Topic 4 - Evidence for Ionic and covalent compounds

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  • Created by: Lauren
  • Created on: 22-12-12 08:50

The Evidence for Ions

Physical properties of the ionic compounds:

  •  High melting temperatures, showing strong forces of attraction between ions
  • Soluble in polar solvents
  • Conduct electricity when molten

Electron Density Maps

  • Electron desity maps of compounds produced from x-ray diffraction patterns show zero electron density between ions - showing complete electron transfer

Migration of ions in Elecetrolysis

  • For example, electrolyisis of green aquaous copper(ii)chromate(vi) attracts a yellow colour (Chromate(vi) ions) to the anode and a blue colour (copper(ii) ions) to the cathode
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Ionic Bonding and Lattices

An ionic bond is an omnidirectional electrostatic force of attration between oppositely charged ions.

  • The forces of attraction are equal in all directions
  • In ionic compounds each ion is surrounded by ions of the opposite charge
  • Ionic compounds form giant ionic lattices in the solid state (also called an ionic cyrstal)

An ion is formed when an tom gains or loses one or more electrons. For example a copper atom loses 2 electrons:

Cu(g)  goes to  Cu2+(g) + 2e-

When ions are formed they tend to have a full outer shell of eight electrons. This is called the octet rule. Ions with full outer shells have the same electronic configuration as noble gases - for example a Ca2+ ion has the same electronic configuration as argon; they are isoelectronic. 

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Trends in Ionic Radii

The ionic radius is the radius of an ion in its crystal form.

  • Cations are smaller than the original atom since the atom loses electrons. Usually a whole electron shell has been lost, and the remaining electrons are pulled in towards the nucleus more tightly 
  • Anions are larger than the original atom since the atom gains electrons and there is more repulsion in the electron coud

Going down a group in the periodic table, the ions become larger - the number of shells is increasing.

Going across a period in the periodic table, the ions become smaller - the number of protons in increasing so there is more attraction of the electron cloud.

Isoelectronic ions have different ionic radii:

  • The additional electrons in anions makes the ions larger because there is greater repulsion and all the electrons are less tightly bound than in the atom
  • The loss of electrons to form cations means the nucleus attracts those electrons more strongly
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Lattice energies and Born-Haber cycles

The formation of an ionic crystal from its elements is exothermic. The lattice energy is the energy relesed when one mole of an ionic crystal is formed from its ions in the gaseous state, under standard conditions. This process can be broken down into a number of stages, each associated with a particular energy change:

  • Atomisation of the metal
  • Ionisation of the gaseous metal
  • Atomisation of the non-metal
  • Ionisation of the gaseous non-metal (electron affinity)
  • Forming of the crystals from the gaseous ions

Having measured each of these, and the standard enthalpy of formation, the lattice energy can be calculated using an enthalpy level diagram known as a Born-Haber Cylce. This is an application of Hess's Law.

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Born-Haber Cycles

When asked to find the lattice enthalpy, when given the other values, move around the cycle in one direction, adding up the energies. When going in the direction of an arrow use the same sign as the energy given, when going against the direction of the arrow use the opposite sign.

Remember the lattice energy is always exothermic so whatever number you get for the lattice energy, it needs to be either made into a negative, or just left as a negative number. 

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Polarisation of Ions

Polarisation of an ion is the distortion of its electron cloud away from completely spherical:

  • A cation will distort an anion
  • A cation has polarising power
  • An anion is polarisable

The polarising power of a cation depends on its charge density:

  • A small cation is more polarising than a larger one - the positive nucleus has more effect across the small ionic radius
  • A cation with a large charge is more polarising than one with a small charge - a large charge has more attraction than a small one

The polarisability of an anion depends on its size alone:

  • A large anion is easily polarised - its electron cloud is further from the nucleus and is held less tighly than on a smaller anion

Polarisation explains the difference in theoretical and experimental values for some compounds lattice enthalpy. For some ion pairs the bonds show considerable covalent character (caused by polarisation).

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Formation of Covalent Bonds

A covalent bond is formed when a pair of electrons is shared between two atoms. This happens when 2 atoms approach eachother and their electron clouds overlap and electron density is greates between the nuclei. The region of high electron density (the covalent bond) attracts each nucleus and therefore keeps the atoms together.

  • Covalent bonding is a strong electrostatic attraction between the nuclei of the bonded atoms and the shared pair of electrons between them
  • The distance between the 2 nuclei is the bond length. It is the separation at which the energy of the system is at its lowest 

Dative covalent bonds are formed when both the shared electrons come from just one of the atoms.

Atoms can share more than one pair of electrons and form double or triple covalent bonds:

  • A double bond results from 2 shared electron pairs
  • A triple bond occurs from 3 shared electron pairs
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The Evidence For Covalent Bonds

The physical properties of giant atomic structures such as diamond provide evidence for the strong electrostatic attraction in covalent bonding. Giant atomic structures are also known as giant molecular structures. They are very hard and have high melting temperatures. The covalent bonds are very strong, holding the atoms together and require a lot of energy to break them before the atoms can move in a liquid.

Electron density maps show high electron density between atoms that are covalently bonded.

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Metallic Bonding

Metals consist of giant lattices of metals ions in a sea of delocalised electrons. The metal ions vibrate around fixed positions in the solid latice, being held in place by the electrons around them. It is the outer electrons of the metal with become delocalised - they are no longer associated with one particular atom.

Metallic bonding is the strong attraction between metal ions and the sea of delocalised electrons.

The typical characteristics of metals can be explained using a simple model of metallic bonding

  • Electrical conductivity - the delocalised electrons are free to move and carry charge
  • Thermal conductivity - The electrons transmit kinetic energy through the metal, by their ability to move
  • High melting temperatures - the positive ions are strongly held together by the attraction of the delocalised electrons; it takes a lot of energy to break them 
  • Maleability and ductility - metals can be hammered into shape and pulled into wire because the layers of positive ions can be forced to slide ofver each other while staying surrounded by the sea of delocalised electrons
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