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Structure and Bonding (Giant ionic and covalent structures (A giant…
Structure and Bonding
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Born-Haber cycle
Lattice energy of an ionic crystal is the energy required to break apart an ionic solid and covert it into its gaseous ions.
Factors affecting lattice energy
- Ion size
The greater the radius of the ion, the less negative the lattice energy because the attractive force is weaker as the outer electrons are further away from the nucleus, and vice versa
- Charge of the ion
The greater the charge of the ion, the more negative the lattice energy because they attract other ions more strongly, and vice versa.
Electron affinity - the amount of energy released when when an electron is added to a neutral atom or molecule in gaseous state to give a negative ion.
Standard enthalpy change of atomisation - the enthalpy change when 1 mole of of element's atoms in the gaseous state is formed from the element under standard conditions (298.15K, 1 atm). It is endothermic.
Ionization energy - the amount of energy required to remove an electron from each atom in one mole of gaseous atoms.
The formula of magnesium chloride is MgCl2 instead of MgCl because it has a more exothermic/negative lattice energy and therefore means that this structure of magnesium chloride is more stable.
Properties of ionic crystals
- High melting/boiling points because the oppositely charged ions are held together by strong electrostatic forces therefore ionic crystals have high lattice energies
Lattice energy cannot be measured directly because of several factors:
- The activation energy is too high
- The reaction is too slow
- There is more than one reaction taking place
Polarity
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Types of bonds
Polar covalent
The pair of electrons are not shared equally. One atom has a greater attraction on the electrons, causing it to have a partial negative charge, whereas the other atom with a smaller pull on the electrons has a partial positive charge.
Non-polar covalent
The pair of electrons are equally shared between the two atoms because they have the same/similar electronegativities.
Ionic
The bonding between oppositely charged ions, usually formed between metals and non-metals, where one atom gives its electrons to the other atom. The ions are held by very strong electrostatic forces and form a giant lattice structure.
Dative covalent bond
The shared pair of electrons come from the same atom unlike in a simple covalent bond
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Polarization of ions
Polarization occurs because the cation attracts the electrons in the outermost shell, however it repels the positive nucleus, causing a distortion of the anion. Polarization power is the ability of a cation to distort an anion. If the degree of polarization is small, the compound is ionic whereas if the degree of polarization is big, a covalent bond forms.
A small cation has a greater polarization power as there is a greater postive charge concentrated in a small area and therefore the compound is more covalentIn a large anion the outer shell electrons are more loosely held, therefore it is more easily polarized and is more covalentA greater charge on the ion means that there is a greater electrostatic attraction between the cation and the electrons of the anion, therefore making it more covalent
Dot and Cross diagrams
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Oxygen molecules form a double bond because each oxygen atom contains 6 electrons and needs 2 more in order to have a full outer shell. Therefore by sharing 2 pairs of electrons, both atoms are able to have full outer shells.
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