[Chemistry SPM] Exam Pointers I

Essay
Shown (below) are the proton numbers of sodium and chlorine atoms.

• Element: Sodium, Na
  Proton Number: 11
  
  
• Element: Chlorine, Cl
  Proton Number: 17
  

a) The atomic radius of a chlorine atom is smaller than that of a sodium atom. Explain. [4_marks]

• Answer: The electron arrangements of sodium, Na, and chlorine, Cl, are 2.8.1 and 2.8.7, respectively. Since both of them have three electron-filled shells, they are found in the third period of the periodic table. However, the positive charge in the nucleus of chlorine is higher than that of the sodium atom. This is because a chlorine atom has more protons than a sodium atom. This leads to a stronger force of attraction between the nucleus and the electrons of the chlorine atom. Hence, the shells filled with electrons are pulled nearer to the nucleus, thus reducing the atomic radius of the chlorine atom.

b) Chlorine reacts with sodium to form a compound.

i) Explain the formation of the compound. [6_marks]

• Answer: The electron arrangements of sodium, Na, and chlorine, Cl, are 2.8.1 and 2.8.7, respectively. To form a sodium ion, a sodium atom donates one valence electron to achieve the stable octet electron arrangement, 2.8.
  
  Na --> Na⁺ + e^-
  To form a Cl^- ion, one atom of Cl receives one electron to achieve the stable octet electron arrangement, 2.8.8.
  
  Cl + e^- --> Cl^-
  The Na⁺ and Cl^- ions then forms an ionic bond as they are attracted to each other by a strong electrostatic force. Hence, an ionic compound of NaCl is formed.

ii) The compound conducts electricity is molten state but not in solid state. Explain. [2_marks]

• Answer: In solid state, no free moving ions are available to conduct electricity since strong electrostatic forces hold the ions together. In molten state or aqueous solution, however, the charged ions can move freely to conduct electricity.