4.5 Metallic Bonding

OBJECTIVES At the end of this presentation you should know that A metallic bond is an electrostatic attraction between a lattice of positive ions and delocalized electrons. The strength of a metallic bond depends on the charge of the ions and the radius of the metal. Alloys usually contain more than one metal and have enhanced properties. At the end of this presentation you should be able to Explain electrical conductivity and malleability of s and p block metals. Explain the trend in the melting point of s and p block metals. Explain the properties of alloys in terms of their non-directional bonding

GIANT METALLIC LATTICES Structure: Giant metallic lattice / network / crystal. Bonding: Described using the “electron sea” model. A lattice of positive ions/ atoms in a sea of delocalized electrons. Electrostatic attraction between the positive metal ions / atoms and the delocalized electrons. Beware! A metal is made of atoms. Each positive center represents all the rest of the atom apart from the valence electron. The electron hasn't been lost, it is just delocalized. Sodium metal is therefore written as Na and not Na +.

Sodium Electron structure 1s 2 2s 2 2p 6 3s 1. When the sodium atoms come together, the one valence electron of one atom shares space with the valence electron on a neighboring atom to form a metallic bond. Each sodium atom is touched by eight other sodium atoms. The 1 valence electron on each atom can move freely and becomes detached from its parent atom, resulting in a +1 positive charge. These free moving electrons are said to be delocalized The sodium atoms are held together by the strong electrostatic force of attraction between the positive sodium ions and the delocalized electrons.

Magnesium Electron structure 1s 2 2s 2 2p 6 3s 2. The two valence electrons of one magnesium atom shares space with the two valence electrons on a neighboring atom to form a metallic bond. The two valence electron on each atom are delocalized and become detached from the parent atom, resulting in a +2 positive charge. The magnesium atoms are held together by the strong electrostatic force of attraction between the positive ions and the delocalized electrons.

Physical Properties of Metals High Melting Points A large amount of heat energy needs to be absorbed to overcome the strong electrostatic attractions between the positive ions and delocalized valence electrons. Conduct Electricity The delocalized electrons are free to move (mobile) enabling them to carry a current.

CONSIDER THE MELTING POINTS OF MAGNESIUM AND SODIUM? Mpt (Na) = 97.8 o C Mpt (Mg) = o C Suggest a reason for the differences in their melting points? *Suggest means propose a hypothesis.

COMPARE THE MELTING POINTS? Mpt (Na) = 97.8 o CMpt (Mg) = o C Mg has 2 electrons in its valence shell and Na has 1 valence electron. In Mg, the two valence electrons become delocalized, so the ‘sea’ has twice the number of electrons per atom than sodium’. The remaining +2 ‘ions’ have twice the charge of the +1 in Na and therefore there is a greater electrostatic attraction between the Mg ions and their ‘sea’ or mobile electrons, increasing the strength of the metallic bond. Each Mg atom has 1 more positive proton in the nucleus than Na which creates a greater attraction for the delocalized electrons, increasing the electrostatic attraction and the strength of the metallic bond. Mg atoms have a smaller atomic radius than sodium atoms, and so the delocalized electrons are closer to the nuclei increasing the electrostatic attraction between the protons and valence electrons, increasing the strength of the metallic bond. *Compare means give an account of the similarities and differences between two items, referring to both throughout.

Look at the trends in the melting points of Na, Mg and Al in the third period of the periodic table. Explain the trend by giving a detailed account of the reasons or causes. When the ____ the ________. This is because...

Physical Properties of Metals Metals conduct electricity The delocalized electrons are free to move (mobile) enabling them to carry a current.

Explain the electrical conductivity of magnesium. *Explain means give a detailed account of the causes and reasons. Magnesium is a ____________ structure. Its bonding can be described as ____________ Magnesium conducts electricity. This is because ___________

PHYSICAL PROPERTIES OF METALS Metals are malleable which means they can be beaten into different shapes. This is because the positive ions don’t have any defined direction (called non-directional) and so they can slide past one another, which leads to a rearrangement of the metal. When a small stress is applied the layers of positive ions roll over each other without breaking the metallic bond.

Explain the malleability of copper. *Explain means give a detailed account of the causes and reasons. Copper is a ____________ structure. Its bonding can be described as ____________ Copper is malleable. This is because ___________

Superconductors In a normal metal the delocalised electrons collide with the positive ions as they move through the metal releasing heat energy. This heat energy creates resistance and slows down the flow of electrons, decreasing a metals electrical conductivity. A superconductor is an alloy with enhanced properties, one of these being the the ability to conduct electricity without releasing so much heat energy.

SILICON, SI Structure: Giant covalent structure. Semi- metal. Bonding: Each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement. There are no delocalised electrons because each valence electron is involved in bonding, therefore silicon does not conduct electricity

SILICON In late 1980’s scientists discovered that silicon although a non- conductor becomes a semi- conductor when the certain elements are incorporated into the lattice structure.

Everyday use of Superconductors To make electromagnets (coil of wire with a current moving through it) Used in medical imaging equipment, like an MRI. In a maglev train magnets made with superconductors induce a current causing the train to levitate or lift off the track, allowing it to travel much faster. rZ4knpg rZ4knpg