The study of chemistry begins with the learning of structure of atom and electronic configuration. It is very important to understand the constitution of atom, especially the arrangement of electrons in it so as to correlate them with the chemical properties of that atom.
Though atomic structure also includes the nucleus part, however, most of the time we deal with the data related to electronic arrangement. You will find more details about particles in the nucleus in nuclear chemistry or physics.
Let us start with the question: What is an atom?
Atom is a basic unit of matter. It is the smallest particle which can take part in a chemical reaction without losing its identity i.e., Atom of one type cannot be converted to another type in a chemical reaction (you will learn the in depth meaning of this in the later sections).
It is not correct to say that the atom is indivisible. On the other hand, it can be further divided into sub atomic particles like protons, neutrons and electrons. However while doing so, the atom loses its chemical identity.
Note: In 1808, Sir Dalton proposed that matter is made up of small particles called atoms. All substances were either made of single atoms or combinations of atoms (also called molecules). He also proposed that atoms were indivisible. However, in the 19th century, experiments showed that atoms were divisible.
In the following sections, the structure of atom is discussed in detail.