#chemistry #textbook #physicalproperties #ioniccompounds
Pg:- 59,60,61
Ch:- 3
Ionic compounds are chemical compounds composed of ions held together by electrostatic forces known as ionic bonds. These compounds generally form between metals and non-metals, where the metal loses electrons to become a positively charged cation, and the non-metal gains those electrons to become a negatively charged anion.
Important features of ionic bonding are:
The electrons involved in the formation of ions are those in the outer shell of the atoms.
Metals atoms lose their outer electrons to become positive ions. In doing so, they achieve the more stable electron arrangement of the nearest noble gas to them in the periodic table.
Generally, atoms of non-metals gain electrons to become negative ions. Again, in doing so, they achieve the stable electron arrangement of the nearest noble gas to them in the periodic table.
1. High Melting and Boiling Points
Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions, known as ionic bonds. A significant amount of energy is required to overcome these forces, resulting in high melting and boiling points. For example, sodium chloride (NaCl) melts at around 801°C.
2. Hardness and Brittleness
Ionic compounds tend to be hard and brittle. The ions in an ionic lattice are arranged in a rigid, crystalline structure, which makes them difficult to deform. However, if a force is applied that distorts the crystal lattice, like charges may be forced closer together, leading to repulsion and causing the crystal to shatter. This explains why ionic compounds are brittle rather than malleable or ductile.
3. Electrical Conductivity
In Solid State: Ionic compounds do not conduct electricity in their solid form because the ions are locked in place within the crystal lattice, and there are no free electrons or ions to carry an electrical charge.
In Liquid State or Solution: When melted or dissolved in water, ionic compounds dissociate into their respective ions, which are free to move. In this state, ionic compounds can conduct electricity because the ions can carry charge through the solution or molten liquid.
4. Solubility in Water
Most ionic compounds are soluble in polar solvents like water. Water molecules are polar, meaning they have partial positive and negative charges that interact with the positive and negative ions of the ionic compound. This interaction weakens the electrostatic forces between the ions, allowing the compound to dissolve. However, not all ionic compounds are equally soluble, as the lattice energy (the energy required to separate the ions) must be overcome by the energy released during solvation (hydration of ions).
5. Crystalline Structure
Ionic compounds typically form crystalline structures. In these crystals, ions are arranged in a repeating three-dimensional pattern known as a lattice. The precise arrangement depends on the relative sizes and charges of the ions. For example, sodium chloride forms a cubic lattice structure. The regularity of this arrangement contributes to the sharp, well-defined edges often seen in ionic crystals.
6. Cleavage
Due to the highly ordered structure of ionic crystals, they tend to exhibit cleavage. This means that when struck, they break along specific planes where the bonding forces are weaker. This is another consequence of the rigid lattice structure.
7. Density
Ionic compounds generally have relatively high densities compared to covalent compounds. This is because ions are closely packed in a crystal lattice, reducing the space between them and increasing the overall mass per unit volume.
Summary
High melting and boiling points due to strong ionic bonds.
Hard and brittle due to a rigid, crystalline structure.
Electrical conductors in molten or dissolved states but not as solids.
Water-soluble in many cases due to interactions with polar solvents.
Crystalline structures with high symmetry.