CHM 111: Introductory Chemistry I

CHAPTER1 

ATOMIC THEORY AND NATURE OF ATOMS 

Chemistry studies the composition of matter and the changes it undergoes. Matter has been defined as anything that has weight and can occupy space. All materials are made up of matter. For centuries, philosopher, chemist and physicist tried to answer the question of what matter was made up of using a variety of experiments and observations. 1.1 THE EARLY IDEAS OF THE ATOM In 440BC a Greek philosopher named Democritus came up with a conceived idea that matter was composed of very tiny and indestructible particles. He stated that these particles were the smallest unit of matter which he called atomos – meaning indivisible in Greek. The theory was not generally accepted because it was a mere philosophy (guess). In 1803, John Dalton proposed the atomic theory as follows: 1. All elements are composed of atoms and atoms are indivisible and indestructible particles 2. Atoms of the same elements are exactly alike (identical) in size, shape, mass and chemistry. While atoms of different elements are different. 3. Atoms combine with atoms of other elements in simple whole-number ratios to form chemical compounds. Dalton‟s atomic theory became one of the foundations of chemistry and the dawn of quantitative analysis, because soon after the theory, scientific investigation became rapid. Curiosity and the urge to understand the world became increased. 1.2 JOSEPH JOHN THOMSON ATOMIC MODEL Discovery of the electrons In 1897, J.J. Thomson discovered the electrons while investigating the electrical conductivity of gases at very low pressure. In his experiment, a very high potential difference (pd) was placed across a glass tube containing gas at a very low pressure. At that experimental condition, a glow coming from the negative terminal was observed and it was attracted to the anode. A hole in the anode allowed the rays to pass through and hit the end of the glass tube‟s surface. The rays was deflected to the north pole by the magnetic field in the glass tube. The beam hit the fluorescent screen at the positively charged plate, when it was exposed to an electric field. However, in the absence of both the magnetic and electrical field or when both fields are on such that the fields cancels each other influence, the beam was not deflected, it travelled in a straight line and hit the fluorescent screen at point directly opposite the source.