The atom is the smallest unit into which matter can be divided without the release of electrically charged particles.
Most of the atom is empty space. The rest consists of a positively charged nucleus of protons (+) and neutrons (neutral) surrounded by a cloud of negatively charged electrons. The nucleus contains more than 99% of the atomic matter. On the other hand, the nucleus size - compared to the atom size is like a baseball to the size of 100 baseball stadium!
Can we actually see atoms?

Development of our knowledge of the atom
In 430 BC, The Greek philosopher Democritus of Abdera developed the idea that everything is made of atoms, meaning literally "indivisible". Democritus believed that atoms were uniform, solid, hard, incompressible, and indestructible and that they moved in infinite numbers through empty space until they stopped. Differences in atomic shape and size determined the various properties of matter. In Democritus philosophy, atoms existed not only for matter but also for such qualities as perception and the human soul. For example, sourness was caused by needle-shaped atoms, while the ckjso sm color white was composed of smoothed-surfaced atoms. The atoms of the soul were considered to be particularly fire.
The Greek atomic theory is significant historically and philosophically, but it has no scientific value. It was not based on observations of nature, measurements, tests or experiments. Instead, the Greeks used mathematics and reason almost exclusively when they wrote about physics.

In 1897, J.J. Thomson's discovery of the electron showed that the 2000-year-old conception of the atom as a homogenous particle was wrong and that in fact the atom has a complex structure. However they didn't change the name "atom"

J.J Thompson Credit:AIP

(indivisible) for what is actually a" non atom" (indivisible).
Thomson's discovery of the negatively charged electron raised theoretical problems for physicists, because atoms as a whole are neutral. Where was the neutralizing positive charge and what held it in place? Between 1903 and 1907 Thomson tried to solve the mystery by adapting an atomic model that had been first proposed by Lord Kelvin in1902. According to this theoretical system, often referred to as the" plum pudding" model, the atom is a sphere of uniformly distributed positive charge about one angstrom in diameter. Electrons are embedded in a regular pattern like raisins in a plum pudding to neutralize the positive charge. The advantage of the Thomson atom was that it was inherently stable: if the electrons were displaced, they would attempt to return to their original positions. In another contemporary model, the atom resembled the solar system or the planet Saturn, with rings of electrons surrounding a concentrated positive charge.

Rutherford refined our knowledge of the atom. With Hans Geiger he run the well-known gold foil experiments and demonstrated Ernest Rutherford Credit:AIP that the atom has a tiny massive nucleus. He beamed alpha particles through gold foil and detected them as flashes of light or scintillation on the screen. The gold foil was only 0.00004 centimeter thick. Most of the particles went straight through the foil, but some were deflected by the foil and hit a spot on a screen placed off to one side. One in 20 000 alpha particles had been deflected 45° or more. Rutherford asked why so many particles passed through the gold foil while a few were deflected so greatly. "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back to hit you," Rutherford said later."On consideration, I realized that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible o get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center carrying a charge."