Rajasthan Board RBSE Class 9 Science Notes Chapter 3 Atomic Structure
- The idea that matter can be divided and subdivided was considered long back in India and Greece. Maharishi Kanad was a great Indian sage in the vedic period suggested that if a pure substance is broken down into smaller pieces, a stage is reached when no further subdivision is possible. He called the smallest particle of any pure substance as ‘anu’. Each anu of a substance has the same properties.
- He further believed that each anu may be made of two or more still smaller particles. He called these particles as ‘parmanu’. According to him, a parmanu is the ultimate smallest unit of matter.
- Some Greek philosophers believed that matter is composed of tiny particles which cannot be destroyed. The Greek philosopher Democritus called such tiny particles as atoms (from the Greek word atoms, meaning uncut). However, there was no experimental evidence to support this model.
- In 1805. an English school teacher John Dalton postulated his famous atomic theory, known as Dalton’s atomic theory.
Laws of Chemical Combination
- Law of Conservation of Mass: Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
- Law of Constant Proportions: This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.
- A British chemist John Dalton proposed the basic theory about the nature of matter, in 1808.
Postulates of Dalton’s theory are:
- All matter is made up of tiny particles called atoms.
- An atom cannot be broken down further. It cannot be created or destroyed in a chemical reaction.
- All the atoms of any element are Identical in all aspects.
- Atoms of different elements have different properties.
- Compounds are formed by the combination of atoms of the same element more than one element in the ratio of small whole numbers.
- Kinds of element and relative number of their atoms always remain constant in a compound.
Electrical discharge tube
Electrical discharge through gases is studied by using a specially designed glass tube, called as discharge tube. It consists of a cylindrical glass tube having a side tube, and two metal electrodes, one at each end. These electrodes can be connected to the respective terminals of a high tension power supply. Air from the inner side of the tube can be pumped out by connecting the side tube to a vacuum pump, and a desired pressure can be maintained inside the tube.
Discovery of charged particles of an atom
Discovery of electrons and cathode rays. The electron was discovered by J.J. Thomson in 1859 during the experiments on passage of electric charge at high voltage through gases, at low pressure in vacuum discharge tube. When a high voltage of 10000-30000 volt is applied on the electrodes in the tube, invisible rays are produced which move from cathode towards the anode. These rays are called cathode rays.
It was found that cathode rays are made up of negatively charged particles known as electrons. Mass of an electron is 9.1096 × 10-31 kg or 5.487 × 10-4 amu. Charge on an electron is 1.602 × 10-19 coulomb. It is taken as one unit (negative) charge. The name ‘electron’ was given by Stony.
Characteristics of cathode rays:
- These rays travel in a straight line.
- They show fluorescence.
- They are effected with electric and magnetic field.
- They are made up of negatively charged particles.
- The e/m ratio of particles of these rays remain the same.
Protons
In the production of cathode rays a discharge tube having perforated cathode was used. When a high voltage of about 10000 volt was applied, it was observed that certain rays, consisting of positively charged particles are moving from anode to cathode. These rays were named as Positive rays or Canal rays, and the particles were called protons. Credit of the discovery of protons is given to Goldstein (1886). Charge on proton is also 1.602 × 10 coulomb, taken as 1 unit positive charge. The mass of proton is 1.6726 × 10-24 g. It is estimated that a proton is 1836 times as heavy as an electron.
Characteristics of Anode or Canal rays:
- These rays always travel in a straight line.
- The c/m values for the particle depend upon the gases enclosed in the discharge tube.
- They are deflected by electric and magnetic field.
- These rays consist of positively charged particles.
- They form image on photographic plate.
Structure of Atom: Scientists had given various explanations about the structure of an atom.
Thomson’s Model of an Atom:
Thomson described an atom as a solid sphere of radius 10-8 cm made of a positively charged substance. Electrons are embeded into the sphere, like plums i.e. dry fruits in a pudding.
i. e., Halwa or Khichdi. The two types of charges are equal in amount and the atom on the whole is electrically neutral.
Rutherford’s Model of an Atom:
In 1911, Ernest Rutherford and his students Marsden and Geiger performed an experiment. They bombarded a stream of a-particles from radioactive source on a very thin foil of gold and observed that 99% of the particles went through the gold foil without any deflection. A few particles were deflected at fairly large angles, and very few (say, one out of every 20000 particles) were returned back. On the basis of their observations, they concluded that atom is like a spherical body.
The maximum mass and whole positive charge of the atom is situated at its center. This very small solid portion is called nucleus. Electrons revolve around the nucleus in huge space at various distances at very high speed. Number of electrons present outside the nucleus is equal to the number of protons present in the nucleus. Thus, atom on the whole is neutral.
Drawbacks of Rutherford’s Atomic Model: According to Rutherford, it has been assumed that electrons move in circular orbits around the nucleus, then it always radiates out energy. So it must gradually move towards the nucleus in a spiral path till it falls into the nucleus. But atom is very stable. Rutherford model does not explain this stability. Also, it must produce continuous spectra. But, actually it produces line spectra.
Discovery of Neutron:
In 1932, Chadwick showed the presence of neutral particles in the nucleus of the atoms on the basis of some experiments. Mass of neutron is found to be nearly equal to the mass of a proton, 1.675 × 10-24 g.
Bohr’s Model of Atom:
- Neils Bphr proposed certain postulates and modified the Rutherford’s model of atom, and rectified its defects.
- Electrons revolve only in certain special circular orbits. Radius and energy of each orbit is constant and definite.
- There is no change in the energy as long as electron remains in the discrete stationary orbit. Energy is absorbed or emitted in the form of bundles known as quanta (quantum means one bundle, quanta is its plural), only when electron jumps from one orbit (energy level) to another orbit.
- The first orbit or energy level nearest to the nucleus is denoted by K for which n = 1, second orbit is denoted by L for which n = 2, third orbit is M for which n = 3, fourth orbit is N for which n = 4 and so on.
Size of Atom:
Atoms are extremely small in size. The size of atom is so small that if millions of atoms are placed one over another they will make a layer as thick as sheet of a paper. The size of an atom is usually expressed in terms of its radius called atomic radius or measured in nanometres (nm)
1 nm = 10-9 m = 10-7 cm = 10 A or 1 m = 109 nm
Atomic number:
- The number of unit positive charges present on the nucleus of an atom of a particular element or the number of protons in the nucleus is called the atomic number of that element. Furthermore, the number of protons is exactly equal to the number of electrons present in a neutral atom. Atomic number is represented by Z.
- Thus, Atomic Number (Z) = Number of Protons = Number of Electrons, in a neutral atom.
- The atomic number is represented by putting ‘Z’ in subscript on left hand side of symbol of element e.g. 2He for helium as He has only two protons in its nucleus. Similarly atomic number of potassium is 19, it means that the potassium atom has 19 protons and 19 electrons in its neutral atom and is depicted by 19K.
Mass number:
- The sum of total number of protons and neutrons present in the nucleus of an atom is called the mass number of that atom.
- Mass number is represented by A and is placed on left side on top of symbol of element.
- Mass Number (A) = Number of Protons + Number of Neutrons = Number of Nucleons
- Thus, for a neutral atom with mass number A and atomic number Z, of an element having
Symbols, i.e. \(_{Z}^{\mathrm{A}} \mathrm{S}\)
Number of electrons = Z, Number of protons = Z, and Number of neutrons = A – Z
For example: \(_{19}^{\mathrm{39}} \mathrm{K}\) has an atomic number (Z) equal to 19 and mass number (A) equal to 39. Therefore for a neutral atom of potassium, Number of electrons = 19, Number of protons = 19 and Number of neutrons = 39 – 19 = 20.
Each element has a characteristic atomic mass. In 1961, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass equal to exactly one-twelfth the mass of one atom of carbon-12. Relative atomic mass of the atom o’f an element is defined as the average mass of the atom as compared to the mass of one carbon – 12 atom.
Relative atomic mass of an element = \(\frac{\text { Mass of an atom of the given element }}{\frac{1}{12} \times \text { Mass of an atom of } \mathrm{C}-12 \text { isotope }}\)
Atomic mass and atomic number of some elements:
| Element | Atomic number | Mass number | Atomic mass in amu |
| H | 1 | 1 | 1.008 |
| He | 2 | 4 | 4.003 |
| c | 6 | 12 | 12.001 |
| N | 7 | 14 | 14.007 |
| 0 | 8 | 16 | 15.999 |
| Na | 1 | 23 | 22.99 |
Avogadro number or Mole concept:
One mole of any substance is the amount that contains the same number of particles as there are in 12 g of carbon-12 isotope. The number of particles present in one mole of any substance is fixed. This number is 6.022 × 1023. This number is called the Avogadro constant or Avogadro number. Mass of one mole of a substance is called its molar mass.
Example, 1 mole H2O = 6.022 × 1023 molecules.
Distribution of Electrons in Orbits:
- The net positive charge and mass of the atom is concentrated in its nucleus. Size of the nucleus is very small, of the order of 10-15 m. Electrons revolve around the nucleus in definite orbits. The number of electrons in different energy levels is as per the following rules.
- Maximum number of electrons in an orbit is given by 2n2, e.g.
- Maximum number of electrons that can be accommodated in the outermost orbit is 8.
- Orbits (or shells) are filled in a step wise, orderly manner. Electrons are not allowed to occupy the next shell unless the inner shell of lower energy level is completely filled.
Isotopes:
Atoms of the same elements having the same atomic number, but different mass numbers are called isotopes.
Applications of Isotopes:
- Radio isotope of Uranium is used as a fuel in nuclear reactors for generation of electricity.
- Specific radioisotopes are used for medical diagnosis and treatment. Iodine – 131 is widely used in treating thyroid, cancer. Cobalt-60 was formerly used for external beam radiotherapy for cancer.
- Isotopic substitution can be used to determine the mechanism of a chemical reaction i.e. the kinetic isotope effect.
- Sodium – 24 is used for study of electrolytes within the body. Some isotopes are used to detect the tumors, blood clots etc.
Isobars:
Atoms of different elements having different atomic numbers, which have the same mass number, are called isobars. For example, \(_{40}^{20} \mathrm{Ca} \text { and }_{16}^{40} \mathrm{Ar}\) are isobars.