Atoms are the building blocks of everything we see. an atom remains the fundamental particle because it is the smallest unit. An atom is the smallest constituent particle of an element which exhibits the chemical properties of an element and also can take part in a chemical reaction. Atoms are tiny and their sizes are about an angstrom. atom means something that is indivisible. When Dalton gave his atomic theory, it was believed that the atom was indivisible and hence the name. But later it was found that atoms have been subdivided into simpler parts. That is called as subatomic particles.
The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms is known as a molecule or it can be defined as The simplest unit of a chemical compound that can exist, consisting of two or more atoms held together by chemical bonds. For example, we cay Ca2 which says that there are 2 molecules of calcium.
Element consist of only one kind of atom which cannot be broken down into a simpler type of matter by either physical or chemical means. For example, the element hydrogen is made from atoms containing just one proton and one electron
A compound is a substance made from two or more different elements that have been chemically joined. Some examples of compounds are water (H2O), table salt (NaCl), table sugar (C12H22O11) and chalk (CaCO3). It can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means).