 |
Introduction |
| |
A story unfolds - Structure of atom : Let us right away meet those who scripted a "mighty story" of a "miniscule" atom.
John Dalton (1766 - 1844) : Born in a poor family of a weaver in England, he rose to become a teacher and later a Principal of a school. In 1793, he left for Manchester to teach physics and chemistry in a college. |
|
Atomic structure |
| |
Although Dalton's theory of the existence of atoms was published in 1808 there was no clear idea of what an atom might look like until the early part of the twentieth century. With the advent of electricity and phenomenon of spectrum, experimentation underwent a change and a flurry of research in the structure of matter has put forth the modern concept of the structure of atom. |
|
Discovery of Neutrons |
| |
Although Rutherford's model was enormously successful at explaining the scattering of alpha particles, there was a problem when the experimental data were used to calculate the mass of the nucleus. From the results of the scattering experiments, it was possible to calculate the charge on the nucleus. However the mass of these protons was only about half the overall mass of the nucleus. In 1920, William Draper Harkins an American physicist suggested that the missing mass could be accounted for if the nucleus contained other particles with mass similar to that of a proton but no charge. He named this particle neutron. James Chadwick finally discovered it in 1932. He bombarded the element beryllium with alpha-particles. |
|
Discovery of Electrons |
| |
In 1885, Sir William Crookes carried out a series of investigations into the behaviour of metals heated in a vacuum. The experiment of Crookes and others showed that a heated cathode produced a stream of radiation, which could cause gases at low pressure to glow and which, make other substances emit light too. The radiation emitted from the cathode was given the name 'Cathode rays'. By mid-nineties it was known that these rays could be deflected by a magnetic field and they carried a negative charge. Some scientists felt that these rays were waves and others were inclined to think they were particles. |
|
Discovery of Protons |
| |
Since the atom is electrically neutral there must be positively charged particles present in the atom to neutralize the negative charges of the electrons. Goldstein experimentally proved the existence of protons in the atom. |
|
Atomic Models |
| |
'Raisin Pudding' Model - Thomson's model of atom : J. J Thomson the discoverer of the electron believed that the atom is a uniform sphere of positive charge with electrons (raisins) embedded in it. |
|
Characteristics of Electrons, Protons and Neutrons |
| |
|
|
X-Rays |
| |
In 1895, Wilhelm Conrad Roentgen, a German physicist observed, while doing some experiments with the discharge tube, that when cathode rays are allowed to fall on a metal target called anticathode placed in their path, a new kind of rays are produced. These radiations are called X-rays. |
|
Bohr's Model of an Atom |
| |
Rutherford's model had a major drawback. It could not explain why ultimately electrons did not fall into the nucleus by taking a spiral path. This was in concurrence with the electromagnetic theory that states 'if a charged particle undergoes accelerated motion then it must radiate energy (lose) continuously'. |
|
Atomic Number and Atomic Mass |
| |
The nuclei of atoms is made up of protons and neutrons. These two components of the nucleus are referred to as nucleons. The electrons occupy the space outside the nucleus. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. This number is the atomic number given by the symbol Z. |
|
Arrangement of Electrons in an Atom (Bohr-Bury Scheme) |
| |
Electrons revolve around the nucleus in different energy levels or shells and each shell is associated with definite energy. The energy of the K shell is the least while those of L, M, N and O shells increases progressively. We also know that any system that has least energy is the most stable.
1st energy level is K shell
2nd energy level is L shell
3rd energy level is M shell
4th energy level is N shell and so on. |
|
Valence Electrons |
| |
The outermost shell of an atom is known as the valence shell. The electrons present in the valence shell are called valence electrons. |
|
Valency |
| |
It is the combining capacity of an element. It is the number of electrons in an atom that actually take part in bond formation. For example, carbon atom has 4 valence electrons.
K L
C(6) 2 4 |
|
Isotopes |
| |
It is interesting to note that atoms of a given atomic number can have different number of neutrons. |
|
Radioactivity |
| |
The discovery of the electron towards the end of the nineteenth century was the starting point of new avenues of research in science, which were to give physicists an insight into the structure and nature of the atoms of matter. |
|
Summary |
| |
Atoms are made up of three fundamental particles: electrons, protons and neutrons.
Discharge tube experiments by William Crookes and J.J. Thomson led to the discovery of electron. |
