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Periodicity of Elements and Periodic Table CHAPTER – 4 Definitions Periodic Table A table of elements obtained by arranging them in order of their increasing atomic number in which elements having similar properties are placed in the same group is called Periodic Table. Group The vertical column of elements in the periodic table are called Groups. Period The horizontal rows of elements in the periodic table are called Periods. Periodicity The repetition of physical and chemical properties of elements periodically is called Periodicity of Properties. Periodic Law Physical and chemical properties of elements are periodic function of their atomic masses. Metal Elements which are good conductors of heat and electricity are malleable and ductile and have a metallic luster are called Metals like Sodium, Potassium, Gold, Copper etc. Non-Metals Elements which are non or bad conductor of heat and electricity are neither malleable or ductile and have no metallic luster are called Non-Metals like Carbon, Nitrogen, Chlorine etc. Metalloids Metalloids are semi metals have the properties which are intermediate between a metal and nonmetal like Boron, Silicon, Germanium, Arsenic, Antimony etc. Law of Triads A German Chemist, Dobereiner (1829), arranged chemically similar elements in groups of three on the basis of their atomic masses called Triads and it was found that atomic mass of the middle element was approximately equal to the average of atomic masses of other two elements. This is known as Law of Triads. Drawback or Defect As very few elements could be arranged in such groups, this classification did not get wide acceptance. Law of Octaves An English Chemist Newland (1864) stated that if the elements were arranged in the ascending order of their atomic masses, every eight element will have similar properties to the first. This is knows as Law of Octaves. Drawback or Defects 1. Noble gases were not discovered at that time and no place was reserved for the undiscovered noble gases. 2. In the same way no blank spaces for the undiscovered elements were present in his table. Mendeleyv’s Period Table and Periodic Law Russian Chemist, Mendeleyv’s (186) who wa working separately from Lother Mayer published a table of elements. According to Mendeleyv’s when the element were arranged in order of their increasing atomic mases, the elements with similar properties were repeated after regular interval and were placed one above the other.A table obtained in this manner is called Periodic Table. Mendeleyv’s stated this periodicity in the form of Periodic Law. Important Features of Mendeleyv’s Periodic Table The important features of Mendeleyv’s Periodic table are: Periods and Groups The horizontal rows which run from left to right in Periodic Table are called Periods and they are twelve in number. The vertical rows which run from top to bottom in periodic table are called groups and they are eight in number. Vacant Spaces Mendeleyv’s left many vacant spaces for the still unknown elements. For example, next to Calcium (40) should be Titanium (48) but it resembled silicon (28) instead of Aluminium (27). He left vacant space for element with atomic mass 44. Discovery of New Element Mendeleyv’s discovered new elements and also guessed their atomic mass and properties. Atomic Mass Correction Mendeleyv’s corrected the atomic masses of certain elements on basis of their properties and provided proper place to them in the periodic table. Defects in Mendeleyv’s Periodic Table The Mendeleyv’s Period Table has following defects: Irregular Position of Some Elements According to Mendeleyv’s Periodic Law Potassium (39) should be placed before Argon (40) but he placed Argon (40) before Potassium (39) which goes against his law. Position of Isotopes Mendeleyv’s periodic table gives no indication about the position of isotopes. Structure of Atom Mendeleyv’s Periodic table gives no idea about structure of atoms. Position of Lanthanides and Actinides Lanthanides and Actinides have not been given proper place in Periodic Table. Coinage and Alkali Metals Alkali metals and coinage metals with different properties are placed in the same group. This defect has been replaced by placing them into two sub groups. Modern Periodic Law and Modern Periodic Table Modern Periodic Law Physical and chemical properties of the elements are periodic function of their atomic number. Mosely (1913) says that atomic mas is not fundamental property. Due to some defects present in Mendeleyv’s periodic law, Mosely introduced the concept of anomic number for the elements. Example When isotopes were discovered, it was thought advisable to arrange the elements on basis of their atomic number instead o increasing atomic mases. Isotopes were needed different position in the Mendeleyv’s periodic table. Hence Mendeleyv’s periodic law was modified. Modern Periodic Table When Mendeleyv’s periodic law was modified and new elements were discovered. This forcd the scientists to change Mendeleyv’s periodic law. The electronic configuration of atoms also played an important role in he arrangement of the modern periodic law. This form of periodic table is called “Long form of Periodic Table” because it contains eighteen groups instead of eight but seven periods instead of twelve. Group I – The Alkali Metals The elements of group I are called “Alkali Metals”. The word alkali is derived from an Arabic word meaning Ashes. Elements of Group I Lithium Sodium Potassium Rubidium Cesium Francium Properties of Group I 1. They are mono atomic. 2. They exist in solid metallic state. 3. Outer most shell of these elements is incomplete having one electron. 4. Elements of this group are highly reactive. 5. Elements of this group have large tendency to form compounds. 6. Elements of this group are strongly electro-positive. Group II – The Alkaline Earth Metals The elements of group II are called Alkaline Earth Metals. These elements occur in nature as silicate mineral and their oxides and hydroxides are strongly basic. Therefore these elements are called Alkaline Earth Metals. Elements of Group II Beryllium Magnesium Calcium Strontium Barium Radium Properties of Group II 1. They are mono atomic. 2. They exist in solid state. 3. Outer most shell of these elements is incomplete having two electrons. 4. Elements of this group are moderately reactive. 5. Elements of this group have moderate tendency to form compounds. Group III – The Boron or Aluminium Family The elements of group III exist in solid state. Elements of Group III Boron Metalloid Aluminium Metal Gallium Metal Indium Metal Thallium Metal Properties of Group III 1. They are mono atomic. 2. They exist in solid state. 3. Outer most shell of these elements is incomplete having three electrons. 4. Elements of this group are quite reactive. 5. Elements of this group have moderate tendency to form compounds. Group IV – The Carbon and Silicon Family Elements of Group IV Carbon Silicon Germanium Tin Lead Properties of Group IV 1. They are mono atomic. 2. They exist in solid state. 3. Outermost shell of these elements is incomplete. 4. Elements of this group are quite reactive. 5. Elements of this group have moderate tendency to form compounds. Group V – The Nitrogen Family Elements of Group V Nitrogen Phosphorus Arsenic Antimony Bismuth Properties of Group V 1. Some are mono atomic and some are di-atomic. 2. Some of them exist in gaseous and some are in solid state. 3. Outermost shell of these elements is incomplete having five electrons. 4. elements of this group are quite reactive. 5. Elements of this group have quite tendency to form compound. Group VI – The Oxygen Family Elements of Group VI Oxygen Sulphur Selenium Tellurium Polonium Properties of Group VI 1. Some are mono atomic and some are di-atomic. 2. Some of them exist in gaseous and some are in solid state. 3. Elements of this group have quite tendency to form compounds. 4. The tendency of forming covalent bond decreases from oxygen to polonium. 5. There is a gradual decrease in the ionization potential down the group. Group VII – The Halogen Family Elements of Group VII Fluorine Gas Chlorine Gas Bromine Liquid Iodine Solid Astatine Radioactive Properties of Group VII 1. They are diatomic except At. 2. Halogens are very active non-metals. 3. Outer most shell of these elements is incomplete having seven electrons. 4. Elements of this group are highly reactive. 5. There is a gradual decrease in the ionization potential down the group. Transition Elements Definition Elements in Group IB, IIB, through VIIB are known as Transition Elements because they show their properties which are transitional between higly reactive and strong electro-positive elements of S-block which form ionic compounds and p-block elements which form largely covalent compounds. Properties of Transition Elements 1. Transition Elements have incomplete inner electron shells. 2. They show variable valency. 3. They show similar behaviour. 4. They all are metals. 5. They have strong inner atomic bonds. Group 0, The Noble Gases The elements of Group VIII A are called “Noble Gases” or “Inert Gases” or “Zero Group Elements”. Elements of Group 0 Helium Neon Argon Krypton Xenon Radon Properties of Group 0 1. They are mono atomic. 2. They exist in gaseous state. 3. Outer most shell of these elements is either complete or contains eight electrons. 4. These elements are mostly chemically non-reactive. 5. These elements have no tendency to form compounds (only a few of these compounds are known). Atomic Radius Definition One half of the distance between the nucleus of two identical atoms when these are in close contact with each other is called Atomic Radius. Unit It is measured in angstrom unit A. Trend in Period The atomic radii decreases from left to right within a period in the periodic table. This is because nuclear charge increases with the increase of atomic number. But the number of shells remains same within a period. Trend in Group Atomic radius increases from top to bottom in a group. This is because, although nuclear charge increases from top to bottom but at the same time on new shell is also added for each successive element down the group. Ionization Energy (I.E) or Ionization Potential (I.P) Definition The minimum energy needed to remove an electron from an isolated, gaseous atom in its ground state is called Ionization Energy. Unit It is expressed in electron volts or kilo-joules permole. 1 ev = 96.49kj Factors Affecting Ionization Energy The ionization energy of elements depends upon the following factors: 1. Effect of Nuclear Charge on I.E The greater the nuclear charge the higher is the ionization energy. 2. Effect of Atomic Size The larger the size of atom the lower is the ionization energy. Trend of I.E in Period Ionization energy increases from left to right in a period due to increase in nuclear change and decrease in atomic size. Trend of I.E in Group I.E decreases from top to bottom in a group due to increase in atomic size. Electronegativity Definition The tendency of each atom in a covalent molecule to attract a shared pair of electrons towards itself is known as its electronegativity. Factors Affecting Electronegativity Electronegativity depends upon the following factors: Atomic size Atomic Number Electron Affinity Ionization Energy Trend or Variation in the Period Electronegativity increases from left to right within a period due to increase in nuclear charge and decrease in atomic size. Trend or Variation in the Group Electronegativity values decreases from top to bottom within a group due to increase in atomic size. Electron Affinity Definition The energy change that occurs when an electron is gained by an atom in the gaseous state is known as Electron Affinity. Electron Affinity for the addition of first electron is negative i.e. energy is released but for further addition of electrons it is positive because energy has to be added to over come repulsion between negative ion and electron. Unit It is measured in KJ/mol or in e.v per atom. Factors Affecting Electron Affinity Atomic Size Nuclear Charge Trend or Variation of Electron Affinity in Group Down the group in the periodic table, electron affinity decreases because the addition of a new shell to each atom decreases its force of attraction. Trend or Variation of Electron Affinity in Period In a period, the electron affinity increases from left to right because the incoming successive atoms have higher nuclear charge and attract electron more towards itself.