The Structure of the Atom
Scientific investigation on the structure of the atom and its subatomic particles is an area which still absorbs scientists. In the process, they have discovered many more new subatomic particles; for example, positron, meson, quarks etc.
We will, however, confine ourselves to the study of three fundamental subatomic particles - the electron, the proton and the neutron. A basic knowledge of the nature and arrangement of these three subatomic particles is important in understanding the structure and behavior of atoms.
"An atom is an indivisible particle and is the building block of all matter present".
Though we call it indivisible, the atom consists of three sub atomic particles: Proton, electron and neutron.
An English chemist, Dalton in 1803, put forward an atomic theory known as Dalton’s atomic theory, according to which all matter is composed of extremely indivisible particles which he named atoms. (Greek – atom = cannot be cut)
However, this concept of indivisibility of atom did not hold long and it was proved by the experiments made by brilliant research workers like J. J. Thompson, Rutherford (1911), Neil’s Bohr (1912), Chadwick and Moseley and others that the atom itself has a complex structure. Atoms consist of several particles called subatomic particles like the proton, electron, neutron, positron, neutrino, meson, etc. Out of these, electrons, protons and neutrons are very important. The atom remains indivisible though, in all its physical and chemical changes.
Atomic structure is the core of chemistry. Atomic structure is the study of the structure of an atom, and also the sub atomic particles present in it. J . J. Thompson Plum pudding model
There are many atomic theories put forward to describe the structure of an atom. The first ever theory put forward, Dalton's atomic theory, paved the way for all the other atomic theories.
Dalton's atomic theory
John Dalton used the Greek concept of an atom and the laws of definite proportions, conservation of mass and multiple proportions to give the atomic theory on scientific basis.
Dalton proposed that the properties of elements differ from one another because their atoms differ. He also recognized that even though they may share the same atoms, compounds have properties that bear no relationship to those elements of which they are composed.
The discovery that atoms contained electrons caused Thompson to predict an atomic structure, according to which the whole atom was considered to be a positive sphere, with negatively charged electrons embedded in it like a plum in a pudding. Thompson's model did not have any nucleus in it.
But, with the discovery of the nucleus and positively charged proton and neutrally charged neutrons, two more important models of atomic structure were put forward:
Rutherford atomic model
Rutherford atomic model, though a major breakthrough with a central nucleus and surrounding electrons, did not explain how an electron keeps revolving around the nucleus without losing energy.
Bohr's atomic structure
Niels Bohr expanded Rutherford's theory further and gave a clear concept of balancing the attractive force and the centrifugal force of the revolving electrons.
The atomic theory put forward by Niel's Bohr, was completely successful, except for certain corrections, like replacement of the orbits of Bohr by orbitals, etc.
The discovery of the electron, proton and neutron was the starting point for new avenues of research in science which gave physicists an insight into the structure and nature of the atoms of matter.
An atom is made up of three elementary particles namely electrons, protons and neutrons. Electrons have a negative charge, protons have a positive charge and neutrons have no charge. Neutrons are neutral. Due to the presence of an equal number of negative electrons and positive protons, the atom as a whole is electrically neutral. Based on the above findings, one can say that the atom has two major divisions.
- The first is the center of an atom, called its nucleus. The protons and neutrons are located in the small nucleus at the center of the atom. Due to the presence of protons the nucleus is positively charged.
- The second are electrons which revolve around the nucleus in different shells (or orbits). The space around the nucleus in which the electrons revolve, determines the size of the atom.
Lot of research has been done on the structure of an atom. To define atomic structure, “The atomic structure is where the protons and neutrons are shown in the nucleus and electrons revolve around in orbitals.”
Definition of an ElectronDefinition of a Proton
Electron is a subatomic particle which bears a mass equal to 0.0005493 amu (which is taken to be equal to zero) and negative charge equal to -1 on it. So, electron is a mass less particle which bears one unit of negative charge. An electron is indicated as: e-10.
A proton is a subatomic particle which bears mass equal to 1 amu and positive charge equal to +1 on it. Proton can also be defined as the subatomic particle which bears one unit mass and one unit positive charge on it. A proton is indicated as P1+1.Definition of neutron
Neutron is a subatomic particle which bears a mass equal to that of a proton and has no charge on it. It is represented as: n10.
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|| Discovery / Landmark Event in Atomic Structure
|| Dalton’s discovery that ‘all matters consists of small indivisible particles called atom’.
|| J. J. Thompson’s plum pudding model
| 1896- 97
|| J. J. Thompson’s discovery of electron.
|| Goldstein’s discovery Of protons.
|| Rutherford’s Nuclear atom
|| Moseley’s determination of atomic number.
|| Bohr’s atom
|| Bohr – Bury scheme of electronic arrangement.
|| Chadwick’s discovery of neutrons.
Carbon contains a total of 6 electrons. So, its atomic number is 6.
Mass number of Carbon = 12.
So, Number of neutrons = Mass number – atomic number = 12 – 6 = 6.
Therefore, Carbon atom has:
Number of protons = 6
Number of electrons = 6
Number of neutrons = 6