Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
ADMISSION OPEN FROM 9TH TO F.SC. SLPS/C BAKHTA THANA Contacts. Principal. Haq Nawaz M.phil, MSc with distinction (UK), Applied chemistry with distinction (UK) 03419252988 email. [email protected] Farooq shah. 03339479169 All subjects from 9th to FSc notes and lectures’ videos are prepared under supervision of experienced lecturers. DALTON ATOMIC THEORY. In 1808, a British school teacher John Dalton gave his atomic theory called Dalton atomic theory. The main points of Dalton atomic theory are given below: 1. Matter is composed of extremely small indivisible particles called atoms 2. Atoms can neither be created nor destroyed 3. Atoms of the same elements are identical in all respects, i.e size, shape, and mass, but differ from those of other elements. 4. Atoms combine in a simple whole number ratio to form a compound. RUTHERFORD GOLD FOIL EXPERIMENT FOR THE STRUCTURE OF ATOM Introduction. After discovery of electrons and protons, the question was raised about the arrangement of these particles inside the atom. In order to answer this question, Rutherford performed experiment in 1911 to find out location of fundamental particles inside atom. Superior Lalazar Public School and college Thana [email protected] Page 1 Apparatus and materials. Alpha particles source like radioactive elements Gold foil Zinc sulphide (ZnS) Screen Movable microscope Lead shield Experiment. Rutherford obtained α particles from radioactive elements and bombarded it on gold foil. In order to observe the extent or scattering of alpha particles, he placed a zinc sulphide screen behind the gold foil. Observation: He observed that: Most of the alpha particles passed undeflected through the gold foil Some of the alpha particles were deflected at larger angles from their original path A few particles deflected backward on their original path or completely bounced back Conclusions or Rutherford atomic model. Based on his experiment, Rutherford presented the following model for the structure of atom. An atom is a neutral particle Major portion of the atom is empty because most of the alpha particles passed undeflected The positive charge particles (proton) are present in the centre of atom called nucleus. Nucleus has positive charge because alpha particles are positive so only repel from positive things. Superior Lalazar Public School and college Thana [email protected] Page 2 The size of the nucleus is very small as compared to the size of atom because most of the alpha particles passed undeflected. The negatively charged electrons revolve around the nucleus such as the planets revolve around the sun in the solar system. Drawbacks or shortcoming or defects in Rutherford atomic model. The following defects were observed in Rutherford atomic model: In Rutherford atomic model, electron is a charged particle revolves around the nucleus in a circular path but according to classical electromagnetic theory the charged particles lose energy continuously. Therefore electrons should lose energy continuously and ultimately fall in the nucleus but it does not happen. If the revolving continuously, electrons then the radiate atomic or emit spectrum energy must be continuous instead of line spectrum. Neil Bohr’s atomic theory. In order to remove the defects of Rutherford atomic model that why electrons do not fall in the nucleus, Neil Bohr’s in 1913 gave his atomic theory. The main postulates of Neil Bohr atomic theory are given below: 1. Electrons revolve around the nucleus in a fixed circular path called orbit or energy level. Each orbit has a fixed size, shape and energy. 2. Electron does not absorb or emit energy when revolves around the nucleus in a fixed orbit. This postulate removed the defect of Rutherford atomic model that why electron not fall in the nucleus 3. Electron only absorbs or emits energy when jumps from one orbit to another and the energy absorbed or released is not continuous but discontinuous in the form of quantum Superior Lalazar Public School and college Thana [email protected] Page 3 (photon). This postulate removed the second defect that why atom gives discontinuous or line spectrum 4. Electron releases energy when jumps from higher orbit to lower orbit but absorbs energy when jumps from lower orbit to higher orbit. 5. The energy absorbed or released is always equal to the difference of the energy of the two orbits. ∆E = E2-E1 = hⱱ Where ‘h’ is planks constant and ‘ⱱ’ is frequency of radiation. 6. Electron only revolves around the nucleus on those orbits for which the angular momentum (mvr) is equal to ‘nh/2π’. mvr = nh/2π where ‘m’ is mass of electron, ‘v, is velocity, ‘r’ is radius of orbit, ‘n’ is any natural number (1,2,3-----) and ‘h’ is a plank constant (6.6262x 10-34J) Spectrum. A range of electromagnetic radiations obtained by passing light through a prism is called a spectrum Continuous spectrum: Those spectrums in which there is no clear boundaries line between two adjacent colours are called continuous spectrum. Example. When white light is passed through a prism, it disperses into its component colours and a spectrum is obtained known as Continuous Spectrum. A continuous spectrum is one in which colours are diffused in one another without any line of demarcation. Line spectrum or discontinuous or atomic spectrum: Those spectrums in which there is clear boundaries line between two adjacent colours and colours are separated called continuous spectrum. Superior Lalazar Public School and college Thana [email protected] Page 4 Example. Electron radiates energy when jumps from higher orbit to lower orbit. This radiated energy gives a line on the spectrum and called line spectrum. Energy levels. A circular path on which electrons revolve around the nucleus is called energy level or shell. It shows how close the electrons are to the nucleus Example. The first energy level is K which is the nearest to the nucleus. Energy sub levels or orbital. The three dimensional region around the nucleus where the probability of finding electrons is maximum is called orbital. The sub shells are denoted by s, p, d, f which stand for sharp, principal, diffused and fundamental respectively. The s-orbital is spherical, p-orbital is dumbbell and d and f orbital are more complex. All main energy levels consist of sub energy levels. Examples. The first shell K (n=1) consists of s-orbital(sub shell), the second shell L (n=2) consists of s and p, the third shell consists of s, p and d. Electronic configuration. The arrangement of electrons in orbits or shells around the nucleus of an atom is called electronic configuration. Explanation: Electrons revolve around the nucleus in a circular path called shell or orbit. The distribution of electrons in shell or orbit is called electronic configuration. The maximum number of electrons in shell or orbit are according to 2n2 formula where ‘n’ is number of shell. Electronic configuration is done with the help of energy level diagram (Figure-1) where lower energy orbitals are filled first. Examples. When ‘n’ is equal to 1 it is called K-shell and maximum number of electrons K-shell can accommodate are 2 i.e 2n2 = 2(1)2 =2 Superior Lalazar Public School and college Thana [email protected] Page 5 Similarly when ‘n’ is equal to 2 it is called L-shell and maximum number of electrons L-shell can accommodate is 8 i.e 2n2 = 2(2)2 = 8 Fig. 1 n Energy level or shell Energy sublevels Number of electrons 1 2 3 4 s s, p s,p,d s,p,d,f 2 2, 6 2,6,10 2,6,10,14 K L M N Total number of electron 2 8 18 32 Isotopes Atoms of the same elements having same atomic number but different mass number are called isotopes of that element (OR) Atoms of the same element, which have same number of protons but different number of neutrons in nucleus are called isotopes. Characteristics: (I) same number of electrons. (2 same number of Protons. (3) Different number of neutrons. (4) Different mass number. (5) Same chemical properties but different physical properties. Superior Lalazar Public School and college Thana [email protected] Page 6 Representation of isotopes. Isotopes can be represented by writing atomic number below and mass number above on the left hand side of element symbol. Examples. 1. Isotopes of Hydrogen: Hydrogen has three isotopes and represented by From above figure, it is clear that all three forms (protium, deuterium and tritium) have same number of proton and electron but different number of neutrons. Ordinary hydrogen is called protium because contains only one proton but no neutron. II. Isotopes of carbon: Carbon has three isotopes and represented by From above figure, it is clear that all three forms (C-12, C-13 and C-14) have same number of protons and electrons but different number of neutrons. The C-12 contains six neutrons, the C-13 contains 7 neutrons and C-14 contains 8 neutrons. Superior Lalazar Public School and college Thana [email protected] Page 7 III. Isotopes of chlorine: chlorine is a member of halogen family and its atomic number is 17. Chlorine has two isotopes and represented by Isotopes Cl-35 Cl-37 No. of electron 17 17 No. of proton 17 17 No. of neutron 18 20 From above figures, it is clear that both forms (Cl-35 and Cl-37) have same number of protons and electrons but different number of neutrons. The Cl-35 contains 18 neutrons, the Cl-37 contains 20 neutrons. IV. Isotopes of Uranium: Uranium has atomic number 92 and has three isotopes which are given below From above figure, it is clear that all forms (U-234, U-235 and U238) have same number of protons and electrons but different number of neutrons. The U-234 contains 142 neutrons, the U-235 contains 143 neutrons and U-238 contains 146 neutrons. Uses of Isotopes: Isotopes arc mainly used in chemical, agricultural and medical research and for diagnosing and treatment of diseases. For instance: 1. Iodine- 131 is used in the treatment of thyroid cancer. Superior Lalazar Public School and college Thana [email protected] Page 8 2. Radium irradiation and cobalt-60 are used in the treatment of cancer and for diagnosis of tumours. 3. Deuterium, heavy carbon {C-13), heavy nitrogen (N-I 5) and heavy oxygen (O-18) are being, used as tracer elements in biochemical and physio-chemical research. 4. Sodium-24 is used for the identification of blood circulation problems in patients. Exercise 1.Choose the correct answer i. (b) proton ii. a) 2 iii. b) 18 iv. c) an electron v. c) radiate energy vi. a) alpha particles vii. d) double positively charged viii. a) reside in an orbit ix. c) different number of neutrons x. d) isotopes 2. Answer theses questions (i) How many sub energy levels are there in L-energy level? Give there names and the number of electrons they can accommodate. Ans. As L-energy level is the second shell of atom therefore n=2. There are two sub energy levels in L-shell. They are named as ‘s’ and ‘p’ which stand for sharp and principal. The s-orbital can only accommodate 2 electrons and p-orbital (px, py and pz) 6 electrons. Therefore, the total number of electrons L-energy level can accommodate will be 8. Diagrammatically: vii. Give electronic configuration of Ne(Z= 10) Ans.10 Ne : 1s2, 2s2, sp6 K=2 L=8 Superior Lalazar Public School and college Thana [email protected] Page 9 7. Give reasons i. Only a few high energy α-particles rebound after striking the metal foil in Rutherford’s experiment. Ans. As major portion of the atom is empty and nuclei only occupies small portion of atom due to which only a few high energy α-particles rebound. The size of positive centre (nucleus) is very small as compared to the whole atom so deflection of only a few alpha particles (double positive) is possible. ii. An atom is a neutral particle. Ans. As the number of electrons (negative) in atom is always equal to the number of protons (positive), therefore atom is a neutral particle because cancel the effects of each other. iii. Electrons continuously revolve around the nucleus but do not fall into the nucleus. Ans. Electrons continuously revolve around the nucleus but do not fall into the nucleus because electron does not absorb or emit energy when revolves around the nucleus in a fixed orbit. When electrons revolve in a fixed orbit the centrifugal force of an electron is balanced by the force of attraction between the nucleus and the electron iv. K shell can only accommodate 2 electrons while other shell can accommodate more than 2. Ans. The maximum number of electrons can be accommodated by the help of 2n2 formula. For K shell n= 1, therefore maximum number of electrons K-shell can accommodate are 2 i.e 2n2 = 2(1)2 = 2. However, for other shell the n value is greater than 1 due to which accommodate more than 2 electrons. For example when n=2 then it is called L-shell and maximum number of electrons Lshell can accommodate is 8 i.e 2n2 = 2(2)2 = 8 v. The mass of an atom is present in its nucleus. Superior Lalazar Public School and college Thana [email protected] Page 10 Ans. The mass of an atom is present in its nucleus because both proton and neutron are heavy particles of atom which are present inside the nucleus. This was the reason that alpha particles bounced back when collided with nucleus because mass of an atom is present in its nucleus. vi. Physical properties of the isotopes are different but their chemical properties are the same. Ans. Chemical properties depend upon number of electrons and its configuration around the nucleus. Therefore, chemical properties of isotopes are same because isotopes have same number of electron and same configuration around the nucleus. However, physical properties such as mass, density etc will be different from each other because isotopes have different number of neutrons. For example, all isotopes of carbon when react with oxygen it form carbon dioxide (same chemical properties) but masses are different such as C-12, C-13 and C-14. vii. Rutherford concluded that most of the space in atom is empty Ans. Most of the space in atom is empty because most of the alpha particles passed undeflected from gold foil. Superior Lalazar Public School and college Thana [email protected] Page 11