BONDING AND STRUCTURE

Standard enthalpy change of Atomization of an element

Enthalpy change when 1 mole of gaseous atom is formed from its element in its standard state under standard conditions

Standard enthalpy change of atomization of a compound is the enthalpy change when 1 mole of gaseous compound is converted into gaseous atoms under standard conditions

Ionization Enthalpy

Ionization enthalpy is the energy required to remove one mole of the most loosely held electrons from 1 mole of gaseous atoms or gaseous ions to infinity

Electron Affinity

The enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms or gaseous ions to form negative ions

Characteristics of Election affinity

  1. most 1st electrons affinities of elements are exothermic. This is because a neutral atom has some attraction for an extra electron because of the incomplete shielding of the nucleus by its own electrons
  2. Electron affinities of elements generally become more negative (ie. The elements show a greater attraction for an extra electron) from left to right across a row in the periodic table, excluding the noble gases. This is because across the period from the left to right, the effective nuclear charge of the atoms increases => attraction for an additional electron increases => electron affinities become more negative
  3. Exceptionally less negative electron affinities occur at Group IIA elements (Be, Mg,…), Group VA elements (N,P,…) and Group 0 elements (Ne, Ar, …) This is because Group IIA elements has a filled ns subshell, Gp VA elements have a half-filled npsubshell and Group 0 elements have filled ns and np subshells, which are all stable electronic configuration and it is difficult to add an electron to these stable electronic configuration
  4. Electron affinities of elements generally become less negative down a group. This is because down a group, the extra electron is added to a principal shell further away from the nucleus and the extra electron is shielded by an additional inner shell. These 2 effects outweigh the increase in nuclear charge down a group => weaker attractive force of the nucleus for the extra electron => Electron affinities become less negative down a group

** NOTE: Down a group, the second element in the group most often has the most negative electron affinity. This is because the 1st member od a group has the smallest atomic radius => electron-electron repulsions are larger in the anion formed by the first member

  1. The 2nd electron affinity of an element is always positive (endothermic). This is because like charges repel and so energy is always required to bring a negatively charged electron to a negatively charged ion.
  2. Electron affinity measures the ease with which an atom forms an anion in the gas phase. Elements with very negative electron affinities gain electrons readily to form anions.

Lattice Enthalpy

The enthalpy change when 1 mole of a solid of an ionic compound is formed from its constituent ions in the gaseous state

Stoichiometry of Ionic compound

Stoichiometry of a compound is the simplest ratio in which the involved atoms or ions are bonded together to form the compound

Stoichiometry of Ionic compounds considered from Enthalpy Changes of Formation

For the formation of ionic compounds, the important considerations are the energy changes involved. So long as the exothermic energy terms of lattice enthalpy and electron affinity can outweigh the contributions like ionization enthalpy, enthalpy change of atomization, ionic compound formation is liable to take place if:

  1. enthalpy changes of atomization are low (less positive)
  2. ionization enthalpies are low (less positive)
  3. electron affinity is more negative and
  4. the highly exothermic lattice enthalpy can outweigh the positive terms (e.g. atomization and ionization) such that the enthalpy change of formation has a high negative value

The high negative value of the lattice enthalpy makes the greatest contribution to the negative value for the enthalpy change of formation

Ionic Radii

Ionic radius is the radius of the effective space occupied by the electron cloud in all direction of an ion in the ionic lattice

Interionic distance = cationic radius + anionic radius

Factors affecting Ionic Radius

  1. the original atomic radius of the atom from which the ion is formed
  2. the sign of charge of ion

electron add/remove => effective nuclear charge experienced by the outer shell/ remaining electron reduced/increase => weaker/stronger attractive force of nucleus on electrons => expansion/ contraction on electron cloud

  1. the magnitude of the charge

charge of cation increase => ionic radius of cation decreases

charge of anion increase => ionic radius of anion increase

  1. presence of other ions in the enviroment (minor factor)

Periodic Trends of Ionic Radii

  1. Down a group, ionic radii increase

This is because down a group, the outermost electrons reside in a principal shell further away from the nucleus and are screened by an extra shell of inner electrons. These two effects outweigh the increase in nuclear charge = weaker attractive attractive force between outermost electron and nucleus => ionic radii increases down a group

  1. In an isoelectronic series, the ionic radius decreases as the atomic number increases. This is because as the total number of electrons is the same, the increasing nuclear charge increases the effective nuclear charge on the electrons => the electron clouds contract in size => ionic radius decreases

Hydration Enthalpy of ion

-dissolution is the process of dissolving a solute in a solvent

-solvation is the attachment of a number of solvent molecules to the solute particle. When the solvent is water, the process is called hydration.

Standard enthalpy change of solution of a substance is the enthalpy change when 1 mole of a substance is completely dissolved in so much volume of a specified solvent (usually water) that further dilution produces no detectable enthalpy change under standard conditions.

Hydration enthalpy of ion is the standard enthalpy change when 1 mole of gaseous ions is dissolved in infinite amount of water (i.e. at infinite dilution) under standard conditions

Factors affecting Hydration Enthalpy of Ions

  1. smaller size of cation/anion => hydration enthalpy of the cation/anion is more negative or has larger magnitude
  2. Higher charge of cation/anion => hydration enthalpy of cation/anion is more negative or has larger magnitude

Octet Rule

In compound formation, an atom tends to gain or lose electrons or to share electrons until there are eight electrons in its valence shell

Dative Covalent Bonding

A dative covalent bond or co-ordinate bond is a covalent bond in which the shared pair of bonding electrons is provided solely by one of the two bonding atoms

Sp – linear

Sp2 – trigonal planar ; v-shape

Sp3 – tetrahedral; trigonal pyramidal; v-shape

Sp3d – trigonal bipyramidal; distorted tetrahedral; T-shape; linear

Sp3d2- octahedral; square pyramidal; square planar

Factors affecting Bond Angle of a Molecule/Polyatomic Ion

  1. Effect of non-bonding electron pairs (lone pairs of electrons)
  2. Electronegativity of bonded atom
  3. Electronegativity of central atom
  4. Effect of size of central atom
  5. Effect of size of bonded atoms