Atoms are composed of three principal kinds of subatomic particles: protons, neutrons and electrons. In the centre of an atom is a tiny, dense core called the nucleus, where protons and neutrons are present. Sub-atomic particles inside the nucleus are also known as nucleons. The electrons in an atom surround the nucleus and fill the remaining volume of atom.
Protons carry a single unity of +ve charge, which is one type of electrical charge. Electrons carry one unit of the opposite charge, a negative charge. These electrical charges have the property that- like charges repel each other and opposite charges attract each other, so negatively charged electrons are attracted to positivelky charged porotons. In fact, it is this attraction that holds the elctrons aaround the nucleus. Neyutrons have no charge and ares saidto be electrically neutral. In a neutral atom, number of protons is equal to the number of electrons, as charges cancel out each other in electrically neutral systems.
The structure of atom wasn't made in one day or a year but it took almost a century to find out about the three sub-atomic particles after which many scientists started speculating the structure of atom and proposing their own theories and calculations on the positions of the sub-atomic particles.
In 600 B.C., Kanada- Indian philosopher and saint conceived of matter as being composed of smallest individual discrete particles. He called them "paramaanu". He didn't tell anyone about "paramaanu" due to fear of 'misuse'.
Around 400 B.C, the Greek philosopher Democritus suggested that all matter is composed of tiny, discrete, indivisble particles that he called atoms. His ideas, nbased entirely on philosophiocal speculations rather than experimental evidence.
The first atomic theory, with an experimental proof, was given by John Dalton in 1808. He regarded atom as the ultimate indivisble particle of matter. Towards the end of nineteenth century, it was proved experimentally that atoms are divisible and consist pf three fundamental particles: electrons, protons and neutrons. The discovery of sub-atomic particles led to proposal of various atomic models to explain the structure of atom.