Question

Correct order of hardness
A. $Be > Cs$
B. $Cs > Be$
C. $Cs > Na$
D. $Na > Mg$

Answer 1

The hardness can be defined as the resistance to deform, identate or penetrate a material. Non-metals are mostly not hard. However, the exception is the non-metal diamond, the toughest substance. Hardness of a material depends on various factors which are as follows:
Plasticity (elastic stiffness)
Strain
strength
toughness
viscousness or viscoelasticity

Complete step by step answer:
The alkaline earth metals are the elements that correspond to the Group $2$ of the modern periodic table. The group $2$ elements are beryllium, magnesium, calcium, strontium, barium, and radium. The elements of group $2$ represent the similar physical and chemical properties to each other.
Metallic element Beryllium has additional valency nature owing to its smallest size. Highest ionization energy, high positive nature and strongest polarizing nature. Because of these, metallic elements differ from alternative element metal properties. It is the hardest metal among alkaline earth metals.
Alkali metals belong to the s-block components occupying the left facet of the table. They occupy the first column of the table. Alkali components are Lithium (Li), Sodium (Na), Potassium (K), metallic element (Ru), caesium (Cs) and metal (Fr) occupying consecutive periods from one to seven. Lithium shows different behaviour from other metals as it shows additional valency nature due to some factors like small size, highest ionization energy, electropositive and polarising in nature.
All metals are hard except sodium and potassium, which are soft and can be cut with a knife.
Values of the hardness of the following metals of the periodic table in the Mohs scale of hardness is as follows:
Caesium $0.2$
Beryllium $5.5$
Sodium $0.5$
Magnesium $2.5$
So the correct order of hardness of the metals is $Be > Mg > Na > Cs$

So, the correct answer is Option A.

Note: Alkali metals without delay lose electrons, creating them count among the foremost reactive components on earth. Alkali metals have a corresponding element $n{s^1}$ electronic configuration. Down the column, the nuclear charge increases and a new orbital gets added to each alkali atom and due to this, there is a changing trend in the physical properties down the column.