We have already read about, what is chemistry, it's basic and applications.
Let's dive in to the world of atom and molecules & understand how things things are made and how they work.
Atom:
Atoms are the building blocks of matter. They are smallest units of a chemical element that retain the properties of that element.
For example : we can understand it from a house, where a house is made up of bricks, here we can consider bricks as atom.
Structure of atom :
A atom consists of three main parts :
1)protons :
Positively charged particles that reside in the nucleus (center) of atom.
2) Neutron :
Particles with no charge that reside in the nucleus along with protons.
3) electron :
Negatively charge particles that orbit around the nucleus.
Atomic number and atomic mass :
If you are new in the world of chemistry or you don't have much knowledge about it, then whenever you hear these words like atomic number and atomic mass, they would confuse you, but after reading this, you will never get confused.
1) atomic number :
The number of protons in an atom's nucleus, which determines the element of an atom.
Note= no. Of protons is equal to no. Of electron in an atom.
Now you should ask a question ⁉️ if number of protons are equal to number of electron in an atom then why we use only protons to determine the atomic number :
So, we use protons because it is a unique, stable, and unchanging properties that defines the element's characteristics, while electrons can change, they can be removed and add through various processes.
{Atomic number =number of protons.}
Atomic mass:
The total number of protons and neutrons in an atom's nucleus.
{mass number = number of protons +number of neutrons} 
