1. 1
   Two hydrogen atoms join together to form a hydrogen molecule, H₂, held together by a covalent bond formed by sharing electrons.

   (a)

   What is shared between the two atoms to form this bond?

   [1 mark]

   • AA pair of electrons
   • BA pair of protons
   • CA single neutron
   • DThe entire nucleus of one atom

   (b)

   Which of the following is a simple molecule formed by covalent bonding, as studied in this specification?

   [1 mark]

   • ACaCl₂
   • BNaCl
   • CMgO
   • DCl₂

   (c)

   Explain why two hydrogen atoms share a pair of electrons to form H₂, in terms of electron arrangement.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A water molecule, H₂O, is formed when one oxygen atom bonds covalently with two hydrogen atoms, each bond formed by sharing a pair of electrons.

   (a)

   In a water molecule, H₂O, an oxygen atom bonds covalently with two hydrogen atoms. How many covalent bonds does the oxygen atom form in this molecule?

   [1 mark]

   • A4
   • B1
   • C2
   • D6

   (b)

   How many electrons make up a single covalent bond?

   [1 mark]

   • A1
   • B2 (one pair)
   • C4
   • D0

   (c)

   Explain how the oxygen atom and the two hydrogen atoms in a water molecule each achieve a stable, full outer shell of electrons.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Methane, CH₄, contains a carbon atom bonded to four hydrogen atoms, while ammonia, NH₃, contains a nitrogen atom bonded to three hydrogen atoms.

   (a)

   State the number of covalent bonds formed by the carbon atom in methane and by the nitrogen atom in ammonia.

   [3 marks]

   (b)

   Explain why carbon forms 4 covalent bonds in methane, while nitrogen forms only 3 covalent bonds in ammonia, in terms of their electron arrangements.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is comparing the covalent bonding found in small molecules, such as H₂, HCl, NH₃ and CH₄, with the covalent bonding found in giant covalent structures, without looking at any diagrams of either type of structure.

   (a)

   Describe covalent bonding, and explain the difference between simple molecular substances and giant covalent structures, without referring to any diagrams.

   [6 marks]

   (b)

   Using the examples H₂, HCl, NH₃ and CH₄, explain how the number of covalent bonds an atom forms relates to how many more electrons it needs to complete a stable outer shell.

   [6 marks]

   Total for question 4: 12 marks

End of questions