Support
Learning Objectives
Contents to learn
the structure of an atom
In 1911 Rutherford proposed the first adequate structure of the atom.
He suggested that an atom consists of a central massive part called the nucleus and electrons revolving round the nucleus.
Most of the space in the atom is empty.
Almost the entire mass of the atom is concentrated in the nucleus which contains protons and neutrons.
The mass of an electron is negligible as compared with that of a proton or a neutron.
The proton being a positively charged particle carries a charge of +e, equal and opposite to the charge on an electron.
Thus the charge on an electron is –e and its numerical value is 1.6 X 10-19 Coulombs.
The mass of an electron is 9.11 X 10-31 kg. The mass of a proton is 1.67 X 10-27 Kg which means that a proton is 1836 times heavier than an electron.
A neutron is a neutral particle as it has no charge. Its mass is nearly equal to that of a proton.
We now believe that the nucleus of an atom is built up of protons and neutrons. The particles that reside inside the nucleus are called nucleons. An atom as a whole is neutral as the number of protons in its nucleus is equal to the number of electrons revolving round it.
Atomic number or charge number of an element is defined as the numbers of electrons revolving round the nucleus which is the same as the number of protons present in the nucleus of an atom.
Thus, Z = number of protons in the nucleus.
The mass of an atom is specified by its mass number which is the total number of protons and neutrons present in the nucleus of the atom. It is denoted by A.
Therefore, A = Number of protons + Number of neutrons
The number of neutrons in the nucleus is given by, N = A – Z
The nucleus of the atom of an element having X as its chemical symbol is represented by zXA, where A is the mass number of element X and Z is its atomic number or charge number.
Hydrogen is the lightest of all the elements and its atoms have the simplest structure.
An atom of Hydrogen consists of one proton in its nucleus and one electron revolving round the nucleus.
Therefore, its Z = 1 and A = 1. The nucleus of Hydrogen is symbolically represented by 1H1.
The next heavier element is helium having two protons and two neutrons. It has Z = 2 and A = 4. Its nucleus is represented by 2He4.
| Atom | Symbol | Number of protons | Number of neutrons | Number of electrons |
| Hydrogen | 1H1 | 1 | 0 | 1 |
| Carbon | 6C12 | 6 | 6 | 6 |
| Calcium | 20Ca40 | 20 | 20 | 20 |
| Uranium | 92U238 | 92 | 146 | 92 |
Rutherford’s atomic model
At the begining of 20th century, scientists knew that the atom contained positive and negative charges, but the structure was a great mystery.
An experiment by Rutherford discovered, what is now called Rutherford scattering experiment.
(The actual work was done by two of his students, Gieger and Marsden)
What they discovered was that almost all of the alpha particles go through the thin metal sheet with no difficulty.
Perhaps one particle in a million was scattered by a relatively massive object that sent it off at a wider angle, and perhaps even back the way it had come.
This told them that the atom consists of a nucleus that contains almost all of the mas of the atom, because very few alpha particles were scattered by the nucleus so it had to be extremely small surrounded by a cloud of extremely light electrons.