Unit 1- Bonding Flashcards
Define giant structures
- network of strong bonds which links all the atoms and ions
- The bonds results in giant structures
Define simple molecular structures?
- Small group of atoms
- Intramolecular forces are strong but intermolecular are weak therefore easily spererated
Define a lattice
- regular arrangement of atoms or ions in crystals
Define polarisation
The distortion of the electron cloud by the cation
What are the physical properties of ionic comounds?
- Hard crystaline substances
- High melting and boiling temperatures
- Often soluble in polar solvents
- Does not conduct electricity when solid state
What is ionic bonding in a crystal?
- Ionic crystals formed when metals react with non metals
- They are giant lattices
- Strong net electrostatic force due to the negative and positive ions in all directions
What is the trends in atomic radii of ionic compounds?
- Positive ions radius are smaller than the atomic radius of the element. Due to a loss in an electron shell
- Negative ions radius is larger than the atomic radius of the element. Due to more electrons added in the outer shell
What are the stages involved in forming an ionic compound from its elements?
- Seperate atoms (enthalpy change of atomisation)
- Remove electrons to form ions ( first ionisation energy)
- Add electrons to form ions ( first electron affinity)
Where does the energy come from in a Born-Haber cycle?
- Energy is released from the oppositely charged ions join together to form a crystal lattice
How are theoretical values calculated ?
- Uses laws of electrostatics that assume the bonding is purely ionic
What are experimental values?
- Experimental lattice energy calulcated from the Born-Haber cycle
What does a difference in the experimental and theoretical value imply?
- Small difference means the bonding is mostly ionic
- If the bonding is stronger and a bigger difference in the values, there is a degree of covalency
What does polarisation lead to?
- Electron sharing- degree of covalency
What affects the polarising power of the positive metal ion (cation)
- Charge
- Radius
What affects the polarisability of a negative non metal ion (anion)
- Charge- Larger the negative charge, more polarisability
- Size- Larger the ion, more polarisability
Is enthalphy change of formation exothermic or endotermic for ionic compounds?
What does this mean?
- Exothermic
- Compounds are more stable, due to a lower level energy
How can enthalpy of formation be calculated for ions that do not exist?
Born-Haber cycle
What gives an ion more preference to be formed ?
- More exothermic enthalpy change of formations
- More stable
- Greater preference
What is a covalent bond?
- Sharing of electrons between two non metals
- Forms molecules and giant structures
- Electrostatic force between the positive nuclei and the shared pair of electron holds the atoms together
What evidence is there for the strength of covalent bonds?
- Giant molecular structures form atomic crystals. The lattice contains covalent bonds where the lattice has a very high melting and boiling temperature therefore the bonding is strong
- Electron denstiy maps of simple covalent structures are directional. Allows for repulsion to shape the molecule showing the strength of the covalent bond
What are dative covalent bonds?
When one atom provides both the electrons for the covalent bond. There is no difference in strength or length of the bond
What is the general metal structure?
- Positive metal ions in a crystal surrounded by a sea of delocalised electrons
- Close packing
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What is metallic bonding?
- The attraction between the positive metal ions and the sea of delocalised electrons
What are the properties of metals?
- High melting and boiling temperatures- Strong metallic bonding due to metallic bonding and the close packing therefore emore energy required to break the bonds
- Good conductors of heat and electricity- Heat energy is transffered to the electrons allowing a greater KE for the electrons to conduct the heat and electricity around the metal
- Malleable- Due to delocalised electrons
