What Is The Atomic Theory?

This theory teaches that atoms are made up basically of three particles: protons, neutrons and electrons. Combinations of these particles in varying numbers make up the elements. An element is a substance that cannot be separated into simpler substances by ordinary chemical means. So, for ordinary chemistry, elements are the building blocks. The next unit in order is the molecule, which may consist of one or more atoms. Then come compounds, made up of the union of two or more elements.
There are ninety-two elements that are commonly found in the natural state. Hydrogen, a gas, is the lightest of these. Platinum is one of the heaviest. Some others have been made artificially, so that the total number of known elements today is more than one hundred. The most abundant element in earth's crust and in its waters is oxygen, essential to both animal and plant life. Oxygen also constitutes about one fifth of the air by volume.
Most elements have an affinity or attraction for others. Very few are considered inert, or practically inactive. There is virtually an endless number of arrangements and combinations, making up every kind of material that exists. The most complex molecules are found in living things. Massive molecules of various proteins, consisting of many hundreds of atoms in a very complicated arrangement, have recently been given much attention by scientists. Massive as these are, for molecules, they can be "seen" only by means of an electron microscope.