Simple Molecules & Covalent Bonds (Cambridge (CIE) IGCSE Chemistry)

The table below includes information about some of the elements in Period 2.

Why does neon exist as single atoms but fluorine exists as molecules?

What determines the order of the elements in a period?

Extended Only

When liquid nitrogen boils the following change occurs.

N₂ (l) → N₂ (g)

The boiling point of nitrogen is very low even though the bond between the atoms in a nitrogen molecule is very strong. Suggest an explanation.

Draw a diagram showing the arrangement of the outer shell electrons in a molecule of nitrogen.

Strontium and sulfur chlorides both have a formula of the type XCl₂ but they have different properties.

The formulae of the chlorides are similar because both elements have a valency of 2.

Explain why Group II and Group VI elements both have a valency of 2.

Draw a diagram showing the arrangement of the outer electrons in one covalent molecule of sulfur chloride. Use x to represent an electron from a sulfur atom. Use o to represent an electron from a chlorine atom.

Explain the difference in electrical conductivity between the following.

i) Solid and liquid strontium chloride

[1]

ii) Liquid strontium chloride and liquid sulfur chloride

[1]

Table 1.1 shows the properties of four substances.

Table 1.1 

Complete Table 1.1 to show the electrical conductivity of solid diamond and molten sulfur.

State one piece of evidence from the table that shows that sulfur is a simple molecular substance.

Aqueous chlorine reacts with aqueous potassium bromide as shown.

Cl ₂ + 2KBr → Br₂ + 2KCl

Complete the dot-and-cross diagram in Figure 1.1  of a molecule of chlorine. Show outer shell electrons only.

Figure 1.1

Alkynes and alkenes are homologous series of unsaturated hydrocarbons. All alkynes contain a C≡C triple bond. Ethene and but-2-ene are alkenes.

Draw a dot-and-cross diagram to show a molecule of ethene, CH₂=CH₂. Show outer shell electrons only.

This question is about phosphorus and compounds of phosphorus. Phosphorus has the formula P_4. Some properties of P₄ are shown.

Name the type of bonding that exists between the atoms in a P₄ molecule.

Explain, in terms of attractive forces between particles, why P₄ has a low melting point.

Explain why phosphorus is a non-conductor of electricity.

Phosphorus(V) oxide, P₄O₁₀, is an acidic oxide. Phosphorus(V) oxide, P₄O₁₀, reacts with aqueous sodium hydroxide to form a salt containing the phosphate ion . Water is the only other product. Write a chemical equation for the reaction between phosphorus(V) oxide and aqueous sodium hydroxide.

Iodine and bromine are both halogens. The bonding in iodine is similar to the bonding in bromine.

Suggest two reasons why they bond in similar ways.

Complete the diagram below to show the bonding arrangement in a molecule of iodine.

Show the outer shell electrons only.

Explain why liquid iodine is unable to conduct electricity.

Iodine has an atomic number of 53 and xenon has an atomic number of 54.

Explain why xenon exists as an atom whereas iodine exists as a molecule.

Separate: Chemistry Only

Selenium and sulfur are in Group VI. They have similar properties.

One of the main uses of selenium is in photoelectric cells. These cells can change light into electrical energy.

i) Name a process which can change light into chemical energy.

[1]

ii) Name a device which can change chemical energy into electrical energy.

[1]

Extended Only

The electron distribution of a selenium atom is 2 + 8 + 18 + 6.

i) Selenium forms an ionic compound with potassium. Draw a diagram which shows the formula of this ionic compound,  the charges on the ions and the arrangement of the outer electrons around the negative ion. Use o to represent an electron from an atom of potassium. Use x to represent an electron from an atom of selenium.

[3]

ii) Draw a diagram showing the arrangement of the outer electrons in one molecule of the covalent compound selenium chloride. Use x to represent an electron from an atom of selenium. Use o to represent an electron from an atom of chlorine.

[3]

iii) Predict two differences in the physical properties of these two compounds.

[2]

Separate: Chemistry and Extended Only

The selenide ion reacts with water.

Se^2– + H₂O → HSe^–+ OH^–

What type of reagent is the selenide ion in this reaction? Give a reason for your choice.

The diagram shows part of the structures of caesium chloride and carbon dioxide.

Describe both caesium chloride and carbon dioxide in terms of:

• bonding:

• arrangement of particles:

Extended Only

Ethanal is manufactured by two different processes.

The structural formula of ethanal is shown.

The C=O group in aldehydes is at the end of the carbon chain. This is a reactive part of the molecule.

Complete the dot-and-cross diagram to show the electron arrangement of a molecule of ethanal. Inner shells have been drawn.

Extended Only

Fluorine forms both ionic and covalent compounds.

Carbonyl fluoride, COF₂, is a covalent compound. The structure of a molecule of COF₂ is shown.

Complete the dot-and-cross diagram to show the electron arrangement in a molecule of carbonyl fluoride. Show outer shell electrons only.

Extended Only

The melting points of magnesium fluoride and carbonyl fluoride are shown.

Explain, using your knowledge of structure and bonding, why carbonyl fluoride has a low melting point.

Phosphorous trichloride, PCl₃, bonds covalently.

Draw a dot-and-cross diagram to show the bonding in PCl₃. Show the outer electrons only.

Compare the bonding in phosphorous trichloride and ammonia.

Extended Only

Like nitrogen, phosphorus can exist as a diatomic molecule, diphosphorus, P₂, but it is unstable.

Suggest what type of covalent bond exists within diphosphorus.

Separate: Chemistry and Extended Only

The bond energy of the bond between the two nitrogen atoms within the nitrogen, N₂, molecule is 945 kJ/mol. 

Suggest the bond energy of the bond between the two phosphorus atoms within the diphosphorus, P₂ molecule.

Extended Only

Sulfur monochloride, S₂Cl₂, has a simple molecular structure.

It was suggested that the structure of sulfur monochloride included a double covalent bond between the two sulfur atoms.

Suggest what is meant by the term double covalent bond?

After analysis of the structure of sulfur monochloride, S₂Cl₂, it was found that only single covalent bonds were present in its structure. Complete the diagram below to show the bonding arrangement in a molecule of sulfur monochloride. Show the outer shell electrons only. 

Predict two properties of sulfur monochloride.

Sulfur monochloride can be produced in the laboratory by feeding chlorine gas into a flask containing sulfur in its elemental form, S₈.  Write a balanced symbol equation for this reaction.

Extended Only

Methane, CH₄, and propane, C₃H₈, are both simple molecular compounds that are gases at room temperature.

Predict which of the two compounds has the higher boiling point. Explain your answer.

A carbon atom can form up to four covalent bonds. A hydrogen atom cannot form four hydrogen bonds.

State how many covalent bonds a hydrogen atom can form and explain why there is a difference.

Germanium also will form up to four covalent bonds. 

Draw the dot-and-cross diagram of the simple molecule digermane,Ge₂H₆.

Extended Only

Explain why digermane does not conduct electricity.