People of long ago, at around 5th century B.C., a Greek Philosopher named Leucippus thought that water has an ultimate particle that cannot be subdivided. His idea was later expanded by his student, Democritus who named these possible indivisible particles as “atomos”. From there, scientists tried to experiment in order to verify its existence and farther they go into figuring out how ”atomos” looks like.
John Dalton theorized that an atom is a solid sphere which we can call “billiard ball model”; J.J. Thomson theorized that atom is like a “raisin pudding” where the pudding is a pool of positive particles and the raisins are the negative particles. We can call this the “watermelon model”, if you cut a watermelon in halves, the randomly distributed seeds represent the negative particles like that of the raisins in the pudding; Ernest Rutherford with his experiment on thin gold foil struck with alpha particles, figured out that atom is like that of the planetary system, where a massive part at the center(analogous to the sun) and some particles are revolving around it(analogous to the planets). We can call this model as “microplanetary system”.
Scientists did not stop there, they continue to experiment and to theorize the existence and structure of atom. In adaptation of Max Planck’s idea that energies are quantized (in specific regions), Neils Bohr theorized that the particles (electrons) around the center (nucleus) of the atom are in circular orbit about a particular distance from the nucleus, in an allowed energy state. Louis de Broglie and Erwin Schrodinger of course did not fully agree with Bohr, so they studied, contributed and arrived to the quantum mechanical model or the electron cloud model of atom. They based their model on Werner Heisenberg Uncertainty Principle.
According to Heisenberg, it is impossible to know simultaneously both the exact location and momentum of an electron. Thus, there is no way that Bohr can tell with certainty at what particular distance the electron is from the nucleus. According to Schrodinger we can only talk of the probability (possibility) of finding an electron at a certain region in space, we can not pinpoint its location exactly.Years by and by, At last! Atoms can now be seen. Thanks to the invention of Scanning Tunneling Electron Microscope. This electron microscope has at least proven that an atom is spherical in shape. Congratulations to Dalton, he really had done a great job in figuring out the shape of an atom. Of course, atom is mostly empty space and thus, not a massive one as theorized by Dalton.
You may enjoy more about atom by listening to "The Atom Song". See link to this song in the comments section below,
Brown, T. L., et al (2000) Chemistry The Central Science, 8th ed. Singapore: Prentice Hall,
Hill and Kolb (1995). Chemistry For Changing Times, 7th ed. New Jersey: Prentice Hall, Inc.
Petrucci, R. H. (2002). General Chemistry Principles and Modern Applications: New Jersey:
Prentice Hall, Inc.