The Wave Particulate Nature of Matter is a very broad topic meaning that it cannot be completely covered in this page rather, we would touch on important or vital areas of that topic. This topic would discuss pieces of evidence to support the existence of three subatomic particles in an atom namely protons, neutrons, and electrons also, we would talk about the arrangement of electrons in the atoms.
Electrical Nature Of Atoms
Electrons were discovered by Sir J.J Thompson in 1897 during his cathode ray experiment. As a chemist, you must have heard of a cathode ray experiment without knowing the definition.
Cathode Ray Experiment: When a gaseous form of a substance is introduced into a long glass-tube between two electrodes at extremely low pressure, and connected to a high-voltage source, the gas becomes conducting. Luminous rays are emitted from the cathode moving to the anode hence, the tube is called a cathode ray tube or discharge tube.
What is then a Cathode Ray? By definition, cathode rays are streams of negatively charged particles moving in straight lines with high velocities from the cathode to the anode. Cathode rays are called electrons.
Oil-Drop Technique Experiment To Determine The Charge On An Electron
R.A. Milikan in 1909 used the oil-drop technique to determine the value of the charge on an electron. The value obtained was 1.60* 10-19C and it was taken as the value of the charge e, on an electron. An electron has only one unit of a negative charge, written as e– . When an atom captures one or more electrons, it becomes a negatively charged particle called a negative ion or anion.
From the following, the value of the charge-to-mass ratio by Thompson(E/m = 1.76 * 108Cg-1) and that of the charge by Milikan(e- = 1.60*10-19C) and the mass, m of an electron was found to be 9.11 * 10-28g
Discovery Of Protons
Goldestein in 1886 discovered the presence of positively charged particles in the discharge tube. They are produced when electrons are removed from neutral gas atoms and such particles are called positive ions or cations. They are usually attracted toward the negative cathode in the discharge tube and when positive ions are allowed to pass through a perforated cathode, they form positive rays.
Some of the properties of positive rays are:
- Positive rays are more massive than the cathode rays; hence, the entire mass of an atom is associated with its positive ion
- The charge-to-mass ratio of a positive ray is not constant; its value depends on the nature of gas in the discharge tube.
Discovery of Neutrons & Nucleus
Lord Ernest Rutherford is a very important name associated with the discovery of neutrons. In 1911, he made the following observations:
- The total mass of the protons and electrons in an atom was not equal to the actual mass of the atoms as obtained from the mass spectrometer
- The total mass of the protons in an atom accounted for about half of its nuclear mass
The experiment that led to the discovery of the Nucelus was accredited to both Lord Ernest Rutherford and Sir J.J Thompson. Sir J.J Thompson described an atom as a uniform-density ball of positively charged mass with electrons scattered randomly within the sphere. During an experiment conducted by Rutherford in 1911, a thin gold film was fired with alpha particles and he observed that almost all the alpha particles pass through without being deflected while a few particles were deflected to various angles and directions with some even deflecting backward.
Neil Bohr’s Theory Of An Atom
Neil Bohr proposed a theory to account for the existence of energy levels in the atom and his theory was based on quantum theory instead of electromagnetic theory. He made five assumptions that are very vital in the study of the wave-particulate nature of matter. The assumptions are:
- The electrons in an atom exist in distinct energy levels and never between energy levels
- The energy of an electron in a particular energy level has a fixed value
- An electron continues to orbit around the nucleus in a particular energy level at a given temperature without losing energy
- When an atom absorbs enough energy, its electrons are promoted from a lower to a higher energy level and an electron falls from a higher to a lower energy level.
- When an electron is in the lowest energy level, it does not emit energy when orbiting around the nucleus.