Atomic Theory

One of the most remarkable discoveries of the 20th century is that the apparently “solid” matter of our everyday world is actually mostly empty space. This matter actually consists of atoms. Atoms are the building blocks of all visible matter in the universe. The word atom is derived from the Greek word atomos; which means, “That which cannot be divided.”

In 1808, a scientist and teacher John Dalton developed the atomic model of matter that we base all modern chemistry upon.  Three of the main ideas of modern atomic theory are:

  • An element is composed of tiny particles called atoms. All atoms of a certain element have similar chemical properties. Atoms of different elements have different properties.
  • In an ordinary chemical reaction, no atom of any element disappears or is changed into an atom of another element.
  • Compounds are formed when atoms of two or more elements combine. In a given compound, the relative numbers of atoms of each kind are definite and constant.

Based on Dalton’s propositions the atom can be defined as the smallest particle of an element that defines its chemical properties.

Inside the atom there are three very important particles: electronsprotons and neutrons.

In the very middle of the atom is the nucleus which contains protons and neutrons. Each element is identified by the number of protons in its nucleus. Carbon, for example, always has six protons. The number of neutrons in an element’s nucleus can vary, however. Carbon can have from two to 16 neutrons, forming different isotopes of carbon. Of these, only the carbon nucleii with six, seven or eight neutrons are naturally occurring. The proton has a mass nearly equal to an ordinary hydrogen atom. It carries a positive charge, which is equal and opposite to the negative charge of the electron. The neutron is an uncharged particle with a mass slightly greater than a proton. In a neutral atom, the number of protons in the nucleus is equal to the number of electrons outside the nucleus. Compared to the mass of protons and neutrons electrons are very small.