Question

Assertion (A): Alkali metals are soft and have low melting and boiling points.
Reason (R): This is because interatomic bonds are weak.
(A) Both (A) and (R) are not true
(B) (A) is true but (R) is not correct explanation of (A)
(C) (A) is true but (R) is true
(D) Both (A) and (R) are true but (R) is correct explanation of (A)

Answer 1

Hint: Alkali metals are reactive elements. Melting point of a substance is the temperature at which it changes from solid to liquid state. High Interatomic strength gives rise to high melting points of the compound, as during melting of compound bonds are broken.

Complete step by step answer:
As you go down the group , there are extra electron shells which cause electron shielding. Because the force of attraction must extend further and through these shells to the outermost shell, the force becomes less so there is less strong bonding occurring. Thus it is easier to break these strong bonds. And because the melting point is a measure of the point at which all the bonds holding a metallic structure together are broken, it will decrease because this point is reached a lot sooner as the bonds are not as strong.
Hence, we can say that all alkali metals are found to be very soft and they have low melting points and boiling points. Alkali metals have low binding energy in the metal crystal lattice as these atoms have only one valence electron. This results in the formation of metallic bonds which are not very strong. Also, alkali metals melting and boiling points decrease down the group.
So, the correct answer is “Option D”.

Note: Alkali metals belong to the s-block elements occupying the leftmost side of the modern periodic table. Alkali metals readily lose electrons, making them count among the most reactive elements on earth.