CHEMISTRY FORM TWO STUDY NOTES TOPIC 5: ATOMIC STRUCTURE & TOPIC 6: PERIODIC CLASSIFICATION

TOPIC 5: ATOMIC STRUCTURE
The Atom
We learned early that matter is made up of small, indivisible particles. Everything around us is made of extremely small particles. These particles are either molecules or atoms. An atom is the smallest indivisible particle of an element that takes part in a chemical change. Atoms are the building blocks of matter. All solids, liquids and gases are made of atoms fitted in different ways.
The present day chemistry is built on the foundations of the Atomic Theory. The idea that elements are made up of atoms is called the Atomic Theory. An English chemist, John Dalton was the first to put forward the Atomic Theory, which for most of the 19th century stated that atoms were hard, extremely small, indivisible and spherical particles like minute lead shots
Dalton Contribution to Atomic Structure
Explain Dalton contribution to atomic structure
The Greek philosopher Democritus (460-370 BC) believed that matter was indestructible and that it is made up of tiny particles called atoms. Our modern understanding is based on the Atomic Theory which was put forward by John Dalton in 1808. His theory re-introduced the ideas of Democritus and other Greek philosophers who suggested that all matter was infinitely divided into very small particles called atoms. These ideas were not widely accepted at that time. They were only revived when Dalton developed them further and experimental science was able to back them up with practical observations.
The Atomic Theory goes back to ancient Greeks, yet we always talk today about Dalton's Atomic Theory. There is a good reason for this. The reason is that, while Greeks put forward the idea that atoms exist they did nothing more. They left the idea vague and untested. Dalton changed this vague imaging into a set of concrete suggestions about atoms which could be tested by experiment. This change from vagueness to precision and experimental test justifies his claim to the theory.
Dalton’s Atomic Theory contains the following main ideas:
  1. Matter is made up of small, indivisible particles called atoms.
  2. Atoms of the same element are all exactly alike in every way and have definite weights.
  3. Atoms are indestructible and they cannot be created.
  4. Atoms of different elements have different weights and posses different properties.
  5. Atoms of different elements combine in small whole numbers to form ‘compound atoms’.
The Modern Concept of Dalton’s Atomic Structure
Explain the modern concept of Dalton’s atomic structure
From the theory, it is observed that each atom has its own mass and that chemical combination takes place between atoms and not fractions of atoms.
Discoveries made in the 20th century, however, showed that certain parts of the theory must be modified. However, Dalton‟s Theory was one of the great leaps of understanding of chemistry. It meant that we could explain many natural processes.
Dalton's Atomic Theory was the first step towards the formation of Modern Atomic Theory. The Dalton's Theory has been subjected to numerous experimentations that have led to some modifications to the theory. However, some ideas in his theory still hold strongly to date. Some modifications to the theory include the following:
  1. The atom is no longer regarded as indivisible, or the smallest particle. Particles smaller than the atom; electrons, protons and neutrons are now known. However, the atom is still the smallest particle which can take part in a chemical reaction.
  2. Atoms of the same element may not be all alike. Some elements have atoms with different atomic masses e.g. carbon 12 and carbon 14. These different atoms of the same element are called isotopes.
  3. In some few cases, atoms of different elements may have the same atomic mass. Both argon and calcium have atomic mass 40. Such atoms are called isobars.
  4. "The compound atoms" of Dalton are known as molecules. A molecule is the simplest particle of matter which is capable of independent existence. Evidence is available where atoms of different elements combine in large integers. An example is in organic and silicon compounds.
  5. Atoms are no longer regarded as indestructible. Radioactive atoms may get destroyed by spontaneous decay or by atomic fission.The atom is therefore the smallest particle of an element which is responsible for the chemical properties of that element, and which takes part in a chemical reaction.

No comments

Powered by Blogger.