chemical bonding worksheet 4 answer

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					                                         Hwa Chong Institution

                                   Chemical bonding (Worksheet 4)

  Section A:

  Answer ‘True’ or ‘False’ to the following statements.

  Question                                                                               T/F
  Metallic bonding is a kind of ionic bonding because ionic bonds exist between the      F
  cations and the delocalized electrons.
  Metallic bonding involves electrons being shared; hence it is a kind of covalent       F
  bonding.
  Metals are malleable because the atoms can slide over each other easily when a         T
  force is applied.
  Metals are malleable because the forces of attraction between metal atoms are          F
  weak.
  In a metal, the electrons are able to move freely and randomly among the cations.      T
  Metallic bonding is weak because the positively charged ions repel one other.          F
  The electrons in metals are able to move only when heat or electricity is supplied.    F
  All metals have high melting and boiling points.                                       F
  Only metals can conduct electricity.                                                   F
  When an electrical current is passed through the metal, electrons repel each other,    F
  thus the metal is able to conduct electricity.
  When an electrical current is passed through the metal, the moving ions in the         F
  metal are able to carry the current from one end to the other.


  Section B:

  Sodium, magnesium and aluminium belong to the same period on the Periodic Table. Some of
  the properties of the metals are given below.

Metals            Melting       Boiling      Electrical Conductivity/   Thermal Conductivity/
                         o              o       -1 -1
                  point/ C      point/ C     cm Ω                       W/cmK
                                                      5
Sodium            98            883          2.1 × 10                   1.41
Magnesium         650           1091         2.3 × 105                  1.56
                                                      5
Aluminium         660           2519         3.8 × 10                   2.37
  Refer to the above table and the Periodic table and answer the questions below.

  1. Write down the electronic configuration of the 3 metals, sodium, magnesium and aluminium.

  Na: 1s22s22p63s1 (2, 8, 1); Mg: 1s22s22p63s2 (2, 8, 2); Al: 1s22s22p63s23p1(2, 8, 3)

  2. How many valence electrons do they have?

                                                                                                1
Na: 1 valence electron. Mg: 2 valence electrons. Al: 3 valence electrons.

3. How many electrons does each kind of metal atom contribute to the ‘sea of electrons’ for
metallic bonding?

The Na atom contributes 1 electron; the Mg atom contributes 2 electrons; the Al atom
contributes 3 electrons to the ‘sea of electrons’.

4. With reference to question 3, explain why the electrical conductivity and thermal
conductivity of the metals increase in the order Na < Mg < Al?

Al contributes the most electrons to the ‘sea of electrons’, followed by Mg and Na. Thus, in Al,
there are more electrons in the ‘sea of electrons’ that can carry electrical charges. Also, when
heated, there are more electrons in Al that can gain energy and collide with neighbouring
electrons, thus heat is conducted more effectively. Thus electrical conductivity and heat
conductivity increases in the order Na < Mg < Al.

5. The melting points and boiling points increase in the order Na < Mg < Al. Is there a
relationship between the number of electrons contributed to the ‘sea of electrons’ and the
melting and boiling points?

Yes. The greater the number of electrons contributed by the metal atom to the ‘sea of
electrons’, the higher the melting and boiling points of the metal.

6. Group I metals have low melting and boiling points. Explain why this is so.

Group I metals have only one valence electrons. As observed in Question 5, the smaller the
number of electrons contributed, the lower the melting and boiling points of the metal. Since
each Group I metal atom contributes only one electron to the ‘sea of electrons’, Group I metals
have low melting and boiling points.

7. Based on what you have learnt, explain why metals generally have high densities? Then
predict and rank sodium, magnesium and aluminium in order of increasing density and justify it.
(Hint: Volume of the metal depends on the size of the cations in the metal.)

Metals generally have high densities because of the metal atoms are packed closely together.
Density of Al> Density of Mg > Density of Na. The number of valence electrons contributed by
aluminium is more than the other two metals. This increases the strength of the metallic bond,
while at the same time, the aluminium cation is smaller than the other two metal cations,
hence aluminium cations can pack more closely together.

Note to students: These are just simplified explanations to the answers, in depth
explanations require more knowledge at a higher level.
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