2 - Chemical bonding and structure Flashcards
What is ionic bonding?
Ionic bonding is the strong electrostatic attraction between oppositely charged ions.
What affects the strength of ionic bonding?
- product of the charges of the ions
- ionic radii
How does ionic charge affect ionic bonding?
- the larger the charges, the larger the lattice energy therefore the stronger the ionic bond.
- the smaller the radii, the smaller the distance between ions, stronger force of attraction (stronger ionic bonds).
trend in ionic radii down a group?
- ionic radii increases.
- due to increasing number of shells of electrons.
- increased shielding effect means also means that electrons are pulled in less by nucleus, which also contributes to ionic radii.
trend in ionic radii across a period?
- ionic radii decreases.
- number of shells of electrons stays the same
- nuclear charge and number of electrons increases.
- increased nuclear charge means that attraction between nucleus and electrons is greater so the electrons are pulled in closer to the nucleus therefore ionic radii decreases.
relationship between ionic bonds and product of charges?
the electrostatic forces of attraction between oppositely charged ions are directly proportional to the product of the charges.
What is it called if ions of different elements have the same number of electrons?
Isoelectronic (same number of electrons).
How are ionic compounds melted and do they have high melting points?
- when an ionic compound melts, the giant ionic lattice is broken.
- in the molten state, the ions are free to move around.
- ionic compounds / giant ionic lattices consist of strong electrostatic forces of attraction between the oppositely charged ions which are strong, so lots of energy is required to break these bonds.
- so, ionic compounds have high melting points.
Why are ionic compounds brittle?
- this is because if a layer of ions is moved in an ionic compound, you end up with ions with the same charges next to each other.
- the layers repel each other and the crystal breaks up.
Can ionic compounds conduct electricity?
- ion fixed in lattice and cannot move in solid state. cannot conduct electricity in solid state.
- in molten, aqueous solution, ions are free to move, can conduct electricity.
How can ionic compounds dissolve in water at room temperature?
- water molecules are polar. Oxygen atoms slightly negatively charged and Hydrogen atoms slightly positively charged.
- the negative (oxygen) end of a water molecule is attracted to the positive ions.
- the positive (hydrogen) end of a water molecule is attracted to the negative ions.
- called hydration.
- provides enough energy to separate the ions in the lattice.
Relationship between water molecules and charge of ion
The higher the charge of an ion, the more water molecules it attracts.
- strength between them measured by enthalpy of hydration.
- always negative (exothermic).
What is metallic bonding?
Metallic bonding is the strong electrostatic attraction between the nuclei of metal cations and delocalised electrons.
What is the structure are giant metals in?
Giant metallic lattices.
Where do delocalised electrons come from and what are they?
- the outer shell electrons of each atom leave to join a ‘sea’ of delocalised electrons which can move freely throughout the structure.
- the sea of electrons binds the positive metal cations together and avoids the repulsion between them.
Why do metals have high melting points?
- metallic bonds are strong and require a large amount of energy to overcome the strong electrostatic forces of attraction between the nuclei of the metal cations and delocalised electrons.
- giant lattice. There are many forces to overcome.
What does the strength of a metallic bond depend on?
- Charge of the metal cation and number of delocalised electrons per cation.
- size of the cation.
How does charge of the metal cation and delocalised electrons affect strength of metallic bond?
The larger the charge, the larger the number of delocalised electrons. This means that the force of attraction between the cations and delocalised electrons is greater.
How does size of cation affect strength of metallic bond?
The smaller the metal ion, the closer the positive nucleus is to the delocalised electrons. This results in a greater force of attraction.
Why does magnesium have a higher melting point than sodium and potassium?
- magnesium has two electrons in its outer shell and both of these get delocalised.
- K and Na only have one electron in their outer shell which gets delocalised.
- so, the sea of delocalised electrons has twice the electron density in magnesium as it does in K and Na.
- Magnesium also has a smaller atomic/ionic radii than K and Na, so the positive nuclei and delocalised electrons are closer together.
- therefore magnesium has stronger metallic bonds and a higher boiling point.
Why can metals conduct electricity?
In the metallic lattice, delocalised electrons are free to move and therefore can carry a charge.
What is the electrical conductivity of a metal depend on?
- number of delocalised electrons per unit volume of metal.
- potassium has larger cations, so number of delocalised electrons per unit volume is lower than sodium. Hence conductivity is lower.
Why are metals good thermal conductors?
- the delocalised electrons transfer kinetic energy throughout the whole metal structure.
- closely packed cations pass kinetic energy from one to the other.
Why are metals malleable and ductile?
- the layers of cations are able to slide over each other, so the structure does not shatter.
- The metallic bonds do not break because the delocalised electrons are free to move throughout the whole metal structure, preventing repulsion between the cations when the layers slide over each other.
Why do metallic bonds exist in significant extent even in molten state?
- metallic bonds are non-directional (the electrons are shared with many neighbouring cations in all directions and not just one).
- so they exist in significant extent even in molten state.
Facts to support that metallic bonds are strong and non-directional?
- high melting and boiling temps. Indicates that they are strong.
- metals are malleable and ductile. This shows that the layers of metal cations can slide over each other without repelling which is a consequence of the non-directional nature of the metallic bond.
ionic bonds and covalent bonds. Non directional or directonal?
- ionic bonds: non-directional.
- covalent: directional. The position of the bonding pair of electrons decides the direction of the covalent bonding.
What is a covalent bond?
A covalent bond is the strong electrostatic attraction between two nuclei and the shared pair of electrons between them.
What kind of structure does graphite, diamond, silicon dioxide have?
Giant covalent lattice
How is a covalent bond formed?
Formed by the overlapping of two atomic orbitals, each containing a single electron.
What type of covalent bonds are there?
- sigma bond: end on overlap of two s orbitals.
- sigma bond: end on overlap of two p orbitals.
- sigma bond: end on overlap of a s and p orbital;.
- pi bond: sideways overlap of 2 p orbitals.
Explain a sigma bond
- end on overlap of orbitals
- can involve s and p orbitals.
- electron density mostly between the two nuclei of the two atoms.
- always a single bond.