Question

Consider the following compounds: KF, NaF, and RbCl. Answer the following questions based on expected periodic trends: (a) Which is expected to have the shortest ionic bonds? (b) Which is expected to have the highest lattice energy? (c) Which is expected to have the lowest melting point?

Answer 1

K, Na and Rb are alkali metal element of the first group. F and Cl belong to the halogen group.

Alakali elements have one extra electron in their outer shell whereas Halogens would be needing one extra to complete the octect and thus achieve the nearest noble gas configuration and thus attain stability.

They as of extremely contrasting characteristics form ionic bonds by donating or taking an electron to or from their respective outer shells to form ions- Cation and Anion.

(a) IONIC BOND

The cation and anion posses charges. The attractive force between these oppositely charged entities is the coulumbic force. All ionic bonds have some covalent character, but the larger the difference in electronegativity between the two atoms, the greater the ionic character of the interaction. The length depends on the electronegativity and size of the ion. The ionic character of the ionic compounds decreases down the group.

Ionic radius increases down the group. Therefore the radius of Na+ < K+ <Rb+ and F- < Cl-

Thus according to these trends, ionic bond length order would be NaF < KF < RbCl.

(b)LATTICE ENERGY

The lattice energy of a crystalline solid is a measure of the energy released when ions are combined to make a compound.  It depends on the electronegativity of individual ions. Electron affinity becomes less negative down a group. As the principal quantum number increases, the size of the orbital increases and the affinity for the electron is less. More ionic the bond, higher the lattice energy.  

Thus according to these trends, lattice energy order would be NaF > KF > RbCl.

(c)MELTING POINT

Ionic compounds have high melting points. The energy to overcome coulumbic attraction in order to allow the ions to move more freely and form a liquid depends on its lattice enthalpy. Higher lattice energies typically result in higher melting points because more thermal energy is needed to overcome the forces that hold the ions together.

Thus according to these trends, the melting point order would be NaF > KF > RbCl.