Question

Why do caesium (Cs) and potassium (K) find applications in photoelectric cells?

Answer 1

Hint : Modern periodic table is the representation of chemical elements arranged in the increasing order of atomic numbers. These are arranged in groups and periods. The chemical elements in the group $1$ are called alkali metals. These are electropositive and can lose electrons easily, and can be used in photoelectric cells.

Complete Step By Step Answer:
Chemical elements are the pure atoms and arranged in the form of a table called a periodic table according to the periodic properties. The chemical elements are arranged in the increasing order of atomic numbers. These are arranged in vertical columns and horizontal rows. The vertical columns are called as groups and the horizontal rows are called as periods. There are a total of $7$ periods and $18$ groups. The elements in group $1$ are called alkali metals. Caesium (Cs) and potassium (K) belong to alkali metals.
The photo electric cell is a device used to generate the current by the process of photoelectric effect. Photo electric effect can be defined as the ejection of electrons when a light energy is given to an atom.
Due to electro positivity, caesium (Cs) and potassium (K) can be used in photoelectric cells. Thus, these both found applications in photoelectric cells.

Note :
The chemical elements in alkali earth metals are hydrogen, lithium, sodium, potassium, rubidium, caesium and francium. These all-elements have only $1$ electrons in their outermost shell. These all elements are electropositive, and can easily lose electrons. The ionization energy is very less compared to other elements due to its electro positivity.