Simple Molecules And Covalent Bonds

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Covalent Bonds

Covalent bonds form 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, achieving a noble gas configuration.
Covalently bonded atoms form molecules.
Dot-and-cross diagrams show the arrangement of electrons: dots for one atom, crosses for the other.
A single covalent bond involves one shared pair of electrons (e.g., H₂, Cl₂, H₂O, CH₄, NH₃, HCl).

H₂ dot-and-cross diagram: single covalent bond formed by one shared pair of electrons.

Molecules & Compounds

Some atoms share more than one pair of electrons to achieve a full outer shell.
A double bond involves two shared pairs (e.g., O₂, ethene, CO₂).
A triple bond involves three shared pairs (e.g., N₂).
In ethene, the two carbon atoms are joined by a double bond.
In carbon dioxide, each oxygen is double-bonded to the central carbon.

O₂ dot-and-cross diagram: a double covalent bond formed by two shared pairs of electrons.

Properties of Simple Molecular Compounds

Simple molecules have low melting and boiling points; most are liquids or gases at room temperature.
Melting and boiling points increase as molecular size increases.
They are poor conductors of electricity because there are no free ions or electrons.
The atoms within a molecule are held by strong covalent bonds; between molecules there are weak intermolecular forces.
Weak intermolecular forces explain low melting/boiling points; larger molecules have stronger intermolecular forces.
Covalent compounds are often insulators (e.g., plastic coating on wires).

Simple molecular structure of water: strong covalent bonds within each molecule, weak intermolecular forces between molecules.

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练习题

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1.Covalent bonds are formed by the sharing of electrons between non-metal atoms.
Easy
True or false?
2.Complete the sentence:
Easy
In a covalent bond, atoms share ____ of electrons to achieve a full outer shell.
3.Name the type of bond formed when two atoms share three pairs of electrons.
Easy
4.Which of the following molecules contains a double covalent bond?
Medium
- AHydrogen
- BChlorine
- COxygen
- DMethane
5.Simple molecular compounds have high melting and boiling points due to strong covalent bonds between molecules.
Medium
True or false?
6.Explain why simple molecular compounds are poor conductors of electricity.
Medium
7.How many electrons are shared in a double covalent bond?
Easy
- A5
- B4
- C6
- D3
8.Match each molecule to the number of covalent bonds between its atoms.
Medium
- Hydrogen (H2)
- Oxygen (O2)
- Nitrogen (N2)
- 1
- 2
- 3

更多题目

9.Arrange the following substances in order of increasing melting point: methane, water, hydrogen chloride.
Medium
- methane
- hydrogen chloride
- water
10.Complete the sentence about the properties of simple molecular compounds:
Medium
As the size of the molecules increases, the intermolecular forces ____ and the melting and boiling points ____.
11.Which statement explains why water (H2O) has a higher boiling point than methane (CH4)?
Hard
- AWater has stronger covalent bonds than methane.
- BWater has stronger intermolecular forces than methane.
- CWater molecules are smaller than methane molecules.
- DWater contains hydrogen bonds, which are covalent bonds.
12.State the type of bond that holds atoms together within a molecule of ammonia (NH3).
Easy
13.Covalent bonds are formed between non-metal atoms only.
Easy
True or false?
14.Which type of bond is formed when two atoms share three pairs of electrons?
Easy
- ASingle bond
- BDouble bond
- CTriple bond
- DIonic bond
15.Which of the following molecules contains a double bond?
Medium
- AH2
- BO2
- CN2
- DCH4
16.Simple molecular compounds have high melting points because of strong covalent bonds between molecules.
Medium
True or false?
17.Match each molecule to the number of covalent bonds between its two central atoms.
Hard
- Oxygen (O2)
- Nitrogen (N2)
- Hydrogen (H2)
- Ethene (C2H4)
- 1 bond
- 2 bonds
- 3 bonds
- 2 bonds
18.Arrange the following molecules in order of increasing melting point: methane (CH4), water (H2O), hydrogen chloride (HCl).
Medium
- methane
- hydrogen chloride
- water
19.Which of the following statements about simple molecular compounds are correct? Select all that apply.
Hard
- AThey have high melting points due to strong covalent bonds.
- BThey are usually gases or liquids at room temperature.
- CThey conduct electricity when molten.
- DThey have weak intermolecular forces between molecules.
- EThe atoms within a molecule are held by covalent bonds.
20.Which of the following are properties of simple molecular compounds? Select all that apply.
Medium
- ALow melting points
- BHigh electrical conductivity
- CWeak intermolecular forces
- DStrong covalent bonds within molecules
- EUsually solid at room temperature
21.The table shows the melting points of four substances. | Substance | Melting point (°C) | |-----------|---------------------| | W | 801 | | X | -182 | | Y | 3550 | | Z | 1414 | Which substance is most likely a simple molecular compound?
Medium
- AW
- BX
- CY
- DZ
22.Which of the following statements about covalent bonding are correct? Select all that apply.
Hard
- ACovalent bonds involve the sharing of electrons.
- BCovalent bonds only form between non-metal atoms.
- CA double bond involves sharing two pairs of electrons.
- DCovalent compounds always conduct electricity when molten.
- EThe atoms in a molecule are held together by intermolecular forces.
23.A molecule of oxygen (O₂) contains a double covalent bond.
Easy
True or false?
24.Arrange the following molecules in order of increasing melting point: methane (CH₄), water (H₂O), hydrogen chloride (HCl).
Medium
- methane (CH₄)
- hydrogen chloride (HCl)
- water (H₂O)
25.A student measures the boiling point of a liquid substance and finds it to be 78°C. Which type of substance is it most likely to be?
Medium
- AIonic compound
- BSimple molecular compound
- CGiant covalent compound
- DMetallic compound
26.A student sets up an experiment to measure the boiling point of a colourless liquid. The liquid is heated and the temperature recorded every 30 seconds. The boiling point is found to be 78°C. Which type of substance is the liquid most likely to be?
Easy
- AA simple molecular compound
- BAn ionic compound
- CA giant covalent compound
- DA metallic compound
27.In an experiment to determine the melting point of substance X, a student obtains a value of 113°C. Substance X is a solid at room temperature, does not conduct electricity, and dissolves in water to form a solution that does not conduct electricity. What is the most likely identity of substance X?
Medium
- AIodine (I₂)
- BSodium chloride (NaCl)
- CCopper (Cu)
- DSilicon dioxide (SiO₂)
28.A student investigates the electrical conductivity of four substances. Which of the following substances would conduct electricity? Select all that apply.
Medium
- ASolid sodium chloride
- BMolten sodium chloride
- CLiquid water
- DGraphite (a form of carbon)
29.A student wants to determine the melting point of a simple molecular solid. Which of the following procedures is most appropriate?
Medium
- AHeat the solid in a test tube over a Bunsen burner and record the temperature at which it first melts.
- BPlace a small amount of the solid in a capillary tube, attach a thermometer, and heat slowly in a water bath, recording the temperature range over which melting occurs.
- CDissolve the solid in water and heat the solution until it boils.
- DPlace the solid on a watch glass and heat it with a hot air gun, noting when it becomes liquid.
30.Which of the following dot-and-cross diagrams correctly represents a molecule of chlorine (Cl₂)?
Medium
- ATwo Cl atoms each with 7 outer electrons, sharing one pair in the overlap region.
- BTwo Cl atoms each with 8 outer electrons, sharing two pairs in the overlap region.
- CTwo Cl atoms each with 7 outer electrons, no overlap, separate shells.
- DTwo Cl atoms each with 8 outer electrons, sharing one pair with square brackets and charges.
31.Which type of elements participate in covalent bonding?
Easy
- Aonly non-metals
- Bmetals and non-metals
- Conly metals
- Dmetalloids
32.Covalent compounds are formed when electrons are transferred from one atom to another.
Easy
True or false?
33.In a dot-and-cross diagram of a covalent molecule, what do the dots and crosses represent?
Easy
- Aelectrons from different atoms
- Bprotons from different atoms
- Cneutrons from different atoms
- Dnuclei of different atoms
34.Simple molecular compounds have high melting and boiling points because of strong covalent bonds between molecules.
Easy
True or false?
35.Which of the following molecules contains a triple covalent bond?
Easy
- AO₂
- BN₂
- CH₂O
- DCH₄
36.Arrange the following substances in order of increasing melting point: methane (CH₄), water (H₂O), hydrogen chloride (HCl), bromine (Br₂).
Hard
- methane (CH₄)
- bromine (Br₂)
- hydrogen chloride (HCl)
- water (H₂O)
37.Covalent bonds are formed by the transfer of electrons from one atom to another.
Easy
True or false?
38.In a molecule of oxygen (O₂), the two oxygen atoms are joined by a double covalent bond.
Easy
True or false?
39.Which of the following dot-and-cross diagrams correctly represents a molecule of methane (CH₄)?
Medium
- AA carbon atom in the centre with four hydrogen atoms around it, each sharing one electron pair, and all outer shells full.
- BA carbon atom with four hydrogen atoms each sharing two electron pairs.
- CA carbon atom with four hydrogen atoms each transferring one electron to carbon.
- DA carbon atom with four hydrogen atoms each sharing one electron, but carbon's outer shell has only six electrons.
40.A substance has a melting point of -95°C and a boiling point of 68°C at room pressure. It does not conduct electricity in any state. Which type of structure does it most likely have?
Medium
- ASimple molecular
- BGiant ionic
- CGiant covalent (diamond)
- DMetallic
41.In a dot-and-cross diagram of a covalent molecule, the dots and crosses represent the nuclei of the atoms.
Easy
True or false?
42.Arrange the following substances in order of increasing melting point. (lowest first)
Hard
- Methane (CH₄)
- Water (H₂O)
- Hydrogen chloride (HCl)
- Bromine (Br₂)
43.The table shows the melting points of four substances. Which substance is most likely a simple molecular compound? | Substance | Melting point (°C) | |-----------|-------------------| | A | 801 | | B | -7 | | C | 3550 | | D | 660 |
Medium
- AA
- BB
- CC
- DD
44.A student is given four white solid substances: A, B, C, and D. She tests their solubility in water and their electrical conductivity in aqueous solution. The results are shown in the table. | Substance | Solubility in water | Conductivity in aqueous solution | |-----------|--------------------|----------------------------------| | A | soluble | conducts | | B | insoluble | does not conduct | | C | soluble | does not conduct | | D | insoluble | conducts | Which substance is most likely a simple molecular compound?
Medium
- AA
- BB
- CC
- DD
45.A student performs an experiment to determine the melting point of a simple molecular solid. Which piece of apparatus is most essential for accurately measuring the temperature?
Easy
- ABunsen burner
- BThermometer
- CBalance
- DMeasuring cylinder
46.Complete the sentence.
Easy
In a covalent bond, atoms share ____ of electrons.
47.Complete the sentence about the properties of simple molecular compounds.
Easy
Simple molecular compounds have low melting and boiling points because only weak ____ forces exist between molecules.
48.Complete the sentence about covalent bonds.
Easy
A single covalent bond involves the sharing of ____ pair(s) of electrons.
49.How many electrons are shared in a triple covalent bond?
Easy
50.State the type of bond that holds atoms together within a molecule of ammonia (NH₃).
Easy
51.Complete the sentence about covalent bonding.
Easy
A covalent bond is formed when two atoms ____ a pair of ____.
52.How many electrons are shared in a single covalent bond?
Easy
53.Explain why simple molecular compounds have low melting points.
Medium
54.Explain why water (H₂O) has a higher boiling point than methane (CH₄).
Hard
55.How many electrons are shared in a triple covalent bond?
Medium
- A5
- B8
- C7
- D6
56.How many electrons are shared in a single covalent bond?
Easy
- A2
- B1
- C4
- D3

闪卡

What is a covalent bond?

A covalent bond is formed when pairs of electrons are shared between atoms, typically non-metal elements, to achieve a full outer shell.

Which types of elements participate in covalent bonding?

Only non-metal elements participate in covalent bonding.

What is a molecule?

A molecule is a group of two or more atoms covalently bonded together.

How are dot-and-cross diagrams used to represent covalent bonds?

Dot-and-cross diagrams show the electronic configurations in simple molecules, with dots representing electrons from one atom and crosses from the other, overlapping in the bond region.

What is a single covalent bond?

A single covalent bond is formed when two atoms share one pair of electrons.

Give an example of a molecule with a single covalent bond.

Hydrogen (H₂) forms a single covalent bond between two hydrogen atoms.

What is a double covalent bond?

A double covalent bond is formed when two atoms share two pairs of electrons.

Give an example of a molecule with a double bond.

Oxygen (O₂) has a double bond between the two oxygen atoms.

What is a triple covalent bond?

A triple covalent bond is formed when two atoms share three pairs of electrons.

Give an example of a molecule with a triple bond.

Nitrogen (N₂) has a triple bond between the two nitrogen atoms.

What are the typical physical properties of simple molecular compounds?

They have low melting and boiling points, are often liquids or gases at room temperature, and have poor electrical conductivity.

Why do simple molecular compounds have low melting and boiling points?

Because the intermolecular forces between molecules are weak, requiring little energy to overcome.

What happens to melting and boiling points as the size of molecules increases?

They generally increase because larger molecules have more electrons, leading to stronger intermolecular forces.

Why are simple molecular compounds poor conductors of electricity?

Because they have no free ions or electrons to carry charge.

What type of bonding is present between atoms within a covalent molecule?

Covalent bonds hold the atoms together within the molecule.

What type of forces exist between molecules in a simple molecular substance?

Weak intermolecular forces act between neighbouring molecules.

How does the strength of intermolecular forces compare to covalent bonds?

Intermolecular forces are much weaker than covalent bonds, typically about one tenth as strong.

Draw the dot-and-cross diagram for chlorine (Cl₂).

Each chlorine atom has 7 outer electrons; they share one pair, so each has a full outer shell of 8 electrons (including the shared pair).

Draw the dot-and-cross diagram for water (H₂O).

Oxygen shares one electron with each hydrogen, forming two single bonds. Oxygen has two lone pairs, and each hydrogen has a duet.

Draw the dot-and-cross diagram for methane (CH₄).

Carbon shares one electron with each of four hydrogen atoms, forming four single bonds. Carbon has a full octet, each hydrogen a duet.

Draw the dot-and-cross diagram for ammonia (NH₃).

Nitrogen shares one electron with each of three hydrogen atoms, forming three single bonds. Nitrogen has one lone pair, each hydrogen a duet.

Draw the dot-and-cross diagram for hydrogen chloride (HCl).

Hydrogen shares one electron with chlorine, forming a single bond. Chlorine has three lone pairs, hydrogen has a duet.

Draw the dot-and-cross diagram for oxygen (O₂).

Each oxygen atom shares two electrons, forming a double bond. Each oxygen has two lone pairs, giving each a full octet.

Draw the dot-and-cross diagram for nitrogen (N₂).

Each nitrogen atom shares three electrons, forming a triple bond. Each nitrogen has one lone pair, giving each a full octet.

Draw the dot-and-cross diagram for carbon dioxide (CO₂).

Carbon shares two electrons with each oxygen, forming two double bonds. Each oxygen has two lone pairs, carbon has no lone pairs.

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