What is the difference between atoms, compound, molecule and element

Atoms are the smallest object that retains the properties of an element and is composed of electrons and a nucleus.

A compound is a substance formed when two or more chemical elements are chemically bonded together. An example is H2O

A molecule is a particle made up of two or more atoms that are chemically bonded together. They can be the same element, like O2 or H2.

Elements are substances whose atoms all have the same number of protons: another way of saying this is that all of a particular element's atoms have the same atomic number.

Elements are chemically the simplest substances and hence cannot be broken down using chemical reactions. Elements can only be changed into other elements using nuclear methods.