By the end of the subtopic, learners should be able to:
Describe metallic bonding as a lattice of positive ions in a ‘sea of electrons'
Explain the electrical conductivity and
Explain malleability of metals.
A. Metallic bonding
Metallic bonding is different from covalent and ionic bonding.
In metallic bonding the metal ions are at a fixed position within a 'sea' of electrons.
Therefore a metallic bond is defined as an attraction between cations and a 'sea' of electrons.
Metallic bonds are formed when atoms lose electrons and the resulting electrons are attracted to all the resulting cations.
The electrons do not belong to a specific cation, so they are termed delocalised electrons.
In Fig 3.3d.1, magnesium atoms lost two electrons each to form magnesium ion and the electrons are attracted to any cation hence they form a 'sea' of electrons.
The opposite charges on the electrons (negative) and cation (positive) results in electrostatic attraction which keeps the structure together.
Fig 3.3d.2: is a simple diagram to illustrate how ions and electrons arrange themselves in a metallic bond.
B. Physical properties of metallic compounds
The physical properties of a compound are affected by the type of bonding in the compound.
These physical properties include Melting/boiling points, malleability and electrical conductivity.
i. Melting and boiling point
Metallic compounds relatively have high melting and boiling points.
The high melting/boiling points are due to the strong metallic bonds though they are not as strong as ionic bonds.
Compounds with cations that are smaller tend to have higher boiling/melting points than those with a larger size. (From the table above Na has a larger boiling/melting point than K)
Metallic compounds with cations that have a higher charge tend to have high boiling/melting points than those with a lower charge. (From the table Mg has a larger boiling/melting point than Na).
Table 3.3d.1: Boiling and melting points of some metallic compounds.
Metal
Na
k
Be
Mg
Melting point/
98
64
1278
649
Boiling point/
883
760
2970
1107
ii. Electrical conductivity
Metallic compounds contain delocalised electrons which move when a voltage is applied.
This means that all metallic compounds conduct electricity in both solid and molten states.
iii. Malleability
Malleability is the ability of substance to change shape when hammered without breaking.
Metallic compounds are malleable because the cations can slide over each other and electrons moves in any direction.
The bond can form easily between the cations and the electrons.
This means that the metal will have a different shape.
