The Atomic Theory of Matter

As you learned from my previous post that matter is anything that occupies space and has mass.  It means what we can see around us are matter.  It can be living matter or nonliving matter.  Do living matters and nonliving matters have similarities in their composition? What are matters made up?  In the fifth century B.C., Democritus (460-370 B.C.) expressed the notion that matter is made up of tiny indivisible particles called atomos, which means indivisible.  But Plato and Aristotle did not agree with his idea that is why the idea faded for many centuries, during those times Aristotelean philosophy dominated the Western culture.

The idea of atoms reemerged in Europe during the seventeenth century when scientific investigation should experimental evidence about "atomism" and give rise to the modern atomic theory formulated by John Dalton, an English scientist and school teacher in 1808.  His assumptions are summarized as follows:

1.  Each element is composed of extremely small particles called atoms.

2.  All the atoms of the given element, having the same size, mass, and chemical properties.  Atoms of different elements are different  and have different properties.

3.  Compounds are formed when one atom combines with one or more than one atoms. In any compound the ratio of the number of atoms is either an integer or simple fraction.

4.  Atoms are not changed to another atoms when combined chemically, rearrangement of atoms only occurs.


This theory states that its the atom that is considered the building blocks of matter.  They are the smallest particles that carries the identity of the element.

The Daltons atomic theory also explains some simple laws of chemical combination, one of these was the law of constant composition or law of definite proportion.  This law was the basis of  assumption no 3.  This law states that different samples of the same compounds always contain its constituents elements in the same proportion by mass.  Another law that supports no. 3 assumption of Dalton was the law of multiple proportion.  This law states that if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. 

The assumption no. 4 also supports another law; the law of conservation of mass, which states that matter can neither be created nor destroyed during chemical reaction.  During chemical reaction, atoms are not destroyed they are just rearranged forming new substances.