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Atom STRUCTURE OF THE ATOM Atom is the smallest particle of an element .It is made up of three fundamental particles: Electrons -Negatively charged Protons -Positively charged Neutrons -No charge (Neutral) Now we will study how these particles were discovered. Different scientists did different experiments and discovered these particles. Discovery Of Electrons J.J. Thomson in 1897 discovered electrons in an atom. This was done through a discharge tube experiment by passing electricity at very high voltage through a gas at very low pressure to break the atoms of gas into smaller particles. The invisible rays, which, formed at cathode during the cathode ray experiment and emitted greenish-yellow light, were called cathode rays. Properties of Cathode Rays 1. 2. 3. 4. 5. Cathode rays travel in straight line. It is due to the fact that cathode rays cast shadows of the objects placed in their path. Cathode rays can produce mechanical effects .It is due to the fact that they can rotate a light paddle wheel if placed in discharged tube. Cathode rays are negatively charged. This is due to the fact that cathode rays are deflected towards the positive plate when an electric field is applied in the path of cathode rays. The nature of cathode rays does not depend on the nature of gas taken in the discharge tube or the material of cathode. The mass of cathode ray particle is very small as compared to mass of atom from which it is formed. Electrons Discovery Of Proton E. Goldstein in 1886 discovered canal rays or positive rays and protons in an atom. This was done through a discharge tube experiment with some different equipments and methods. Passing electricity at very high voltage through a gas at very low pressure to break the atoms of gas into smaller particles A stream of positively charged particle shot out from anode of a discharge tube when a current is passed through a gas at very low pressure are called anode rays. Properties Of Anode Rays 1. 2. 3. 4. 5. Anode rays travel in straight lines as they cast shadows of the objects placed in their path. Anode rays can produce mechanical effect as they can rotate a light paddle when placed in their path. Anode rays are positively charged as they are deflected towards negative plates when an electric field is applied. The nature of anode rays depends on the gas taken in the discharge tube. The mass of anode ray particle is almost equal to the mass of the atom from which it is formed. Protons The proton is positively charged particle found in the atoms of all elements. The protons are located in the nucleus of an atom. Mass Of Protons Relative mass = 1amu or 1840 times to that of an electron Absolute mass = 1.6 × 10 −24 grams Charge Of Proton Relative Charge = +1 Absolute Charge = 1.6 × 10 −19 C The electron is a negatively charged particle found in the atom of all elements. Electrons are located outside the nucleus in an atom. Neutron Mass of Electron How neutrons were discovered? 1 Relative mass of electron = of mass of hydrogen atom 1840 Absolute mass of electron = 9 × 10 −28 grams Charge of Electron Relative charge of electron = −1 Absolute charge = 1.6 × 10 −19 coulomb Because electron has smallest negative charge of 1 unit, that is why it is taken as unit of negative charge. The neutron is a neutral particle found in the nucleus of an atom. After the discovery of protons and electrons the mass of the atom was started measured on their basis. But the problem was that with this method wrong atomic masses were obtained. For example an oxygen atom contains 8 protons and 8 electrons. Now the mass of electron is negligible. So the atomic mass of oxygen should be only 8, which is the mass of protons. This is wrong because the actual mass of carbon is 16. Then a scientist named Chadwick discovered that there is another fundamental particle in an atom, which is called neutron. This particle is bearing rest of the mass of the atom. Mass Of Neutron Relative mass = 1 amu Absolute mass = mass of proton Charge Of Neutron There is no charge on neutron. 1. 2. Atomic Nucleus To show the structure of an atom various models were given by various scientists. These models show the presence and location of the three particles present in it with the presence of nucleus also. Now we will discuss some of the models of atom one by one. JJ Thomson’s Model Of An Atom This model is also called Plum Pudding Model or Apple Pie Model. JJ Thomson gave the first model of the atom. In this model electrons were uniformly distributed in the entire atom. The mass of the atom was supposed to be uniformly distributed. Thomson Proposed That (i) An atom consists of a positively charged sphere and the electrons are embedded in it. (ii) The negative and positive charges are equal in magnitude. So the atom as a whole is electrically neutral. 3. Most of the alpha-particles pass straight without any deflection indicating that most of the space inside the atom is empty.. A few alpha-particles are deflected through small angles and some are deflected through large angles indicating that the positive charge of the atom occupies very small space. A very few alpha-particles turn back completely indicating that all the positive charge and mass of the gold atom were concentrated ina very small volume within the atom. The Conclusions Made Were 1. There is a lot of empty space in an atom as most of the alpha-particles passed straight. 2. There is a center of positive charge in atom, which repels positively charged particles and deflects their path. 3. This center of positive charge in an atom is known as nucleus. 4. The nucleus is very dense and hard which does not allow alpha-particles to pass through them that are why a few alpha-particles completely turned back. Rutherford’s Structure Of An Atom (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. Drawback Of Rutherford’s Atomic Model Of An Atom Rutherford Model Of An Atom Rutherford did an experiment called Alpha Particle Scattering or Gold Foil experiment. This experiment was done by striking alpha particles (Helium ions with charge of +2 & mass of 4) on thin gold foil. It was supposed that when electrons will revolve in a high speed around the nucleus they would loose energy. When energy of electrons will decrease their speed will also be decreased. Due to this the electrons will be attracted towards nucleus more strongly. By coming more and more close to the nucleus following a spiral path ultimately they will fall into the nucleus. Such an atom is not stable. The Bohr’s Atomic Model According to Bohr’s atomic model the electrons revolve around the nucleus in particular orbits having fixed energy levels. When electrons revolve in fixed orbits they do not loose energy and in turn do not fall into the nucleus and that is why they are stable. The number of electron shells (energy levels) in which electrons revolve is also fixed. The nucleus of an atom consists of protons and neutrons and is positively charged. An atom remains neutral due the presence of equal number of protons (positive charge) and electrons (negative charge). The Orbits, Shells, Energy Levels In An Atom Observations were: - There are six energy levels in an atom, which are denoted by the numbers 1,2,3,4,5,6. There are four shells which are denoted by K, L, M and N. Example 4 2 He 12 6C 16 8O Valence Electrons And Valency Valence Electrons The electrons, which are present in the outermost shell of an atom, are called valence electrons. Only the valence electrons take part in chemical reactions. Valency So it is concluded that: 1. No. of energy levels = No. of shells = No. of Orbits 2. The orbit nearest to the nucleus has the minimum energy and the orbit farthest from the nucleus has the maximum energy. Rules To Be Followed For Writing The Number Of Electrons In Different Energy Levels Or Shells. (iv) The maximum number of electrons present in a shell is given by the formula 2n2, where n is the orbit number. Hence the maximum number of electrons in different shells are as follows: - Valency is number of electrons, which are loosed, gained or shared by the atom of an element in order to become stable. In other words, valency is the combining capacity of an element with another element or compound. Valency is determined by valence electrons. The combining capacity of the atoms of other elements that is their tendency to react and form molecules with the atoms of same or other elements, is thus explained as an attempt to attain a fully filled outermost shell. To make an element stable the outermost shell of its atom should be filled completely by the maximum number of electrons, which can be accommodated in that shell. Three things to be considered are: 1. If the maximum capacity is 8 and there are less than 4 electrons in the outermost shell, then atom loses these electrons to become stable. Because less energy is required in losing less than 4 electrons than to gain more than 4 electrons to remove incomplete shell. Example For the first energy level n=1 number of electrons = 2 × (1)2 = 2 × 1 = 2 Example Sodium’s atomic number = 11 Sodium’s electronic configuration = 2, 8, 1 Now it has to lose this electron to become stable. Because losing 1 electron is easier than to gain 7 more electrons needed to become stable. As it is losing its 1 electron in from outer most shell, its valancy is 1. For the second energy level n=2 number of electrons = 2 × (2)2 = 2 × 4 = 8 With this, we can find the maximum number of electrons in an electron shell as follows: Electron Shell K Shell L Shell M Shell N Shell 2. Maximum Number Of Electrons = 2 2 × (1)2 = 2 × 1 2 × (2)2 = 2 × 4 = 8 2 × (3)2 = 2 × 9 = 18 32 2 × (4)2 = 2 × 16 = Example Sulphur’s atomic number = 16 Sulphur’s electronic configuration = 2, 8, 6 Now it has to lose this electron to become stable. Because gaining 2 electrons is easier than to lose 6 electrons to become stable. As it is gaining 2 electron in outer most shell, so its valency is 2. (ii) The maximum number of electrons that can be accommodated in each shell or orbit is 8. (iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is the shells are filled in a step wise manner. Atomic Number The number of protons, which are equal to number of electrons, is called atomic number. It is denoted by Z. Example Z = 1 for hydrogen => 1 electron & 1 proton Z = 6 for carbon => 6 electrons & 6 protons Mass Number The total number of protons and neutrons present nucleus of an atom is known as mass number. An element is represented as Mass Number Symbol Atomic Number If the maximum capacity is 8 and there are more than 4 electrons in the outermost shell, then atom gains electrons to become stable. Because less energy is required in gaining less than 4 electrons than to lose more than 4 electrons to fill the maximum capacity. 3. If the maximum capacity is 8 and there are 4 electrons in the outermost shell, then atom can gain or loose electrons to become stable. As it can loose or gain 4 electrons from outer most shell, its valancy is 4. Metals And Non-metals 1. Elements having 1, 2, or 3 valence electrons in their atoms are metals, except hydrogen and helium. Though, hydrogen has 1 and helium has 2 valence electrons in their outer most shells, but both are non-metals. 2. Elements having 4, 5, 6 or 7 valence electrons in their atoms are non-metals, e.g. carbon, nitrogen. Isotopes Isotopes are the atoms of the same element having same atomic numbers but different mass numbers or atomic masses. All the isotopes of an element have same chemical properties because they have same number of valence electrons. Atomic number of the isotopes is same because number of protons and electrons are same in all atoms. Mass number is different because the number of neutrons is different in atoms of the same elements. The isotopes of the elements are having same chemical properties but different physical properties Example Carbon has two isotopes C-12 C-14 12 6C 14 6C In both atoms of carbon above, atomic number, number of protons and number of electrons are same. But mass number differs because number of neutrons is different. Number of neutrons in C-12 = 12 – 6 = 6 neutrons Number of neutrons in C-14 = 12 – 6 = 8 neutrons Oxygen has three isotopes 16 8O 17 8O 18 8O Atomic number = 8 Atomic masses = 16, 17, 18 Having 8, 9, 10 neutrons respectively Why some elements are having fractional atomic masses? The mass of an atom of any naturally occurring element is taken as the average mass of all the naturally occurring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it. But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then average mass is calculated. That is why the average atomic mass of chlorine atom having two isotopes 35Cl 17 , 37Cl 17 is 35.5 u. This does not mean that one atom of chlorine has fractional mass 35.5 u. It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is 35.5u. Uses of Isotopes 1. Radioactive isotopes like Arsenic 74, Sodium 24 are used as ‘tracers’ in medicine to defeat the presence of tumors and blood clots in human body. 2. Radioactive isotopes like Cobalt -60 are also used for the treatment of Cancer. 3. Radioactive isotopes like U-233. U-238 can undergo fission process for the production of energy. 4. Radioactive isotopes are used in industry to detect the leakage in underground pipeline, gas pipes and water pipes. 5. Radioactive isotopes like C-14 are used to do carbondating to measure the age of dead plants and animals (fossils). Isobars Atoms of different elements with different atomic numbers but same atomic mass are called isobars. Example Let us consider two elements calcium having atomic number 20 and argon having atomic number 18 but both have the same atomic mass as 40. It means that the total number of nucleons is same in the atoms of this pair of elements and so both are called as isobars.