy
- A3
- B4
- C5
- D6

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