Question

# Which shows the highest lattice energy?A.$RbF$B.$CaF$C.$NaF$D.$KF$

Hint: Try to remember the definition of lattice energy and observe the options. They are all $1A$group elements, so think about the physical properties of $1A$ group elements and the abnormal behavior of these elements.
Complete answer: Lattice energy is defined as the energy necessary to break the crystal lattice of an ionic solid into gaseous atoms or ions. If the size of the element is smaller than the atoms in that element are stronger as they have a very strong intermolecular force of attraction, which will be easy to break the crystal lattice. In $1A$ group elements from top to bottom the size of atoms increases. In the given elements sodium has a smaller size, so $NaF$ has high lattice energy.
$1A$ group elements have only one electron in its valence shell. They have low melting and boiling points. They form bases when added to water. They are reactive. $1A$ group elements form an alkaline substance when they react with water so they are named as alkali metals. Alkaline substances are also called bases whose $pH$is greater than seven. With only just one electron in their valence shell, they are unstable, so when they contract other elements they easily donate electrons to get stable and form a strong bond.