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ce tu ts .c om CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES w. s ci en CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES Chapter outline WHY DO WE NEED TO CLASSIFY ELEMENTS • GENESIS OF PERIODIC CLASSIFICATION • MODERN PERIODIC LAW AND THE PRESENT FORM OF THE PERIODIC • NOMENCLATURE OF ELEMENTS WITH ATOMIC NUMBERS > 100 • ELECTRONIC CONFIGURTIONS OF ELEMENTS ANDTHE PERIODIC TABLE • ELECTRONIC CONFIGURATIONSAND TYPES OF ELEMENTS: s-, p-, d-, f- BLOCKS • PERIODIC TRENDS IN PROPERTIES OF ELEMENTS w w • www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES GENESIS OF PERIODIC CLASSIFICATION .c om Periodic Classification of Elements Electrons are filled in various shells and subshells in a fairly regular fashion. Therefore, properties of elements are repeated periodically. Such trends in their physical and chemical properties were noticed by chemists in the nineteenth century and attempts were made to classify elements on their basis long before structure of atom was known. In this lesson we shall study about the earlier attempts for classification, the first successful ts classification which included all the known elements at that time namely Mendeleev’s periodic table, tu and about the long form of modern periodic table which is an improvement over Mendeleev’s work. ce OBJECTIVES After completing this lesson, you will be able to state the importance of periodic classification en and periodic table. State main features of Mendeleev’s periodic table state modern periodic law. • Describe the features of the long form of periodictable. • State s, p, d and f-blocks and their properties. • Define various periodic properties. • Discuss the trends in various periodic properties in the periodic table. • understand lanthanide contraction and actinide contraction. w. s ci • w Concept of group elements based on their properties There are about 111 known elements and 7 elements yet to be confirmed. Among these 118 w elements, 92 elements are available in the elemental form. Remaining 26 elements are manmade elements. Classification of these elements is very important to understand common properties and periodic trends among these 118 elements. Scientists like Johnn Dobereiner (1829), John Alexander Newlands (1865) and Lothar Mayer (1869) gave different kinds of classifications. But they were not become popular. 2 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES DOBEREINER’S TRIADS In 1829, Dobereiner, a German scientist made some groups of three elements each and called them triads. All three elements of a triad were similar in their properties. He observed that the atomic mass of the middle element of a triad was nearly equal to the arithmetic mean of atomic masses of other two elements. Also, same was the case with their other properties. Let us take the example of .c om three elements lithium, sodium and potassium. They form a Dobereiner’s triad. 7 Sodium, Na 23 Potassium, K 39 tu Lithium, Li ts Atomic mass Element Dobereiner Mean of the atomic masses of the first (Li) and the third (K) elements: 7 + 39 by 2 = 23. ce The atomic mass of the middle element, sodium, Na is equal to 23. Dobereneir’s idea of classification of elements into triads did not receive wide acceptance as he could arrange only a few elements in this manner. The next breakthrough in classification of elements came en in the form of Mendeleev’s work. ci MODERN PERIODIC LAW AND THE PRESENT FORM OF THE PERIODIC TABLE w. s Based on his own chemical properties and experiments, he listed the elements in the increasing order of atomic masses and showed the periodic trends. MENDELEEV’S PERIODIC LAW “The properties of the elements and their compounds are a specific function of their atomic weights”. w Mendeleev assigned each element a number called its atomic number. This number is useful to determine the position of the element in a series w of increasing atomic masses. The elements are arranged in horizontal rows called “periods” and vertical columns called “groups”. The first three periods of the Mendeleev’s table are called as short periods. And the other periods are known as long periods. The groups are subdivided into subgroup A and subgroup B. we have learnt about the Mendeleev’s periodic classification. Although it was a very successful classification but it also had some defects in it. 3 Mendeleev www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES MODERN PERIODIC TABLE Henry Moseley, an English physicist discovered in the year 1913 that atomic number, is the most fundamental property of an element and not its atomic mass. .c om Atomic number,(Z), of an element is the number of protons in the nucleus of its atom. The number of electrons in the neutral atom is also equal to its atomic number. This discovery changed the whole perspective about elements and their properties to such an extent that a need was felt to change the periodic law also. Now we shall learn about the changes made in ts the periodic law. Henry Moseley Modern periodic law tu “The physical and chemical properties of the elements are periodic functions of their atomic numbers”. ce The periodic table based on the modern periodic law is called the Modern Periodic Table. Many versions of this periodic table are in use but the one which is most commonly used is the Long Form of Modern Periodic Table. w w w. s ci en Look at the long form of the modern periodic table. Periodic table The properties of elements are due to the distribution of electrons in orbitals of the atom. The long form of periodic table is based on the Bohr’s model of atom. It is the graphical representation of aufbau principle. It successfully explains the periodicities of properties of elements. Now let us learn the main features of this periodic table. It contains 7 horizontal rows called ‘periods’ and 18 vertical columns called ‘groups’. Let us study the details of periods first. 4 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES .c om NOMENCLATURE OF ELEMENTS WITH ATOMIC NUMBERS > 100 ts Periodic Table showing different Group of Elements Introduction tu The long form of periodic table clearly indicates the electronic configurations of the elements, similarities in physical and chemical properties and differences in these properties. When the elements are arranged according to their atomic numbers or the electronic configurations, ce a gradual change in their properties can be seen. The gradation is very useful in deriving information w w w. s ci en about the elements and their compounds. Periodic Table PERIODICITY In the periodic table, the properties of elements change gradually with a change in their electronic configurations. This trend repeats itself at regular intervals. This repetition of a character is called “periodicity” and such properties are called periodic properties.As the atomic number increases the properties of elements repeated at regular intervals. 5 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES For example: From potassium (K 19) to Krypton (Kr 36) atomic number increases. Again from Rubidium (Rb 37) to ts .c om Xenon (Xe 54) same properties of above elements are repeated. tu Example of periodicity Reason: ce The periodicity is due to the similar outer electronic configuration. Potassium (19) general electronic configuration is ns1 . Krypton (36) electronic configuration is ns2 np6. Again from Rubidium (37) ns1 and en Xenon (54) ns2 np6 same configuration is repeated and hence properties also repeated. Periodic properties “Any property which is periodic in nature is called periodic property”. ci Uses of periodic properties: We can predict the nature of chemical bonds formed by the elements and compare their reactivity. w. s Molecular shapes and crystal structures can be identified easily. Important periodic Properties: Some of the important periodic properties are: w Atomic size and ionic size Electron affinity w Ionization potential Electro negativity Valency Metallic nature Reducing (or) Oxidizing nature Electro positivity Nature of Oxides Oxidation states 6 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES ATOMIC SIZE ts .c om Atomic & ionic radii calculated from inter nuclear distance. Internuclear Distance tu Inter-nuclear distance ce “The distance between two nuclei of neighboring atoms in a compound is called inter-nuclear distance”. Inter-nuclear distance are measured with X-ray diffraction methods (or) spectroscopic methods. Type of chemical bond Oxidation states Number of bonds formed w. s ATOMIC RADIUS ci Co-ordination number en Atomic & ionic radii are influenced by: “The distance between the centre of the nucleus and the electron cloud of outer most energy level”. The atomic radius depends on many factors like number w of bonds formed by the atom, nature of bonding , oxidation state of the atom. w Types of atomic radii: Depending on the type of bond, three types of atomic radii are there 1. Covalent radius 2. Metallic or Crystal radius Atomic radius 3. Vander Waal’s radius 4. Ionic radius 7 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES COVALENT RADIUS “ Half the equilibrium distance between the nuclei of two atoms held together by a covalent bond is covalent radius”. .c om Consider chlorine molecule. The inter-nuclear distance between two chlorine atoms is 1.98 A. (1.98 angstroms). 1.98 A is also called as bond length. Covalent radius of chlorine is 1.98 divided by 2 = 0.99 A. Therefore, covalent radius of chlorine is 0.99 A. Characteristics: Covalent Radius ts 1. It is applicable to homo-atomic molecules like Cl2, H2 ,Br2 etc. Bondlength Ao Covalent Radius C-C 1.54 AO 0.77 AO 1.34 AO 0.67 AO 1.20 AO 0.6 AO en C≡C ce No. of bonds C=C Examples: tu 2. It changes with type of bonds. o For Carbon - Carbon single bond - Covalent radius of carbon is 0.77 A o ci For Carbon - Carbon double bond - Covalent radius of carbon is 0.67 A o For Carbon - Carbon triple bond - Covalent radius of carbon is 0.6 A w. s 3. For metals, atomic radius is slightly more than covalent radius. w w Metal Atomic radius Ao Covalent Radius Ao K 2.31 AO 2.03 AO Ba 2.17 AO 1.98 AO Zr 1.57 AO 1.45 AO 4. For non-metals, atomic radius is equal to covalent radius. Non - Metal Atomic radius Ao Covalent Radius Ao C 0.77 AO 0.77 AO p 1.10 AO 1.10 AO S 1.04 AO 1.04 AO 8 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES metallic or crystal RADIUS “Half the inter-nuclear distance between two adjacent metal atoms in a metallic crystal”. Consider sodium crystal. The inter-nuclear distance .c om between two sodium atoms is 3.72A° (angstroms) and metallic radius of sodium is 3.72/2 = 1.86 A°. Therefore, Metallic radius of sodium atom is 1.86 A. Characteristics: 1. It is applicable for metal atoms. Metallic Radius ts 2. Atoms are treated as spheres. tu vander waal’s RADIUS “Half the equilibrium distance between the nuclei of two atoms bound by Vander Waal’s ce forces”. Consider Chlorine atoms of two different molecules. The chlorine atoms not form chemical bonds. When the atoms are close to each other, some weak force of attraction develops between them (atoms), that molecules is 3.6A°. en force is known as Vander Waal’s force. The distance between two adjacent chlorine atoms in different VanderWaal’s Radius w w w. s ci Vander Waals radius of chlorine = 3.6/2 = 1.8 A°. Characteristics: 1. It is observed only in solid molecular substances. 2. It is 40° larger than covalent radius. 9 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES ionic RADIUS “The distance from the nucleus of the ion up to the point where its influence of electron cloud ceases”. It is determined by x-ray diffraction of ionic crystals. .c om Characteristics: 1. For negative ions, size of atom increases with charge; so ionic radius also increases. For example: 1. The atomic radius of Chlorine atom = 0.99 A°. The ionic radius of Chlorine ion cl- = 1.81 A°. Increase in size = 0.82 A°. ts As the number of electrons gained increases, the ionic radius increases proportionally. tu 2. Atomic radius of oxygen = 0.66 A°. Ionic radius of O-2 = 1.40 A°. For positive ions size of atom decreases with charge; so ionic radius decreases. As the number of ce electrons removed from the atom increases, cationic radius decreases more i.e. the cation (cat-ion) becomes smaller. en See the different kinds of atomic radii and their important properties. PERIODS ci ELECTRONIC CONFIGURTIONS OF ELEMENTS ANDTHE PERIODIC TABLE There are seven rows in the periodic table. Each row is called a period. The periods have been w. s numbered from 1 to 7 (Arabic numerals).In each period a new shell starts filling up. The period number is w w also the number of shell which starts filling up in it. Periods 10 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES For example: In elements of 3rd period, the third shell (M shell) starts filling up as we move from left to right. The first element of this period sodium Na (2,8,1) has only one electron in its valence shell (third shell) while the last element of this period, argon Ar (2,8,8) has eight electrons in its valence shell. The gradual filing of the third shell can be from the above table and illustrations. Each period starts with an ce (a) tu ts .c om alkali metal and ends with a noble gas element. (b) Example of Periods (a) Sodium(Na) valence shell (b) Argon(Ar) valence shell It is the shortest period and discontinuous period. w. s ci en 1st period: It contains two elements - Hydrogen & Helium. 2nd period: It contains eight elements from Lithium(3) to Neon (10). It is first short period. w w 3rd period: It also contains eight elements from Sodium, Na(11)to Argon, Ar (18). It second short period. This period elements are called typical elements. 11 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES 4th period: It contains 18 elements from Potassium, K (19) to Krypton, Kr (36). It is first long period. It contains first transition series scandium (21) to zinc (30). .c om 5th period: It also contains 18 elements from Rubidium (37) to Xenon (54). It contains second transition series from Yttrium (Y) (31) to Cadmium (Cd) (48). It is second long period. ts tu 6th period: It contain 32 elements from Cesium, Cs (55) to Radon, Rn (86). It contains lanthanides from Cerium, Ce (58) to Lutetium (lew-tee-shee-əm) Lu (71). en ce It is the longest period. It contains 14 rare earth elements. 7th period: It is incomplete period & most of the elements are radioactive. The elements from Francium (fran-see-əm) Fr (87) to Hahnium, Ha (105) belongs to this group. It contains actinides Thorium, Th (90) to Lawrencium, Lw (103). w w w. s ci 12 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES GROUPS There are 18 vertical columns in the periodic table. Each column is called a group. The groups have been numbered from 1 to 18 (in Arabic numerals) from left to right. Group 1 on extreme left position contains alkali metals (Li, Na, K, Rb, Cs and Fr) .c om Group 18 on extreme right side position contains noble gases (He, Ne, Ar, Kr, Xe and Rn). (a) 1 GROUP Alkali metals ts (Li, Na, K, Rb, Cs, Fr) (b) tu 18 GROUP Noble gases ce (He, Ne, Ar, Kr, Xe and Rn) en Groups showing (a) Alkali metals (b) Noble gases ci All elements present in a group have similar electronic configurations and have same number of valence electrons. You can see in case of group 1 (alkali metals) and group 17 elements (halogens) that as one w. s moves down a group, more and more shells are added. Groups are further classified into a, b and zero group elements. A GROUP ELEMENTS w 1st 2nd & 13th to 17th columns are called ‘A’ group elements.This group contains ‘s and p block elements’. s and p blocks excluding zero group are called representative elements. w B GROUP ELEMENTS 3rd to 12th columns are called ‘B’ group elements. This group contains d-block elements. These are called transition elements. ZERO GROUP ELEMENTS There are noble gases. Chemically nonreactive. 13 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES A-Group Elements .c om B-Group Elements Zero Group Elements Lanthanides Actinides ts Periodic Table showing different Group of Elements INNER TRANSITION ELEMENTS tu LANTHANIDES ELEMENTS 14 elements with atomic numbers 58 to 71 (Cerium to Lutetium (lew-tee-shee-əm)) are called lanthanides ce and they are placed along with the element lanthanum (La), atomic number 57 in the same position (group 3 in period 6) because of very close resemblance between them. However, for convenience sake ci en they are shown separately below the main periodic table. w. s ACTINIDES ELEMENTS 14 elements with atomic numbers 90 to103 (Thorium to Lawrencium) are called actinides and they are placed along with the element actinium (Ac), atomic number 89 in the same position (group 3 in period 7) because of very close resemblance between them. They are shown also separately below the main w w periodic table along with lanthanides. ADVANTAGES OF MODERN PERIODIC CLASSIFICATION Elements are arranged in the increasing order of their atomic number, which is fundamental feature of all elements. 14 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES E.g. Argon (Ar40), Potassium (K39), and Copper (Cu 59), Nickel (Ni 58). Tellurium (Te128), Iodine (I127) anomaly behavior is rectified. The element of a group resembles closely with each other in properties. 8th group gets sufficient place in the table.Table clearly indicates metails, active metals, transitional metals, non-metals, metalloids, .c om lanthanides & actinides. LIMITATIONS The position of hydrogen still remains uncertain. • The inner transition elements are placed separately below the table. • The 14 lanthanides differ from the usual pattern of electronic configuration. ts • tu ELECTRONIC CONFIGURATIONS AND TYPES OF ELEMENTS: s, p, d, f BLOCKS Introduction ce Based on the electronic configurations and differentiating electron i.e. electron in the outermost orbit, the elements in 1. s block elements. 2. p block elements. ci 3. d block elements. en the periodic table are classified into 4 blocks.They are: 4. f block elements. Electron in the outermost orbit w. s See the s, p, d and f blocks in the long form of periodic w w table. s, p, d and f block eements 15 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES s-block elements are placed at the left side of the periodic table, p-block is at the right side and the d-block in between the s- and p-blocks. f-block is placed separately at the bottom of the main table. Elements with one electron in their outer shells have the configuration of ns1. They belong to .c om group 1 elements (hydrogen, alkali metals). Elements with two electrons in their outer shells have the configuration of ns2. They belong to group 2 elements (alkaline earth metals). Elements with three electrons (two in ‘s’ orbital and one in ‘p’ orbital) have the configuration of ns2 np2. They belong to group 3 elements. In a similar way the elements having four, five, six, seven and eight electrons in their outer shells have the general configuration ns2npx (where x = 2,3,4,5 or 6) and belong to 4th, 5th, 6th, 7th and ts Zero group respectively. S-Block elements tu S-Block elements ns1 GROUP IA (b) en (a) ce ns2 GROUP II A S-block elements (a) ns1 Group I A (b) ns2 Group II A ci s-block elements P-block elements “The differentiating electron in an element enters into s - sub shell”. w. s 1 A, 2 A group elements belong to this block. The general electronic configuration is ns1 and ns2 1a group elements Electrons Atomic No. 3 4 3+8 12 11 + 8 20 19 + 18 38 Potassium (K) 37 + 18 56 Rubidium (Rb) 55 + 32 88 Elements “One differentiating electron is present in s-sub w shell”. General electronic configuration ns1 w Elements are Lithium (Li) Sodium (Na) Cesium (Cs) 1 A Group elements Francium (Fr) 16 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES See the 1A group elements and their electronic configuration. The oxides, hydroxides of 1A Group are strong alkalies, so these are called “alkali metals”. 2 a group elements “Two differentiating electrons are present in the Elements Atomic No. 4 4 4+8 12 12 + 8 20 20 + 18 38 38 + 18 56 .c om s - sub shell”. Electrons General electronic configuration ns 2 Elements are Beryllium (Be) Magnesium (Mg) Calcium (Ca) ts Strontium (Sr) 56 + 32 Barium (Ba) Radium (Ra) 88 tu 2 A Group elements See the 2A group elements and their electronic configuration. ce The oxides and hydroxides are alkaline and found in the surface of the earth. So these are called “alkaline earth metals”. en properties of s-block elements These are highly electro positive and low electro-negative elements. They are highly reactive metals. Hence, they are not available freely in For example: ci nature but are available as their compounds. w. s Sodium is available as “Sodium chloride” (NaCl). The chemical reacting and metallic nature increases in a group with increase in atomic mass. These metals are having lowest ionization energy. w These are powerful reducing agents. They posses +1, +2 oxidation states. w They exhibit flame coloration tests. Example: S block elements Lithium - Crimson red Sodium - Golden yellow Potassium - Lilac color Cesium - Sky Blue. 17 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES p-block elements The differentiating electron enters into p - sub shell. As the p-sub level can accommodate a maximum of 6 .c om electrons, there are 6 groups of elements. 13th to 18th group elements belong to this block. P block elements tu 3A group elements (13th group boron family) ts The general electronic configuration ns2 np1-6 One differentiating electron is present in the p - sub III A Group shell. Boron Family ce General electronic configuration ns2 np1 The elements are Gallium (Ga) Indium (In) Thallium (Tl) ci Alluminium (Al) en Boron (B) Boron (B) Alluminium (Al) Gallium (Ga) Indium (In) Thallium (Tl) w. s See the 13th group elements and their electronic configuration. 4A group elements (14th group carbon family) 3 A Group elements IV A Group Carbon Family w Two differentiating electrons are present in the p - sub shell. General electronic configuration ns2 np2 The elements are Carbon (C) w Silicon (Si) Carbon (C) Germanium (Ge) Silicon (Si) Germanium (Ge) Tin (Sn) Tin (Sn) Lead (Pb) Lead (Pb) See the 14th group elements and their electronic configuration. 4 A Group elements 18 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES 5A group elements (15th group nitrogen family) Three differentiating electrons are present in the p - sub shell. V A Group Nitrogen Family Nitrogen (N) General electronic configuration ns2 np3. The elements are Nitrogen (N), Phosphorus (P), .c om Phosphorus (P) Arsenic (As) Arsenic (As), Antimony (Sb) Antimony (Sb) Bismuth (Bi) Bismuth (Bi) See the 15th group elements and their electronic configuration. VI A Group tu 6A group elements (16th group oxygen family) ts 5 A Group elements Oxygen Family General electronic configuration ns2 np4 Oxygen (O) ce Four differentiating electrons are present in the P - sub shell. The elements are Oxygen (O) Selenium (Se) Tellurium (Te) Polonium (Po) en Sulphur (S) ci See the 16th group elements and their electronic configuration. Sulphur (S) Selenium (Se) Tellurium (Te) Polonium (Po) 6 A Group elements w. s 7A group elements (17th group Halogen family) VII A Group Five differentiating electrons are present in the P - sub shell. General electronic Configuration ns2 np5 Halogen Family Fluorine (F) The elements are w Chlorine (Cl) Fluorine (F) Bromine (Br) w Chlorine (Cl) Bromine (Br) Iodine (I) Iodine (I) Astatine (At) Astatine (At) See the 17th group elements and their electronic configuration. 19 7 A Group elements www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES 8A group elements (zero group nobel gases) VIII A Group These elements having octet of electrons in valence shell. Nobel Gases General electronic configuration ns2 np6 Helium (He) These are chemically inactive as their valence shell are completely Neon (Ne) filled with ns np configuration. 6 .c om 2 Argon (Ar) These are Helium (He) Krypton (Kr) Neon (Ne) Xenon (Xe) Argon (Ar) Radon (Rn) Krypton (Kr) 8 A Group elements ts Xenon (Xe) Radon (Rn) properties of p-block elements tu See the noble gas elements and their electronic configuration. These elements are metals, non-metals, and metalloids. • They show variable oxidation states. • They do not impart any color to the flame. • These elements are placed at the right side of the periodic table. • Both s-block and p-block elements are referred as “representative elements” or “main group en ce • ci elements”. d-block elements w. s These are elements of group 3 to 12 in the periodic table. The elements in which the differentiating electron enters the penultimate or (n-1) shell of the atom of the element i.e. [(n-1)d sub shell] are called d-block elements. The d-block elements are called “transition elements” as they are placed between s-block and p-block elements. w The general electronic configuration (n-1)d1 to 10 ns1 to 2 It has four series: w 3d series: From Scandium (Z = 21) to Zinc (Z = 30) in the 4th period. 4d series: From Yttrium (Z = 39) to Cadmium (Z = 48) in the 5th period. 5d series: From Lanthanum(Z = 57) and Hafnium (Z = 72) to Mercury (Z = 80). They form the 6th period of the periodic table. 6d Series: In-complete series. It starts from Actinium (Z = 89). These elements appear in the seventh period of the periodic table. 20 www.sciencetuts.com ts D-block Elements .c om CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES properties of d-block elements They are all metals. • They form alloys. • They are good conductors of heat & electricity. • These elements form complex compounds. • They exhibit variable valence and show Para-magnetic nature in their oxidation states. F-block elements en ce tu • The two rows of elements present at the bottom of the periodic table are f - block elements. The ci differentiating electron enters to f-orbital of anti penultimate (n - 2) shell of atom. These are called “inner transition elements”. w. s The general electronic configuration (n-2) f1-14 (n-1) d0-1 ns1-2, where n = 6 or 7. It has 2 series and each series has 14 elements. 4f - series or Lanthanides: From cerium (Z = 58) to Lutetium (Z = 71). The elements of this series are called Lanthanides or rare earths. w 5f series or Actinides: From Thorium (Z = 90) to Lawrencium (Z = 103) w The elements of this series are called actinides. F-block Elements 21 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES Periodic trends in properties of elements Electron affinity “The amount of energy released when an electron is added to a neutral isolated gaseous atom in its ground state”. Equation: .c om Atom (gaseous state) + electron (e-) → Anion (gaseous state) + Energy (E) ( Where X = Atom , x- = Anion , E = Energy ) X (g) + e- → x- (g) + Energy (E) Units: Electron volts /(per) atom (or) Kilo Joules / mole (or) Kilo calories / mole. Electron affinity depends on size and effective nuclear charge. ts Note: It cannot be measured directly. tu It can be measured indirectly using Born-Haber Cycle. second Electron affinity Equation: (Endothermic reaction) en x- (g) + e- → x2-(g) + Energy ce “The energy absorbed when an electron is added to uni-negative gaseous ion”. Reason for absorption of energy: ci Repulsion between negatively charged ion and electron requires some energy to absorb which causes the endothermic reaction. w. s Example: First electron affinity of oxygen is 33.9 Kilo calories per mole - Here energy is released. (Exothermic reaction) O (g) + e- → O- (g) + 33.9 Kcal w Second electron affinity of oxygen is 119 Kcal/mole energy is absorbed. w O -(g) + e- → O2- (g) + 119 Kcal (Endothermic reaction) Factors influencing electron affinity: Size of atom Nuclear Charge Electronic Configuration 22 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES variation of Electron affinity Groups: On moving down the group, electron affinity decreases as the atomic size increases. In representative elements the first element of every group has lower electron affinity than second element. The order is as follows: F(g) + e- → F- + Energy Reason: The repulsion between added electron and Element At.No Electron Affinity F 9 3.62 17 3.78 Br 35 3.54 I 53 3.08 electrons already present is more. Note: Why fluorine is powerful reducing agent than Because fluorine has less electron affinity and high tu bond dissociation energy. Cl ts chlorine? .c om Fluorine < Chlorine, Oxygen < Sulphor, Nitrogen < Phosphorous etc. Periods: On moving from left to right in a period, Nitrogen, Neon elements has low electron affinity en values. As they have stable electronic configuration. B C O F Li Electron Affinity kj/mole 57 -66 15 121 -31 142 333 size and increase in nuclear charge. Beryllium, Be N Element ce electron affinity increases due to decrease in atomic Zero group elements have no electron affinity. Because zero group elements has completed valence shell and so does not accept electrons. Hence electron affinity value is taken as zero. Hence called ci zero group elements. w. s Electro negativity This concept was proposed by Pauling in 1932. “The tendency of an atom of an element to attract the shared electron pairs of more towards itself in diatomic molecule or in a polar covalent bond”. w Electro negativity depends on: 1. The charge of nucleus w 2. The distance of the outer electrons from the nucleus. 3. Shielding effect and 4. Nature of bonded atoms as it is a property of bonded atoms. Scales to measure electro-negativity: There are two scales to measure the electro-negativity. 1. Mulliken Scale. 2. Pauling’s Scale. 23 www.sciencetuts.com CLASSIFICATION OF ELEMENTS AND PERIODICITY IN PROPERTIES mulliken scale It was proposed by R.S. Mulliken in 1934. This method is applicable only for monovalent atom. According to this scale, electro-negativity (E.N) of an element is the average of ionization energy and electron affinity. E.N = (I1 + E1 ) / 2. Where I1 = Ionization Energy, E1 = Electron Affinity .c om Electronegative values measured on this scale are 2.8 times more than values measured on Pauling scale. Limitation: The difficulty in the measurement of electron affinity values. When ionization energy and electron affinity values are in Kilo Joules per mole, then use E.N = (I.P + E.A) divided by 544 ts pauling’s scale It was proposed by Pauling in 1931.It is based on bond energy values. E A-B = half into (EA-A + EB-B) tu Expression: Let A-B molecule. The bond energy of molecule A-B theoretically, ce The bond energy of A-B molecule experimentally E1A-B. This bond energy exceeds theoretical value. The difference in bond energies is given by ∆ (delta) ∆ = E1A-B - EA-B en Where ∆ → Ionic - Covalent Resonance Energy → It depends upon the electro chemical nature of atoms A and B. → It is measured in Kilo Joules per mole. ci Now, XA → Electro-negativity of atom A w. s XB → Electro-negativity of atom B Pauling Relation is XA - XB = 0.208 √∆ w Note: He calculated electro-negativities of elements with reference to fluorine. On Pauling scale fluorine has higher electro negative value of 4.0. w Uses of Electro-negativity: 1. In the identification of nature of bond i.e. If electro-negative difference is 1.7 or greater than 1.7, then the bond is ionic. If it is less than 1.7, then the nature of bond is covalent. 2. Metallic or non-metallic nature of the elements can be identified. 3. To assign the oxidation state. (i.e. - ve or + ve) 4. To write the formula of the compound i.e. more electro-negative should be written on right hand side. 24 www.sciencetuts.com