Thomson’s and Millikan’s contributions to atomic theory. Thomson, through his cathode ray tube experiments, discovered the electron Millikan, through his oil drop experiments, determined the charge and mass of the electron
Properties of Canal rays: (a) Canal rays are positively charged. (b) Canal rays have charge equal in magnitude but opposite in sign to that of electron. (c) Canal rays have mass approximately 2000 times that of electrons.
Thomson model : He proposed that atom is sphere of positive charge having negatively charge embedded in it. The amounts of positive and negative charges on the atom are equal.
Rutherford’s model : He proposed that atom has a central positively charged nucleus which possess entire mass of the atom and is very small in comparison to the size of the atom. The electrons revolve around the nucleus in definite circular path.
Rutherford put forward the nuclear model of an atom, which had the following features: (i) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. (ii)The electrons revolve around the nucleus in well-defined orbits. (iii)The size of the nucleus is very small as compared to the size of the atom.
What experimental evidence led Rutherford to conclude that an atom is mostly empty space? The great majority of the alpha particles went straight through the gold foil, as though nothing were there.
Drawbacks of Rutherford’s model of the atom : Rutherford had proposed that electrons move around a positively-charged nucleus at very high speed in circular orbits. To remain in a circular orbit, electron would have to be accelerated centripetally (tending to move toward a centre). But according to electromagnetic theory, if charged body (electron) is accelerated around another charged body (nucleus) then there would be continuous radiation of the moving body (i.e. electron). This loss of energy would slow down the speed of electron and eventually electron would fall into nucleus. But Rutherford’s model could not explain such a collapse. Bohr’s model : (i) Atom has central positively charged nucleus with the entire mass concentrated in it. (ii) Electrons revolve around the nucleus in well-defined circular known as discrete orbits of electrons. (iii) While revolving in these orbits electrons do not radiate energy.
Bohr Bury Rules : (i) The maximum no. of electrons present in a shell is given by the formula 2n^2. (where n is shell no.) (ii) The maximum no. of electrons that can be accommodated in the outermost orbit is 8. (iii) Electrons are not accommodated in a given shell, unless the inner shells are filled.
According to Neil Bohr : (i) An atom is made up of two particles-electrons and protons. Electrons have negative charge and protons have positive charge. An atom has equal number of electrons and protons. Therefore, atoms are neutral. (ii) The protons are located in a small nucleus at the centre of an atom. Therefore, the nucleus is positively charged.1 (iii) The electrons revolve around the nucleus in fixed circular parths called energy levels or shells represented either by letters K, L, M, N, O and P or 1, 2, 3, 4, 5 and 6. (iv) Each energy level is associated with a fixed amout of energy. The shell nearest to the nucleus has minimum energy and the shell farthest from the nucleus has the maximum energy. (v) The change in the energy level of an electron takes place only when it jumps from a lower energy level to a higher energy level or when it comes down from a higher energy level to a lower energy level.
Isotopes and Isobars: Elements having same atomic number but different atomic masses are called isotopes. Examples of isotopes C–12 , C–13 and C–14 ; O -16, O-17, and O-18Elements having different atomic numbers but same atomic mass are isobars. Examples of isobars Calcium and Argon
Question: The atomic masses of some elements are in fractions and not in whole numbers This is because because most of the elements have more than one natural isotope. These isotopes have different atomic masses. Since, the atomic mass of an element is the average relative mass of all the natural isotopes of that element; therefore, most of the elements have fractional atomic masses.
Question: Why are the the isotopes of an element are chemically similar Isotopes of an element have same atomic number as well as electronic configuration. Since the chemical properties of elements are related to their electronic configuration the elements with similar configuration will have similar properties. Thus the isotopes of an element are chemically similar.
Question: The average atomic mass of bromine comprising of two isotopes 79Br35 and 81Br35, is 80 u. Calculate the percentage of these two isotopes in bromine.
Let the percentage of one isotope, say 79Br35 be x, so the percentage of the other isotope,81Br35 will be (100 – x).
Now, Mass of 79Br35 isotope = 79 u and Mass of 81Br35 isotope = 81 u
So, average atomic mass of bromine = X % of 79 + (100 –x ) % of 81
But the average atomic mass of bromine is given to be 80 u.
So, X % of 79 + (100 –x ) % of 81 = 80
On solving we get x = 49.5
Thus, the percentage of isotope 79Br35 is = 49.5 %
The percentage of isotope 81Br35 is = (100 – 49.5) = 50.5 %