Metallic bonding Flashcards
Properties of metallic molecules
- Lustrous – they have a shiny surface when polished
- Good conductors of heat and electricity.
- Malleable – can be bent/moulded into any desired shape.
- Ductile – can be drawn into wires
- Generally have high densities
- Generally have high melting points
- They are the most common type of element on the periodic table.
- Metals form cations.
- They have low electronegativity.
The model for metallic bonding
The model for metallic bonding consists of positively charged metal cations and a sea of delocalised electrons that are negatively charged.
How metallic bonding works
- Metals have a low electronegativity so they easily lose their valence electrons, giving them a positive charge and making them cations.
- The lost electrons become delocalised, meaning that they are free to move within the structure, not belonging to the metal ions. Electrons that are not free to move, such as the ones located in the metal ions are localised (not free to move).
- An electrostatic bond/attraction forms between the positively charged metal cations and the negatively charged delocalised electrons, holding the structure together.
Why metals conduct electricity
- When an electric current is applied, the delocalised electrons that are free to move, are electrostatically attracted to the side of the metal and attached to the positive terminal. This flow of electrons allow a electricity to flow through.
Why metals conduct heat
- The delocalised electrons vibrate and bump into each other and transfer energy. By heating metals you are giving more energy to the delocalised electrons, and as they are free to move they can transmit that heat energy across the lattice quickly.
Why metals are malleable and ductile
- When a metal is being shaped, the positive ions are pushed across each other, as they are free to slip over layers, but the delocalised electrons move to surround the ions so the metal doesn’t break apart. Instead it can be bent or drawn into wire.
Why metals are lustrous
- The delocalised electrons are able to reflect the light back, giving metals a lustrous appearance.
Why metals have a high melting and boiling point
- This is due to the very strong electrostatic attractions between the cations and delocalised electrons. Also, metals with more outershell electrons have higher MP and BP as there is a greater attraction with the increased number of electrons.
How the reactivity of metals vary in the periodic table
- Period - reactivity decreases as you go from left to right.
- Group - reactivity increases as you go down a group.
Work hardening
- Work hardening involves hammering or working metals (without warming) so that the crystals are rearranged with smaller gaps between the crystals. This causes the metal to become stronger, harder and more brittle.
Heat treatment- annealing
- Metals are heated until they are red hot and then cooled slowly.
- As a result of this process larger metal crystals are able to form.
- The metal will become softer and more malleable as larger crystals are formed with big gaps between them.
Heat treatment- quenching
- Metals are heated until they are red hot and then cooled quickly (usually in ice water)
- Smaller crystals form, making the metal harder but more brittle.
Heat treatment- tempering
- Quenched metals are warmed again to a slightly lower temperature then allowed to cool slowly.
- This process is repeated several times.
- As the crystals become smaller and smaller the gaps between them reduce, thus the metal is harder and less brittle.
Alloys
- By mixing a metal with other metals or some non-metals we make an alloy.
- Therefore an alloy is a substance with metallic properties, that consists of 2 or more elements, (one of which is a metal).
- Metals are alloyed in order to change their properties to make them stronger and more rust resistant.
Alloys- substitutional and interstitial
Substitutional alloying- atoms being used to create the alloy are usually of the same size. They can replace each other in the metallic crystals. Example of substitution alloying – Sterling silver (Silver and Iron).
Interstitial alloying- adding atoms of a smaller size than the original metal so that they fit in the spaces between the larger atoms. Example of interstitial alloying is Steel (Iron and Carbon).
