Structure of matter
a. Evidence of the Prticulate Nature of Matter
The idea that matter is made up of minute particles called atoms dates back to the ancient Greeks. According to the Greek philosopher Democritus, a given piece of substance, say a piece of yam, can be cut into smaller and smaller bits, until eventually a smallest piece of that substance would be obtained which not be further subdivided. This smallest, indivisible, piece was called an atom. The atomic theory of matter assumes that all matter is made up of tiny particles called atoms and that these are all at times in a rapid state of motion. The nature of this motion and its activity depend upon the temperature of matter and other factors.
The experimental evidence of this particle or atomic nature of matter is the Brownian movement, named after the Biologist Robert Brown who was credited with its discovery in 1827. While observing tiny pollen grains suspended in water under a microscope, he noticed that the tiny pollen grains moved about in zig-zag paths even though the water appeared to be perfectly still. The pollen grains were supposed to be jostled or knocked about here and there by the vigorously moving molecules of water.
Another evidence in favour of the particle nature of matter was obtained from analysis of chemical reactions. This crucial piece of evidence is known by chemists as the Law of Definite Proportions which states that when two or more elements combine to form a compound, they always do so in the same proportions by weight.
For example, the compound water (H2O) is always formed by two parts of hydrogen (H2) and sixteen parts of oxygen (O) and the common salt (NaCl) is always formed from 23 parts of sodium and 35 parts of chlorine by water.
A third evidence that matter is composed of extremely small is provided by the process of diffusion. If we place a few drops of liquid bromine at the bottom of a gas jar on top of which is placed a cover glass, after sometime the brown bromine vapour will be seen in the upper part of the gas jar in spite of the fact that bromine vapour is mush denser than air and should remain at the bottom of the jar. We can explain this diffusion or spreading out of the bromine by assuming that liquid bromine is composed of particles and that the particles can move about easily.
All the evidence we have given that matter is composed of particles are indirect evidence. We have no direct evidence for the existence of these particles since we cannot observe them with even the most powerful microscopes.
b. Simple atomic structure
An atom is the smallest indivisible particle of an element which can take place in chemical change.
Activity
1. Get a tuber of yam and cut out a piece of this tuber about the size of a cube of sugar.
2. Divide this tube into two halves.
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