G.C.E (A/L) Chemistry Resource Book 2021 PDF

G. C. E. (Advanced Level) CHEMISTRY Grade 12 Resource Book Unit 1: Atomic Structure Unit 2: Structure and Bonding Unit 3: Chemical Calculations Unit 6: Chemistry of s, p and d Block Elements Department of Scie...

G. C. E. (Advanced Level) CHEMISTRY Grade 12 Resource Book Unit 1: Atomic Structure Unit 2: Structure and Bonding Unit 3: Chemical Calculations Unit 6: Chemistry of s, p and d Block Elements Department of Science Faculty of Science and Technology National Institute of Education Maharagama www.nie.lk i Chemistry Resource Book Grade 12 © National Institute of Education First Print 2021 Second Print ISBN: 978-955-654-913-3 Department of Science Faculty of Science and Technology National Institute of Education Sri Lanka Published By : Department of Printing and Publications National Institute of Education Maharagama Sri Lanka ii Message from the Director General The National Institute of Education takes opportune steps from time to time for the development of quality in education. Preparation of supplementary resource books for respective subjects is one such initiative. Supplementary resource books have been composed by a team of curriculum developers of the National Institute of Education, subject experts from the national universities and experienced teachers from the school system. Because these resource books have been written so that they are in line with the G. C. E. (A/L) new syllabus implemented in 2017, students can broaden their understanding of the subject matter by referring these books while teachers can refer them in order to plan more effective learning teaching activities. I wish to express my sincere gratitude to the staff members of the National Institute of Education and external subject experts who made their academic contribution to make this material available to you. Dr. (Mrs.) T. A. R. J. Gunasekara Director General National Institute of Education Maharagama. iii Message from the Director Since 2017, a rationalized curriculum, which is an updated version of the previous curriculum is in effect for the G.C.E (A/L) in the general education system of Sri Lanka. In this new curriculum cycle, revisions were made in the subject content, mode of delivery and curricular materials of the G.C.E. (A/L) Physics, Chemistry and Biology. Several alterations in the learning teaching sequence were also made. A new Teachers’ Guide was introduced in place of the previous Teacher’s Instruction Manual. In concurrence to that, certain changes in the learning teaching methodology, evaluation and assessment are expected. The newly introduced Teachers’ Guide provides learning outcomes, a guideline for teachers to mould the learning events, assessment and evaluation. When implementing the previous curricula, the use of internationally recognized standard textbooks published in English was imperative for the Advanced Level science subjects. Due to the contradictions of facts related to the subject matter between different textbooks and inclusion of the content beyond the limits of the local curriculum, the usage of those books was not convenient for both teachers and students. This book comes to you as an attempt to overcome that issue. As this book is available in Sinhala, Tamil, and English, the book offers students an opportunity to refer the relevant subject content in their mother tongue as well as in English within the limits of the local curriculum. It also provides both students and teachers a source of reliable information expected by the curriculum instead of various information gathered from the other sources. This book authored by subject experts from the universities and experienced subject teachers is presented to you followed by the approval of the Academic Affairs Board and the Council of the National Institute of Education. Thus, it can be recommended as a material of high standard. Dr. A. D. A. De Silva Director Department of Science iv  Guidance Dr. (Mrs.) T. A. R. J. Gunasekara Director General National Institute of Education Supervision Dr. A. D. A. De Silva Director, Department of Science National Institute of Education Mr. R. S. J. P. Uduporuwa Former Director, Department of Science National Institute of Education Subject Leader Mrs. M. S. Wickramasinghe Assistant Lecturer, Department of Science National Institute of Education Internal Editorial Panel Mr. L. K. Waduge Senior Lecturer, Department of Science Mrs. G. G. P. S. Perera Assistant Lecturer, Department of Science Mr. V. Rajudevan Assistant Lecturer, Department of Science Writing Panel Dr. Russel C. L. de Silva - Senior Lecturer, Department of Chemistry, University of Kelaniya (Unit 1) Dr. M.A.B. Prasantha - Senior Lecturer, Department of Chemistry, University of Sri Jayewardenepura (Unit 2) Dr. M.N. Kaumal - Senior Lecturer, Department of Chemistry, University of Colombo (Unit 3 and 6) External Editorial Panel Prof. S. P. Deraniyagala - Senior Professor, Department of Chemistry, University of Sri Jayewardenepura Prof. M. D. P. De Costa - Senior Professor, Department of Chemistry, University of Colombo Prof. H. M. D. N. Priyantha - Senior Professor, Department of Chemistry, University of Peradeniya Prof. Sudantha Liyanage - Dean, Faculty of Applied Sciences, University of Sri Jayewardenepura Mr. K. D. Bandula Kumara - Deputy Commissioner, Education Publication Department, Ministry of Education v  Mrs. Deepika Nethsinghe - SLTS-1 (Rtd), Ladies College, Colombo 07 Mrs. Muditha Athukorala - SLTS-1, Prajapathi Balika Vidyalaya, Horana Miss. C. A. N. Perera - SLTS-1, Princess of Wales’, Moratuwa Mrs. V. K. W. D. Salika Madavi - SLTS-1, Muslim Ladies College, Colombo 04 Mrs. H.M.D.D. D. Manike - SLTS-1, Viharamhadevi Balika Vidyalaya, Kiribathgoda Mr. S. Thillainathan - SLTS-1 (Rtd), Hindu Ladies College, Colombo 06 Miss. S. Veluppillai - SLTS-1 (Rtd), Hindu Ladies College, Colombo 06 Mrs. M. Thirunavukarasu - SLTS-1 (Rtd), Hindu Ladies College, Colombo 06 Mrs. S. Rajadurai - SLTS-1 (Rtd), St. Peters' College, Colombo 04 Language Editing Dr. Chandra Amarasekara Consultant, National Institute of Education Mr. M. A. P. Munasinghe Chief Project Officer (Rtd.), National Institute of Education Cover Page Mrs. R. R. K. Pathirana Technical Assitant, National Institute of Education Supporting Staff Mrs.Padma Weerawardana Mr. Mangala Welipitiya Mr. Ranjith Dayawansa vi  Content Message from the Director General ……………………………………………………………..…… iii Message from the Director ………………………………………………………………………........ iv Subject Committee………………………………………………………………………………..…... v 4.0 Atomic structure………………………………………………..………….....…01-42 1.1 The atomic theory of matter 02 1.1.1 Properties of cathode rays (Experimental observations) 1.1.2 The nucleus of the atom 1.1.3 Properties of positive rays (Experimental observations) 1.1.4 Rutherford’s gold foil experiment 1.1.5 Atomic number, isotopes and mass number 1.1.6 The atomic mass scale 1.1.7 Average atomic mass and relative atomic mass of an element 1.1.8 Ions 1.2 Electromagnetic radiation and wave-like properties of matter 13 1.2.1 Quantization of energy 1.3 Electronic energy levels of atoms 17 1.3.1 The hydrogen spectrum 1.3.2 Shapes of orbitals 1.3.3 Orbitals and quantum numbers 1.4 Electron configuration 23 1.4.1 The Aufbau principle 1.4.2 The Pauli exclusion principle 1.4.3 Hund's rule 1.4.4 Condensed electron configurations 1.5 Building of periodic table 28 1.6 Periodic trends shown by s and p block elements 32 1.6.1 Sizes of atoms and ions 1.6.2 Ionization energy 1.6.3 Electron gain energy 1.6.4 Electronegativity 2.0 Structure and bonding………………………………………………..………...43-86 2.1 Covalent bonds 44 2.1.1 Lewis dot diagrams and Lewis dot-dash structures 2.2 Dative covalent bonds 51 2.3 Valance Shell Electron Pair Repulsion theory (VSEPR theory) 52 2.3.1 Hybridization of atomic orbitals 2.3.2 Formation of double and triple bonds 2.3.3 Resonance structures 2.3.4 Effect of electronegativity and geometry on the polarity of molecules 2.3.5 Dipole moment 2.3.6 Factors affecting the magnitude of electronegativity 2.4 Ionic bonds/ ionic interactions 75 2.5 Metallic bonds 78 vii 2.6 Secondary interactions 79 3.0 Chemical calculations………………………………..……………..…….……87-120 3.1 Oxidation number 88 3.1.1 Basic rules that applied in the determination of the oxidation states of an atom in a molecule/ polyatomic ion or in a compound 3.1.2 Use of oxidation numbers to understand electron transfer between atoms in redox reactions 3.2 Nomenclature of inorganic compounds 93 3.2.1 Names of ionic compounds derived from monoatomic ions 3.2.2 Names of ionic compounds derived from elements that form more than one type of cation 3.2.3 Names of simple covalent compounds 3.2.4 Polyatomic ions 3.2.5 Inorganic acids 3.3 Atomic mass, mole and Avogadro constant 97 3.3.1 The connection between atomic mass unit, moles and Avogadro constant 3.3.2 Calculation of average atomic mass of elements 3.3.3 Mole 3.3.4 Molar mass 3.4 Types of chemical formulae 99 3.4.1 Chemical calculations using chemical formulae 3.4.2 Determination of the empirical and molecular formula of a compound 3.4.3 Determination of molecular formula using the empirical formula mass and molecular mass 3.5 Composition of a substance in a mixture 102 3.5.1 Composition given in fractions 3.5.2 Percentage composition in a solution (homogeneous mixture) 3.5.3 Molality 3.5.4 Molarity 3.6 Balancing chemical reactions 105 3.6.1 Balancing a chemical reaction by inspection method 3.6.2 Balancing a chemical reaction by the redox method 3.6.3 Balancing simple nuclear reactions 3.7 Preparation of solutions 113 3.8 Calculations based on chemical reactions 115 4.0 Chemistry of s, p and d block elements……………………………………...…….. 121-175 s Block Elements 4.1 Group 1 elements 122 4.1.1 Group trends 4.1.2 Reactions of Group 1 elements 4.1.3 Thermal stability of salts 4.1.4 Solubility of Group 1 salts 4.1.5 Flame test 4.2 Group 2 elements 126 4.2.1 Group trends 4.2.2 Reactions of Group 2 elements (alkaline earth metals) viii 4.2.3 Thermal stability of salts 4.2.4 Solubility of Group 2 salts 4.2.5 Flame test p Block Elements 4.3 Group 13 elements 131 4.3.1 Group trends 4.3.2 Aluminium 4.4 Group 14 elements 133 4.4.1 Group trends 4.4.2 Diamond and graphite 4.4.3 Carbon monoxide and carbon dioxide 4.4.4 Oxoacid of carbon 4.5 Group 15 elements 136 4.5.1 Group trends 4.5.2 Chemistry of nitrogen 4.5.3 Oxoacids of nitrogen 4.5.4 Ammonia and ammonium salts 4.6 Group 16 elements 141 4.6.1 Group trends 4.6.2 Hydrides of Group 16 4.6.3 Oxygen 4.6.4 Sulphur 4.6.5 Oxygen containing compounds 4.6.6 Hydrogen peroxide 4.6.7 Sulphur containing compounds 4.6.8 Oxoacids of sulphur 4.7 Group 17 elements 149 4.7.1 Group trends 4.7.2 Simple compounds of Group 17 4.7.3 Reactions of chlorine 4.8 Group 18 elements 153 4.8.1 Group trends 4.8.2 Simple compounds of group 18 elements 4.9 Periodic trends shown by s and p block elements 155 4.9.1 The valence electron configuration 4.9.2 Metallic character 4.9.3 Reactions of third period oxides with water, acids and bases 4.9.4 Acid, base and amphoteric nature of hydroxides and hydrides 4.9.5 Nature of the halides across the third period d Block Elements 4.10 Transition elements 159 4.10.1 Occurrence 4.10.2 Properties of fourth period d block elements 4.10.3 Oxides of d block elements 4.10.4 Chemistry of some selected d block oxides 4.10.5 Coordination compounds of transition metal ions ix 4.10.6 Nomenclature of simple complex ions and compounds 4.10.7 Factors affecting the colour of the complexes 4.10.8 Importance of d block elements 4.10.9 Identification tests for selected cations of d block elements x  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure 1. Atomic Structure Content 1.1 The atomic theory of matter 1.3.2 Shapes of orbitals 1.1.1 Properties of cathode rays (Experimental 1.3.3 Orbitals and quantum numbers observations)  The principal quantum number (n) 1.1.2 The nucleus of the atom  The angular momentum quantum 1.1.3 Properties of positive rays (Experimental number (l) observations)  The magnetic quantum number (ml) 1.1.4 Rutherford’s gold foil experiment  The spin quantum number (ms) 1.1.5 Atomic number, isotopes and mass number 1.1.6 The atomic mass scale 1.4 Electron configuration 1.1.7 Average atomic mass and relative atomic 1.4.1 The Aufbau principle mass of an element 1.4.2 The Pauli exclusion principle 1.1.8 Ions 1.4.3 Hund's rule 1.4.4 Condensed electron configurations 1.2 Electromagnetic radiation and wave- like properties of matter 1.5 Building of periodic table  Electromagnetic radiation · Properties  The long form of the periodic table [speed (c), wavelength (λ), frequency (ν), energy (E)] 1.6 Periodic trends shown by s and p block 1.2.1 Quantization of energy elements  Electromagnetic spectrum 1.6.1 Sizes of atoms and ions  c=νλ  van der Waals radius ℎ  E = h ν, λ =  Covalent radius 𝑚𝑚𝑚𝑚  Wave- particle dual nature of matter  Metallic radius  Periodic trends in atomic radii 1.3 Electronic energy levels of atoms  Electron configurations of ions  Variation of successive ionization  Periodic trends in ionic radii energies of elements 1.6.2 Ionization energy 1.3.1 The hydrogen spectrum  Periodic trends in first ionization energies  Existence of electrons in energy levels 1.6.3 Electron gain energy 1.6.4 Electronegativity 1  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure Introduction Chemistry is the study of the properties and behaviour of matter. Matter is the physical material of the universe; it is anything that has mass and occupies space. Although the materials in our world vary greatly in their properties, everything is formed from only about 100 elements and, therefore, from only about 100 chemically different kinds of atoms. (118 elements have been discovered so far but the heavier atoms are short lived and not found naturally.) 1.1 The atomic theory of matter Philosophers from the earliest times speculated about the nature of the fundamental components from which the world is made. Empedocles (~ 440 BC) believed that the four elements-earth, fire, air and water made up all things. The Hindus believed that the four elements stated above makeup the world and space. However, Democritus (460– 370 BC) and other early Greek philosophers described the material world as being made up of tiny, invisible, indivisible particles that they called ‘atomos’, meaning “indivisible” or “uncuttable.” Later, however, Plato and Aristotle formulated the notion that there can be no ultimately indivisible particles, and the “atomic” view of matter faded for many centuries during which Aristotelean philosophy dominated the Western culture. It was in 1808 that an English scientist and school teacher, John Dalton (1766-1844), formulated a precise definition of the indivisible building blocks of matter that we call atoms. Dalton’s atomic theory was based on four postulates. 1. Elements are made out of extremely small, indivisible particles called atoms. 2. All atoms of a given element are identical in mass and size, but the atoms of one element are different from the atoms of all other elements. 3. Atoms of one element cannot be changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions. 4. Compounds are formed by union of two or more atoms of different elements in a simple numerical ratio. Dalton’s atomic model is called the "Golf ball model". (a) (b) Figure1.1 (a) John Dalton and (b) the golf ball model 2  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure Johnstone G. Stoney (1826-1911) named the fundamental unit carrying electricity as “electrons” in 1891 but did not have any experimental evidence of its existence. During the mid-1800s, scientists began to study electrical discharge through a glass tube pumped almost empty of air. This device was an invention of the British chemist and physicist Sir William Crookes (1832-1919) and is called Crookes tube or cathode ray tube. Figure 1.2 A cathode ray tube The experiment of Crookes and the others showed that when two electrodes are connected to a high-voltage source, the heated negatively charged plate, called the cathode, produced a stream of invisible radiation. Although the rays could not be seen, their presence was detected because they cause gases at low pressure to glow and which made other substances to fluoresce, or to give off light. The radiation emitted from the cathode was given the name 'cathode rays'. Later it was known that these rays could be deflected by a magnetic field and they carried a negative electrical charge. Some scientists felt that these rays were waves and others were inclined to think they were particles. The British scientist J. J. Thomson (1856–1940) observed that cathode rays are the same regardless of the identity of the cathode material or the gas in the tube. In 1897 he described cathode rays as streams of negatively charged particles. He used a cathode tube with an anode that had a hole at the centre. Using experimental measurements obtained from that cathode tube he then calculated a value of 1.76 × 108 coulombs per gram (C g-1) for the ratio of the electron’s electrical charge to its mass. Anode (+) Cathode (-) High voltage Fluorescent screen Figure 1.3 Thomson’s cathode ray tube 3  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure 1.1.1 Properties of cathode rays (Experimental observations)  Cathode rays travel in straight lines. When an opaque object like a metal cross is placed in the path of cathode rays in a discharge tube, a shadow with sharp edges of the metal cross is formed at the end opposite to the cathode. The placement of the shadow proves that cathode rays emit from the cathode and they travel in a straight line. Cathode Cathode Paddle rays rays wheel Cathode Cathode Anode Anode Shadow of (metal object) the metal Figure 1.4 Cathode ray properties  Cathode rays are a beam of particles having mass and possess kinetic energy. On placing a light paddle wheel in the path of cathode rays in a discharge tube, the blades of the paddle wheel rotate. This was considered evidence that electrons (cathode rays) have momentum. (However, there is doubt on this conclusion as heating of the tube can also make the paddles move.)  When an electric field is applied in the path of cathode rays, they are deflected towards the positively charged plate. Hence the cathode rays are composed of negatively charged particles. They are affected by magnetic fields showing a deflection perpendicular to the magnetic field. The direction of deflection is similar to the deflection of any other negatively charged particles. Therefore, electron can be concluded as a negatively charged particle too. Cathode Anode Cathode rays Electric field Figure 1.5 Interaction of cathode rays with external electrical fields 4  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure  The nature of the cathode rays does not depend on the nature of the gas taken in the discharge tube or the material of the cathode.  The ratio of the charge to mass (e/m ratio) of cathode ray particles obtained from different gases was found to be exactly the same. Electron s Positively charged sphere Figure 1.6 J. J. Thomson and his model Using his findings in 1899 J.J. Thomson postulated the “plum-pudding” model of atomic structure. In 1909, Robert Millikan (1868–1953) succeeded in measuring the charge of an electron as 1.602 × 10-19 C by performing the oil drop experiment. The mass of the electron could be calculated by using the experimental values for the charge of electron and Thomson’s charge-to-mass ratio. 1.602 ×10−19 C Electron mass = 1.76 ×108 C g−1 = 9.10 × 10−28 g Figure 1.7 Robert Millikan and mass of an electron This mass is about 1/1837 of a hydrogen atom which is the lightest atom. The relative charge of an electron is -1. 1.1.2 The nucleus of the atom The German physicist, Eugen Goldstein experimentally proved the existence of positive charges in matter. In his experiments, a perforated cathode was used in a discharge tube along with air at very low pressure. When a high voltage of about 10,000 volts was applied across the electrodes, a faint red glow was observed behind the perforated cathode. When the high voltage is applied, its electric field accelerates the small number of ions present 5  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure in the gas. These collide with atoms of the gas, knocking electrons off of them and creating more positive ions. These ions and electrons in turn strike more atoms, creating more positive ions. The positive ions are all attracted to the cathode, and some pass through the holes in the cathode. Goldstein called these positive rays "canal rays", because they were produced by the holes or channels in the cathode. Although the rays are not exactly formed at the positive electrode or anode, since they are formed away from the cathode close to the anode, they were also known as anode rays or positive rays. Figure 1.8 A cathode ray tube with a perforated cathode 1.1.3 Properties of positive rays (Experimental observations)  They travel in straight lines and cast a shadow of the objects placed in their way.  They can move a paddle wheel placed in their path.  These rays are positively charged and when an electric field is applied in the path of the rays, they are deflected towards the negative plate of an electric field.  The nature of the positive rays depends upon the gas taken in the discharge tube. Different gases give different types of positive rays, which contain particles having different masses and different charges. Therefore, the e/m ratio is not constant for positive ray particles obtained from different gases. In 1907 a study of how this "ray" was deflected in a magnetic field, revealed that the particle making up the ray were not all the same mass. The lightest ones, formed when there was some hydrogen gas in the tube, were calculated to be about 1840 times heavier than an electron. The mass of any other positive particle is a multiplication of the mass of the lightest positive particle. Therefore, it should be a subatomic particle. They were named as protons. The relative mass of a proton is 1, hence, the mass of a proton is 1.6 × 10-24 g or 1.007276 u (atomic mass units) or Da (Daltons). (The unit was earlier given the name amu.) The proton has a charge equal and opposite to that of an electron. Hence the absolute charge of a proton is 1.6 × 10-19 coulomb and its charge is positive. Proton is the smallest positive charge carrying particle in an atom and the relative charge of proton is +1. 6  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure Following the discovery of radioactivity in 1896 by the French scientist Henri Becquerel (1852–1908), the British physicist Lord Ernest Rutherford (1871-1937) showed that radioactive materials produce three types of emissions alpha (), beta () and gamma (). The paths of  and  radiation are bent by an electric field. Alpha () rays consist of positively charged particles, called particles, and therefore are deflected away from the positively charged plate. Beta () rays or   particles have the identity of electrons and are deflected away from the negatively charged plate. The third type of radioactive radiation consists of high–energy rays called gamma () rays. Like X rays,  rays have no charge and are not affected by an external electric or magnetic field. Lead block  rays  rays  rays Radioactive Electrically substance charged plates Slit Figure 1.9 Behaviour of alpha (), beta () and gamma () rays in an electric field (a) (b) Figure 1.10 (a) Henri Becquerel and (b) Lord Ernest Rutherford 1.1.4 Rutherford’s gold foil experiment In 1908-09, Rutherford together with his associate Johannes Hans Wilhelm Geiger (1882-1945) a German physicist and an undergraduate named Ernest Marsden, carried out a series of experiments using very thin foils of gold and other metals as targets for particles from a radioactive source. 7  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure Source of alpha particles Deflected Thin gold foil Scattered  particles particles Fluorescent screen Beam of particles Circular fluorescent screen Most particles are undeflected Thin gold foil Figure 1.11 Rutherford’s gold foil experiment They observed that the majority of particles penetrated the foil either undeflected or only with a slight deflection. They also noticed that a few particles were scattered (or deflected) at a large angle. Very few  particles bounced back in the direction from which it came. To explain the results of the experiment, Rutherford devised a new model of atomic structure, suggesting that most of the atom must be empty. This structure would allow most of the  particles to pass through the gold foil with little or no deflection. The atom’s positive charges, Rutherford proposed, are all concentrated in the nucleus, a dense central core within the atom. Whenever an  particle came close to a nucleus in the scattering experiment, it experienced a large repulsive force and therefore a large deflection. Moreover, an  particle traveling directly toward a nucleus would experience an enormous repulsion that could completely reverse the direction of the moving particle. Nucleus Electron Figure 1.12 Rutherford’s model (1911) 8  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure Subsequent studies, mainly based on mass spectroscopy revealed that the masses of atoms were much greater than the masses of protons and electrons present. Therefore, another subatomic particle should be present to contribute towards the mass of the atom. In1932 Sir James Chadwick (1891-1972) a British scientist discovered the ‘neutron’. The charge of a neutron is 0 (zero) and its mass is 1.6749 ×10-24g or 1.008665 amu. (a) (b) Figure 1.13 (a) James Chadwick and (b) Niels Bohr Since Rutherford’s time, physicists have learned more and more about atomic nuclei. In 1913 Niels Henrik David Bohr (1885-1962) a Danish physicist, combined the ideas at that time and suggested that the atomic nucleus was surrounded by electrons moving in orbit, like planets around the sun. He postulated that the electrons in order to remain in orbit, the electrostatic attraction between the nucleus and electron must be equal to the centrifugal force. In other words, the electrons have to travel in a constant speed around the nucleus keeping the distance from the nucleus constant. The model he introduced is known as the Rutherford–Bohr model or the Bohr model. Particles found in the nucleus are called nucleons, including the protons and neutrons in to the atom. A nuclide is the nucleus of an atom that has specific numbers of protons and neutrons (all nucleons). Therefore, nuclides are composite particles of nucleons. Nucleus Electrons Figure 1.14 The Bohr model 9  G. C. E. (A/L) CHEMISTRY: Unit 1 Atomic Structure 1.1.5 Atomic number, isotopes and mass number Henry Gwynn Jeffrey Moseley (1887-1915), an English physicist and a co-worker of Rutherford, found that the number of positive charges on the nucleus increases in atoms by single electron units. The atoms of each element have a characteristic number of protons. The number of protons in an atom of any particular element is called that element’s atomic number. Atomic Number of protons Number of electrons number (Z) = in the nucleus = in an atom Since an atom has no net electrical charge, the number of electrons it contains is equal to the number of protons found in its nucleus. All atoms of carbon, for example, have six protons and six electrons, whereas all atoms of oxygen have eight protons and eight electrons. Thus, carbon has atomic number 6 and oxygen has atomic number 8. British scientists J. J. Thomson and Francis William Aston (1877-1945) perfected the ‘mass spectrometer' which they used in 1912-13 to discover the first isotopes (of neon). Atoms of a given element can differ in the number of neutrons they contain and therefore their mass can also vary. The number of protons plus neutrons (nuclide) in an atom is called its mass number. Mass number (A) = Number of protons (Z) + Number of neutrons In the atomic symbol used to represent a particular atom the mass number is given at the top left of the element symbol and the atomic number may be given at the bottom left. However, since the symbol also gives the same information, the atomic number usually may not be shown in the symbol. Figure 1.15 Atomic symbol of carbon 10