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Early Atomic Theory
Sometime around the year 400 BC, a Greek philosopher name Democritus developed a new theory about what matter is made of. He theorized that if a person were to cut an object in half, then cut one of those pieces in half, and keep halfing the object again and again, eventually they would be left with a piece of the object so small that it couldn’t be cut any more. He called this particle “atomos”, which means “uncuttable”. For more than 2,000 years, Democritus’ ideas about the smallest individual particle of matter went largely unchanged. Today, we call this particle an atom, and our knowledge about atoms has evolved rapidly in the era of modern chemistry.
English scientist John Dalton developed the foundation of modern atomic theory in the early 1800’s, making three main statements about atoms:
All matter is composed of atoms, making atoms the “building blocks” of the universe.
Atoms of each element are unique in size, mass, etc...
Atoms are the smallest individual unit of matter and cannot be subdivided (cut) or destroyed.
Sub-Atomic Particles
Over the last 200 years, scientists have added to and modified our understanding of atoms from Dalton’s atomic theory. Using a magnet to bend a glowing beam of particles in a cathode ray tube, English physicist J. J. Thomson detected negatively charged electrons, the first discovery of a sub-atomic particle. A few years later, another British physicist named Ernest Rutherford observed that positively charged alpha-particles sometimes deflect at severe angles when they are shot at a very thin piece of gold foil only a few atoms thick. This confirmed a theory proposed by Japanese physicist, Hantaro Nagaoka, that atoms have a dense and positively charged nucleus at their center. Rutherford later realized that the alpha-particles were actually positively charged sub-atomic particles he called protons, which reside within the nucleus of atoms. While their positive charge is equal but opposite to the negative charge of electrons, protons have 2000 times more mass than electrons. A third sub-atomic particle was discovered in the early 1930’s by English physicist James Chadwick. This particle, also located within the nucleus and equal in mass to the proton but without an electrical charge (neutral) was called a neutron.
The Modern Atomic Model
Our modern understanding of the structure of the atom is often referred to as the electron cloud model. In this model, there is an extremely small and dense nucleus at the center of the atom. The nucleus consists of positively-charged protons and neutral neutrons which account for nearly all of the atom’s mass. Orbiting the positively-charged nucleus in the electron cloud are negatively-charged electrons which contribute almost no mass to the atom. Because the nucleus is so small and the electrons are spread out and orbiting frantically within the electron cloud, the overwhelming majority of the atom is actually empty space.
HOW SMALL IS AN ATOM?