Diamond and graphite are both made only of carbon atoms but have very different properties. They are

Explanation

Allotropes are different structural forms of the same element in the same physical state, and here diamond is a rigid three dimensional network while graphite is made of layers with delocalised electrons between them. That is why diamond is the hardest natural substance and an insulator, while graphite is soft, slippery and conducts electricity. Isotopes differ in neutron number, not in bonding.