Metallic bonds are formed when metals give electrons to the sea of delocalized electrons, resulting in positive ions held together by negative electrons.
The strength of metallic bonding depends on two factors:
- the charge of the metal ions
The greater the charge on the metal ions, the greater the attraction between the ions and the delocalized electrons and the stronger the metallic bonds. A higher melting point is evidence of stronger bonds in the substance.
- the size of the metal ions
The smaller the metal ion, the closer the positive nucleus is to the delocalized electrons. This means there is a greater attraction between the two, which creates a stronger metallic bond.
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SODIUM AND MAGNESIUM
Na: melting point of 371K
Mg: melting point of 922K
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PRACTICE QUESTIONS
1.i) Describe the bonding in magnesium and explain why it is a good conductor of electricity.
Magnesium is a lattice structure of Mg2+ ions that have lost two electrons each to the sea of delocalized electrons. These delocalized electrons hold all of the magnesium ions together with electrostatic attraction, as opposite charges attract. This is metallic bonding.
Magnesium is a good conductor of electricity because of all of the delocalized electrons that hold it together, which are free to move around and carry electrical charge.
ii) State the type of bonding that exists in solid magnesium.
metallic bonding
iii) Explain fully why the melting point of magnesium is higher than that of sodium.
Magnesium loses 2 electrons to the sea of delocalised electrons to become stable, while sodium only loses 1 electron. Therefore, magnesium has more delocalised electrons between the magnesium ions to hold them together, resulting in greater electrostatic attraction in the lattice structure. As sodium has less delocalised electrons, it has weaker electrostatic attraction, so less energy is needed to break the bonds, so it has a lower melting point. Also, magnesium has a higher charge, so it is more strongly attracted to the delocalised electrons.
2. Metals usually have high melting points and boiling points because there are:
- strong attractions between the ions
- strong attractions between the delocalised electrons
- strong attractions between the ions and the delocalised electrons
- strong intermolecular forces
3. The melting temperatures of the elements of period 3 are given in the table below. Use these values to answer the questions that follow.
The metallic bonds in sodium are weaker as the ions have a lower charge, so they have weaker attraction to the delocalised electrons. Also, there are fewer delocalised electrons in sodium than in magnesium, so there is less electrostatic attraction. As the forces of attraction are weaker in sodium, less energy is needed to break the bonds, so it has a lower melting point.
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