There is more than enough information in the public domain to be found from the simplest of things to the most difficult papers one can imagine. To understand the basics quickly we have linked to the video  "A is for Atom". Despite that it is an aged video, it still gives a good general idea of what physics is about in the current paradigm.

After the acceptance of the neutron (which was very much influenced by the need to make Quantum Mechanics work), we basically end up with our current understanding (in essence at least) of the Atom. An atom has several key attributes that are observed:

• a positively charged nucleus
• the nucleus is made up of positive protons and neutral neutrons
• the atom has outer electrons that are attracted to the positive nucleus in orbitals
• the outer electrons do not fall into the nucleus
• the ratio of neutrons and protons and electrons for an atom on average is 1:1:1

Note: Current nuclear physics explains the nucleus in quantum mechanical (QM) terms. QM states there is no way to know the structure of the nucleus because the nucleons are said to occupy a range of locations. QM requires four fundamental forces of nature - strong nuclear force, weak nuclear force, gravity and the electro-magnetic force.