Covalent bondingIB MYP Sciences: Subtopic test

1. 1
   Chlorine gas, Cl₂, is added to the water in public swimming pools in Singapore to kill bacteria. A chlorine atom has seven electrons in its outer shell, and a molecule of chlorine contains two chlorine atoms joined by a covalent bond.

   (a)

   How many electrons are shared in the single covalent bond in a chlorine molecule?

   [1 mark]

   • ATwo, which is one shared pair
   • BOne
   • CSeven
   • DFourteen

   (b)

   Chlorine is a gas at room temperature. Which statement explains this?

   [1 mark]

   • AThe covalent bonds inside each chlorine molecule are weak
   • BThe atoms are held in a giant lattice
   • CChlorine atoms have a full outer shell, so they do not attract each other
   • DThe forces between chlorine molecules are weak, so little energy is needed to separate them

   (c)

   Describe, in terms of outer electrons, how the covalent bond forms between two chlorine atoms.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A fertiliser factory in Gujarat makes ammonia, NH₃, from nitrogen and hydrogen. A nitrogen atom has five electrons in its outer shell and a hydrogen atom has one. Ammonia is a gas at room temperature.

   (a)

   How many pairs of outer electrons on the nitrogen atom in a molecule of ammonia are not used in bonding?

   [1 mark]

   • A0
   • B1
   • C2
   • D3

   (b)

   How many shared pairs of electrons are in one molecule of ammonia?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (c)

   Explain why ammonia has a low boiling point and does not conduct electricity.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student in Nairobi compares two forms of carbon, diamond and graphite. She tries to scratch a glass slide with a sample of each. The diamond scratches the glass. The graphite does not scratch the glass and instead leaves a grey mark on it. She also finds that graphite conducts electricity but diamond does not, and both have very high melting points.

   (a)

   Explain why graphite conducts electricity but diamond does not.

   [3 marks]

   (b)

   Evaluate the student's method for comparing the hardness of diamond and graphite. State a conclusion from the results, one limitation of the method, one improvement and one variable to control.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Silica (silicon dioxide, SiO₂) is the main compound in sand. Large amounts of sand are quarried from river beds and beaches to make glass for windows, bottles and phone screens. Carbon dioxide, CO₂, is also a covalent compound of a Group 4 element with oxygen, but at room temperature it is a gas.

   (a)

   Explain, in terms of structure and bonding, why silica is a hard solid with a very high melting point but carbon dioxide is a gas at room temperature, and why neither conducts electricity.

   [6 marks]

   (b)

   Discuss and evaluate the quarrying of sand for making glass.

   [6 marks]

   Total for question 4: 12 marks

End of questions