An element is a substance that consists of atoms of one type only. Everything in the physical world is really made up of about hundred elements. Eight of the elements (amongst the hundred) constitute more than ninety percent of matter. Two or more atoms join (bond) to form a molecule of a substance known as a compound. Consider, water, where elements of Hydrogen and Oxygen bond to form molecule of compound Water. The elements combine in different ways and the resultant products of the reaction may be very different from what you started with.
You know that an atom comprises of electrons orbiting the protons and neutrons in the nucleus. What is important to note, is that, it is these negatively charged electrons that give a substance most of its physical and chemical properties. (To know more about shell/s ‐ electrons occupying an orbital around nucleus ‐ containing electrons of particular energy, do read inside pages). In fact, they also determine how an atom will relate to other atoms and the ways in which they combine to form molecules with entirely different chemical behavior.