Ionic compounds have a strong molecular force of attraction and they are generally found in solid states. In these ionic solids, the molecules are arranged in a three-dimensional grid-like structure also known as a lattice structure. The amount of energy required to completely separate one mole of the solid ionic compound into constituent gaseous ions is known as lattice enthalpy.
Lattice enthalpy is a measure of the strength of an ionic compound. Whenever an atom loses an electron, it requires energy and whenever an atom gains an electron, it releases energy. Thus the strength of an ionic compound depends upon the ease with which the positive and negative ions are formed from the respective neutral atoms. The ease of formation of positive and negative ions depends on the ionization enthalpy and electron affinity as described below:
- The formation of positive ions involves the removal of electrons. Whereas, ionization enthalpy is the amount of energy required to remove an electron from an atom. So atoms with lower ionization enthalpy will have higher lattice energy.
- The formation of negative ions involves the gain of an electron. As the electron gain enthalpy is defined as the amount of energy released when an electron is added to an atom, thus larger the electron affinity, more is the lattice energy.
Factors affecting lattice enthalpy:
- Charge on the ion: The ions in the lattice crystal are attracted due to an electrostatic force of attraction present between them. As we know that this force is directly proportional to the magnitude of charge, hence higher the charge stronger is the lattice.
For e.g. Potassium Chloride and Calcium Chloride have the same crystal lattice arrangement but still lattice enthalpy of latter is greater than the former. This is because calcium ions have 2+ charges while potassium ions have only 1+ charge and as we know that electrostatic force of attraction is directly proportional to charge, so in the case of calcium chloride, this force is stronger.
- Size of atom: Smaller atoms have smaller interatomic distances and so have a stronger binding force. This results in higher lattice enthalpy. For example, if we go down in the group 16 of the modern periodic table from fluoride to iodide, the lattice energy keeps on decreasing for their sodium salts.
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