The basic building blocks of matter are atoms. The atom is considered the smallest constituent unit of matter that has the properties of a chemical element.  For the existence of atoms, ions and molecules play a vital role. These ions and molecules combine for the formation of matter that we can see, feel, and touch.

To understand the structure and behaviour of an atom, a large number of experiments were conducted. These experiments were used for concluding a hypothetical model of an atom that is similar to the actual atom. The examples of an atom are O and Ne.

Size of an Atom

To understand the size of an atom, consider a sheet of paper. A layer of an atom is as thick as this sheet of paper is. There are millions of atoms stacked together for forming a thin layer of paper.

However, the size of an atom is assumed to be the distance between adjacent atoms which is equal to half of the radius of an atom. The atomic radius is measured in nanometers.

Therefore, the size of an atom is 1 m = 109 nm

Composition of Atoms

An atom is composed of three basic particles, and they are:

  • Neutrons
  • Protons
  • Electrons

NOTE: Hydrogen is an exception as there are no neutrons present in it.

The structure of atom is explained based on these basic particles. The protons and neutrons are present inside the nucleus of an atom, while the electrons revolve around the nucleus. The protons are positively charged while the electrons are negatively charged. Neutrons do not carry any charge. Therefore, we can say that the nucleus is positively charged due to the presence of protons.

What is a Molecule?

A molecule is defined as the smallest unit of a compound that contains the chemical properties of the compound. Examples of a molecule are HCl and O2.

Difference between Atoms and Molecules

Let’s understand the difference between atoms and molecules from the below pointers:

Based on the definition:

An atom is defined as the most fundamental and the smallest part of an element. While a molecule is defined as the two or more atoms that are chemically bonded together.

Based on structure:

The structure of an atom is that it is the smallest particle whereas the structure of a molecule is that it is a combination of two or more atoms.

Based on examples:

The examples of atoms are O, S, P, H and the examples of molecules are O2, P4, S8, H2O.

Based on reactivity:

Atoms in most of the elements have a certain level of reactivity. Noble elements are the only exception. Whereas in a molecule, the reactivity level is less when compared to a few valence points that are filled with electrons of combined elements.