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Which of the following elements has the lowest melting point ?
A. Sn
B.Pb
C.Si
D. Ge

Answer
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Hint:
The element with the lowest melting point is the one which has the highest atomic number among these. It is a silvery coloured metal. On moving top to bottom in a group in the periodic table, the melting point decreases.

Complete step by step answer:
Before jumping to conclusion let’s first see what is the melting point and how it varies in the periodic table.
Melting point is the temperature at which a molecule changes from solid-state to the liquid state. At the melting point, the solids and liquids are in an equilibrium state. The melting point of a substance depends on the pressure applied to it. And now for the elements given to us.
All these elements given are from the Carbon family. We know that the nucleus attracts the outer electrons and is thus responsible for attraction among the atoms. Further, on moving top to bottom in a group in the periodic table, the atomic size increases with the addition of a new shell every time. As a result, the attraction of the nucleus decreases. Thus, now the electrons can be easily pulled out and as a result, these can freely move in the structure. When this happens, the packing structure will get disturbed and the state will change and move solid to liquid. As a result, the melting point can be early reached. So, on moving down a group, the melting point decreases. The top to bottom order in the Carbon family is – C, Si, Ge, Sn and Pb. Thus, the order of decreasing melting points can be written as –
C > Si > Ge > Sn > Pb
But this is not the actual order. The correct order is –
C > Si > Ge > Pb > Sn
The lead (Pb) has a high melting point in comparison with tin (Sn) because tin (Sn) forms a distorted 12 coordinated structure and not a pure one.
Further, lead atoms are bonded by metallic bonds while the tin atoms are bonded covalently. And we know that covalent bonds are weaker in nature and as a result, these will be easy to break. So, tin has a low melting point while the metallic bonds in lead will contribute to it's the higher melting point.

So, the element with the lowest melting point is the option A. Sn.

Note:
Silicon due to its smaller size as compared with Pb and Sn; can combine with other atoms forming bigger sized compounds. So, it has a high melting point.
The covalent bonds require less energy to break. So, they break very easily. The metallic bonds are formed by complete transfer of electrons. They require high energy to break. So, these form strong bonds.