A compound consists of either many identical atoms or more than 1 type of atom. They can be broken down or converted into a simpler one. An element is a substance which contains only one kind of atom. It cannot be broken down further or converted into simpler substance by any chemical means.
Heterogeneous mixture is a non uniform mixture of different atoms without any chemical bonding in between them. Homogeneous mixture is a uniform mixture of different atoms without any chemical bonding in between them. From the definitions it is clear that substance X is (B) Element.
The majority of the mass of the atom is located in the nucleus. Remember that the nucleus contains both protons and neutrons and therefore, most of the mass of the atom.
