Statement I: Diamond is very hard and has high melting point. Statement II : In diamond, each carbon is covalently bonded to four other carbon atoms to form a three-dimensional network.

Statement I is true as diamond is the hardest known natural substance and possesses a very high melting point [c5][t6]. Statement II is also true, describing the structural basis for these properties: in diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral network [c1][t6][t8]. This giant covalent structure involves strong sp3 hybridized bonds that extend throughout the entire lattice [t1][t2]. Because these strong covalent bonds must be broken to melt the substance or deform its shape, the 3D network directly explains the extreme hardness and high thermal stability of diamond [t5][t6]. Thus, Statement II provides the correct scientific explanation for the physical properties mentioned in Statement I, as the macroscopic strength is a result of the microscopic bonding arrangement.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Allotropes of carbon > p. 61
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Table 3.1 > p. 40