PERIODIC CLASSIFICATION OF ELEMENTS Important terms & condition: Classification of elements: the arrangement of elements in such a manner that elements with similar Properties are grouped together while elements with dissimilar properties are separated. EARLY ATTEMPTS OF CLASSIFICATION OF ELEMENTS:- DOBEREINERS S TRIADS Johann Wolfgang Dobereiner was a German chemist. His effort is considered as one of the earliest attempts to classify the elements into groups. He found that when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element; which comes in the middle; is the arithmetic mean of rest of the two. On this basis, he arranged three elements in one group which is known as Triad. This arrangement of elements is known as Dobereiner s Triads. Limitation of Dobereiner s Triads:- Dobereiner could find only three such triads (group of three elements) and he could not even put all the elements known at that time in his triads. The rules of Dobereiner s triads could not be applied to the elements which had very low or high atomic mass. Such as; if F, Cl and Br are put together in a triad, in increasing order of their atomic masses, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br. After the advancement of techniques of measuring atomic mass more correctly Dobereiner s Law became obsolete. NEWLANDS LAW OF OCTAVES:- Newlands found that every eighth element has similar physical and chemical properties when they are arranged in increasing order of their relative masses. This law is known as Newlands Law of Octaves which states that "any given element will exhibit analoogus behavior to the eighth element following it in the table". This means every eight element has the similar chemical and physical properties. For example; Sodium is the eighth element from Lithium and both have similar properties. The arrangement of elements in Newlands Octave resembles the musical notes. In musical notes, every eighth note produces similar sound. Because of this; Newland s classification of elements was popularly known as just Octaves.
Limitation of Newlands Octaves:- Newlands Octaves could be valid upto calcium only; as beyond calcium, elements do not obey the rules of Octaves. Newlands Octaves was valid for lighter elements only. It appears that Newlands did not expect the discovery of more elements than 56 which were discovered till his time. More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group. Iron; which has similar property as cobalt and nickel, was placed far from them. Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties. In spite of above limitations; Newlands was the first scientist who arranged the elements in order of their increasing relative atomic masses. MENDELEEV S PERIODIC TABLE: Dmitri Ivanovich Mendeleev, a Russian scientist arranged the elements in increasing order of their relative atomic masses. He was honoured with Noble prize in 1906 for his Periodic Table. Mendeleev s Periodic Law states that the properties of elements are the periodic function of their relative atomic masses. Mendeleev (1834-1907), a Russian chemist arranged the elements in order of increasing atomic masses, similarity in physical and chemical properties of elements. Properties of hydrides and oxides of different elements were studied and elements with similar properties were grouped together. He classified the elements in table Consisted of vertical columns called groups and horizontal rows called periods. There were 7 groups in table and group is subdivided into subgroups A and B except group 7 which has three sets of elements in 4th, 5th and 6th period. ( R is used to represent any of the elements in a group) Mendeleev periodic table is mainly based on two facts- (i)atomic mass (ii)similarity of chemical properties Mendeleev arranged all 63 elements; which were discovered till his time; in the order of their increasing relative atomic masses in a tabular form. It is known as Mendeleev s Periodic Table. He divided the table in eight columns and seven rows. The columns are known as groups and rows are known as periods. Explanation of Mendeleev s Periodic Table:-
Elements are arranged in the periodic table in the increasing order of their relative atomic masses. Mendeleev divided his periodic table in eight groups and seven periods. Groups from I to VII are meant for normal elements and group VIII is for transition elements. Groups from I to VII have been divided in two sub groups, while group VIII is meant for three elements. Periods from 4th to 7th have been divided in two series: 1st series and 2nd series. Elements having similar properties have been kept in the same group. For example; lithium, potassium, rubidium, etc. are in 1st group. Two general formulae; one for oxides and second for hydrides; have been given for the elements of each group in the periodic table. For example: R2O for oxides and RH for hydrides, of the elements; of 1st group. Using the given general formulae; the formula of oxides and hydrides can be written for the elements of each group. For example; hydrogen, sodium, potassium, etc. belong to the first group. The general formula of oxides for the elements of 1st group is R2O. Accordingly they form H2O, Na2O, K2O, etc. Merits of Mendeleev s Periodic Table:- Mendeleev left some blank spaces in his periodic table in order to place the elements having similar properties in the same group. For example; titanium has been placed in IVth group, leaving a blank space adjacent to it in IIIrd group. Similarly, arsenic has been placed in Vth group; leaving two adjacent spaces blank. These spaces have been occupied by scandium, gallium and germanium after their subsequent discovery. Prediction for the elements to be discovered in future: Mendeleev predicted the discovery of some elements and named them as eka-boron, eka-aluminium and ekasilicon. He gave the name of these elements prefixing the word eka to the name of the preceding elements. Scandium, Gallium and Germanium have been discovered later and took the place of eka-carbon, ekaaluminium and eka-silicon, respectively in the gap left in the Mendeleev s Periodic table; as their properties were exactly similar to the predicted elements. Position of Noble gases which were discovered later:- Noble gases were discovered much later after Mendeleev. After the discovery of noble gas, they were placed in a separate group called Zero Group, after VIII group, without making any disturbance to the arrangement of any elements in the Mendeleev s Periodic Table. Noble gases are chemically un-reactive and present in very low concentration in the atmosphere. Limitation of Mendeleev s Periodic Table
Position of Hydrogen:- Hydrogen has been placed in 1st group with alkali metals, since hydrogen makes compound in the same way as alkali metals do. On the other hand, hydrogen exists as diatomic molecule; similar to halogen and hydrogen makes covalent compounds also as halogens do. Thus, on the basis of properties of hydrogen similar to halogen, hydrogen may be placed with halogens but Mendeleev did not explain about this anomaly. Position of Isotopes:- Elements having same atomic number but different atomic masses are known as isotopes. Although isotopes were discovered after Mendeleev, but it became a challenge to accommodate those isotopes in Mendeleev s Periodic Table without disturbing the order of elements. Wrong Order of Elements:- Mendeleev placed many elements in wrong order of their increasing atomic masses in order to place elements having similar properties in similar group. Example: The atomic mass of nickel is less than that of cobalt; in spite of that cobalt is placed before nickel. The atomic mass of Chromium is 50.20 and the atomic mass of vanadium is 50.94. In spite of this, chromium is placed after vanadium. In spite of above limitations and anomalies, the Mendeleev s Periodic Table was one of the wonderful discoveries.
MODERN PERIODIC TABLE:-
Law of Modern Periodic Table states that properties of elements are the periodic function of their atomic numbers. In the modern periodic table, elements are arranged in order of their increasing atomic numbers. In 1913 Moseley found that frequency of X-ray emitted by different elements is directly proportional to atomic number. These studies show that properties of elements depend upon atomic number but not atomic mass. So atomic number is the basis of classification of element. Moseley gave modern periodic law which stated as The physical and chemical properties of the elements are periodic function of their atomic number In this table, elements have been arranged in order of increasing atomic number. This table also consists of vertical rows called groups and horizontal rows called periods which are discussed as: a) There are 18 groups designated as 1 to 18. b) All the elements in a group have same no. of electrons in outermost shell. c) The elements of each group have similar chemical properties due to same valence electrons. d) There are in all seven periods. e) First three periods with 2,8,8 element called short periods. The next three periods with 18,18,32 element called long period while seventh period is an incomplete period. f) The number of elements in these periods are based on filling of electron into various shell on the formula 2n2 (n-number of shell) For Ex:- K-shell (n=1)-(2.1)2=2 so first period has two elements L-shell (n=2)-(2.2)2=8. So 2nd period has eight elements. M-shell (n=3)-(2.3)2=18, but the outer most shell can have only 8-electron so third period also has eight elements. Advantages of long form of periodic table:- 1. The position of the elements are linked with their electronic configuration. 2. Position of isotopes of an element is justified since they have same atomic number. 3. Similar properties of element in a group is also justified due to same valence electrons. 4. Each group is an independent group and idea of sub-group discarded. GROUPS: Elements in group 1 are called alkali metals. Elements in group 2 are called alkaline earth metals. Elements in group 17 are called halogens. Elements in group 18 elements are called inert gases or noble gases. Significance of group in the periodic table is that an element in a group has same no. of valence electrons, valence and thus identical chemical properties. PERIODS : 1st period 2 elements and is called very short period. 2nd period 8 elements and is called short period. 3rd period - 8 elements and is called short period. 4th period 18 elements and is called long period. 5th period 18 elements and is called long period. 6th period 32 elements and is called very long period. 7th period incomplete period. The number of shells present in the element indicates the period to which it belongs. Valency: it is defined as the combing capacity of an atom of an element to acquire noble gas configuration. It is equal to the number of electrons lost, gained or shared during the formation of a chemical compound. (i) In groups the elements present have same valence.
(ii) In periods: valence increases 1 to 4 and then decreases to zero. Explanation of Modern Periodic Table:- Elements are arranged in order of their increasing atomic numbers. The vertical columns are known as groups and horizontal columns are known as periods; in the modern periodic table. There are 18 groups and 7 periods in the modern periodic table. Elements having same number of valence electrons are placed in the same group. For example; elements having valence electrons equal to 1 are placed in the 1st group, elements having valence electrons equal to 3 are placed in the 13th group, elements having valence electrons equal to 2 are placed in 2nd group except helium which is placed in 18th group, since it is an inert gas. Elements having same number of shells are placed in the same period. Groups in Modern Periodic Table:- 1st group:- Alkali metals are placed in the 1st group in the modern periodic table. Hydrogen is also placed in the 1st group although hydrogen is not an alkali metal. 2nd group:- Alkaline earth metals are placed in the 2nd group in the modern periodic table. Elements placed in 1st and 2nd groups in the modern periodic table are collectively known as light metals. 3rd to 12th group:- Transition elements are placed from 3rd to 12th group in the modern periodic table. 13th group:- Metals are placed in the 13th group; except boron which is a metalloid. 14th group:- Carbon, silicon, germanium, tin and lead are placed in this group. Among them, carbon is a non-metal, silicon and germanium are metalloids and tin and lead are metals. 15th group:- Nitrogen, phosphorous, arsenic, antimony and bismuth are place in the 15th group; among which nitrogen and phosphorous are non-metals, arsenic and antimony are metalloids and bismuth is a metal. 16th group:- Oxygen, sulphur, selenium, tellurium and polonium are placed in this group, among which oxygen, sulphur and selenium are non-metals, tellurium is metalloid and polonium is a metal. 17th group:- Non-metals are placed in the 17th group. Since, halogens are placed in this group hence this group is also known as group of halogen. 18th group:- Noble gases are placed in the 18th group. This group is also known as zero group. Periods in Modern Periodic Table: At present there are seven periods in the Modern Periodic Table. 1st period: This is known as very short period as there are only two elements, i.e. hydrogen and helium. 2nd and 3rd period: There are total 8 elements in each of the 2nd and 3rd periods. These periods are known as short periods. 4th and 5th period: There are total 18 elements in each of the 4th and 5th periods. These periods are known as long periods. 6th period: There are total 32 elements in 6th period. This period is known as very long period. 7th period: This period is known as incomplete period. Blank spaces in this period are supposed to be filled by the elements discovered in future. Position of elements in the Modern Periodic Table: Elements are placed in groups according to their valence electrons and placed in periods according to the number of shells present in them. Helium has valence electrons equal to 2, but it is placed in group number 18 because it is a noble gas and has
completely filled outermost shell. Example: Hydrogen: Atomic number 1 Valence electron = 1 Number of shell = 1 Position in periodic table 1st group 1st period Sodium: Atomic number 11 Valence electron = 1 Number of shell = 3 Position in periodic table 1st group 3rd period. Carbon: Atomic number 6 Valence electron = 4 Number of shell = 2 Position in periodic table 14th group 2nd period. Neon: Atomic number 10 Valence electron = 8 i.e. equal to 0 (zero) Number of shell = 2 Position in periodic table 18th group 2nd period. Trends in Modern Periodic Table: Valence electrons: Number of valence electron remains the same while moving from top to bottom in a group. Number of valence electron increases while moving from left to right in a period. Valency: Valency remains the same on moving from top to bottom in a group. Valency first increases upto 4 on moving from left to right in a group and then decreases upto zero. ATOMIC SIZE / ATOMIC RADII: Atomic size: Atomic size increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
Atomic radii: Atomic radius increases on moving from top to bottom in a group and decreases on moving from left to right in a period. On moving down the group the group the atomic radii increases. Because on moving down the group a new energy shell is added which increases the distance between the outermost electrons and the nucleus. Although the nuclear charge also increases, but it is compensated by the additional shell being added thus, increasing the size of the atom. Hence, In periods atomic radius decreases in moving from left to right due to increase in nuclear charge which have tendency to attract electron closer to the nucleus and reduces the size of atom. Across the period the atomic radii decrease. Due to the increased nuclear charge, the pull on the electrons increases and hence, they are pulled Closer to the nucleus thus, decreasing the atomic size. Hence, in groups: Atomic size increases down the group because new shells are added down the group which increases distance between nucleus and outer most electrons. METALLIC CHARACTER OF ELEMENTS: Metallic character decreases on moving from left to right in a period. Tendency to lose electron: Tendency to lose electrons decreases on moving from left to right in a period and tendency to lose electrons increases on moving from top to bottom in a group. Tendency to lose electron is also known as electropositive character, thus electropositive character decreases on moving from left to right in a period and increases on moving from top to bottom in a group. Tendency to gain electron: Tendency to gain electrons increases on moving from left to right in a period. Tendency to gain electron is called electro-negativity, thus electro-negativity of elements increases on moving from left to right in a period. It is defined as the distance from the Centre of nucleus to the outermost shell of the atom. It is generally expressed in Pico meter (Pm). Oxides and its nature: Metals react with oxygen to form oxides by loss of electrons. These oxides on dissolution in water form bases. REACTIVITY OF ELEMENTS: Down the group reactivity of metals increases as the tendency to lose electrons increases due to increased atomic size. Reactivity of non- metals decreases down the group Because of the increased atomic size and the tendency to gain electrons decreases. On moving across the period, the reactivity first increases due to the decrease in the metallic character and increase in non metallic character. METALLIC AND NON-METALLIC PROPERTIES The metallic character of an element is expressed in terms of its electron releasing tendency while non-metallic character in term of electron accepting tendency. In group: Metallic character of the element increases down the group due to increasing atomic size or because outermost electrons are farther away from the nucleus.so they can be easily lost. In periods:- Metallic character of the element decreases along a period due to decrease in atomic size along a period or outermost electrons are closer to nucleus. So they cannot be easily lost. NOTE:- Non-metallic elements follow the same reverse trend. In modern periodic table a zig - zag line separates metals from Non-metals. The border line elements, B, Si, Ge, As, Sb, Te and Po are called metalloids or semi-metal.
SUMMARY QUESTION BANK
Q. 1. What is periodicity? Ans. Periodicity is the repetitive occurrence of properties after definite interval. Q. 2.Who showed for the first time that there is periodicity in properties of elements? Ans. It was Newland s classification that showed for the first time that there is periodicity in properties of elements. Q. 3. Are the properties of elements placed in a group same? Ans.Yes, the chemical properties of the elements placed in a group are same. There is slight variation (increase or decrease) in physical properties of the elements in the group. Q. 4. Give reason for the need of classification of elements. Ans. The continuous discovery of new elements and their compounds led to confusions. It became difficult to study, remember, and recall the properties of all the elements. Thus, the need to classify and place them in certain groups was felt. Q. 5.Hydrogen can be placed in group 1 and group 7 of periodic table. Why? Ans. Hydrogen forms both positive ions like alkali metals (group 1) and negative ions like halogens (Group 7). Thus, it can be placed both in group 1 and group 7. In Mendeleev s Periodic table, the position of hydrogen was not clear. Q. 6. Name two elements whose properties were correctly predicted by Mendeleev. Mention their present day name. Ans.Two elements whose properties were correctly predicted by Mendeleev are: Eka-aluminium gallium Eka-silicon germanium Q. 7.State Mendeleev's periodic law. Why did he leave gaps in his periodic table? Ans.Mendeleev formulated a periodic law stating that the properties of the elements are the periodic function of their atomic masses. He predicted that there were elements that were not discovered at that time. Therefore, when he came across a position where no element fitted (known at that time), he left a gap to be filled later. Q. 8.An element Z is of second group of the periodic table. Write the formula of its oxide. Ans. The element is in second group. Thus, its valency is 2. Oxygen has a valency of -2 in its oxides. Thus, the formula of element Z oxide is ZO. Q. 9. Noble gases did not find a place in Newland's Octaves. Explain. Ans. Newland s classification arranges the elements such that the properties of elements repeat after every eight element. After the discovery of noble gases, they became the ninth element (and not the eight), which had the similar property. Q. 10.Give formula for the following: (i) Bromide of element X of second group. (ii) Oxide of element Y of third group. (iii) Chloride of element Z of fourth group. Ans. (i) XBr2 (ii) Y2O3 (iii) ZCl4 Q. 11. How many elements are present in (i) Second period (ii) Six period Ans. (i) 8 elements (ii) 32 elements Q. 12.Name (i) A Non metal solid at room temperature (ii) A Metal liquid at room temperature Ans. (i) Iodine (ii) Mercury
Q. 13.Arrange the following elements in the decreasing order of metallic character. (i) Si, Be, Mg, Na, P (ii) B, Al, Mg, K Ans. (i) Na> Mg> Si> P>Be (ii) K> Mg>Al>B Q. 14.How in modern periodic table position of elements in groups and periods is decided? Ans. The elements are positioned in the periodic table such that the elements with same number of valence electrons are present in same group and element with the same number of occupied shells are placed in same period. Q.15.Why metallic character decreases across a period and increases down a group? Ans. As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease. Down the group, the effective nuclear charge experienced by valence electrons is decreases because the outermost electrons are move farther away from the nucleus. Therefore, these can be lost easily. Hence metallic character decreases across a period and increases down a group. Q. 16.Among the elements of second period Li to Ne pick out the element. (i) With the largest atomic radius (ii) that is the most reactive non metal (iii) that is the most reactive metal (iv) Which is a metalloid. Ans.(i) Lithium has largest atomic radius. (ii) Fluorine is the most reactive non metal. (iii) Lithium is the most reactive metal. (iv) Boron is a metalloid. Q. 17.Elements A, B, C, D, E have following electronic configurations- A: 2,3 B: 2,8,3 C: 2,8,5 D: 2,8,7 E: 2,8,8,2 (i) Which of these belong to same group? (ii) Which of these belong to same period? Ans.(i) Element A and B belong to same group. (ii) Element B, C and D belong to same period Q. 18.(i) Name an element with five electrons in the outer shell. (ii) Name an element which tends to lose two electrons. (iii) Name an element that would tend to gain two electrons. (iv) Name the group having a non metal liquid as well as non metal gas at room temperature. (v) Name the group having element with zero valency. (vi) Name the metalloid present in group 14. Ans. (i) Nitrogen is an element with five electrons in the outer shell. (ii) Magnesium is an element which tends to lose two electrons. (iii) Oxygen is an element that would tend to gain two electrons. (iv) Halogen Group (v) Zero groups (Group 18 noble gases) (vi) Silicon
Q19.Differentiate between metals and non metals? Metals They are generally hard, solid and lustrous. They are malleable i.e. can be beaten into sheets. They are ductile i.e. can be drawn into wires. They are not brittle. They are good conductors of heat and electricity. They have generally high melting and boiling points. Non Metals They are generally gaseous and non lustrous. They are not malleable. They are not ductile. They are brittle. They are bad conductors of heat and electricity. They have generally low melting and boiling points. Metallic character increase as we go down the group. Non metallic character increase as we move from left to right along the period. Q20. Define atomic radius of an element. How does it vary along the period and group? Ans.The distance between the nucleus and the outermost orbit of the atom is called the atomic radius of the atom of an element. Variation along the period: - The atomic radius decreases as we move left to right along the period. This is due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom. Variation along the group: - The atomic radius increases down the group. This is because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge. MULTIPLE CHOICE QUESTIONS Question (1): The property of an element in the periodic table depends on its,. 1. atomic size 2. atomic mass 3. electronic configuration 4. number of protons Ans: 3 Question (2): An element has configuration 2, 8, 1. It belongs to,. 1. 1 group and 3rd period 2. 3 group and 1st period 3. 1 group and 8th period 4. 17 group and 3rd period Ans: 1 Question (3): The number of electrons in the valence shell is equal to its. 1. atomic mass 2. group number 3. period number 4. atomic volume Ans: 2 Question (4): The non-metallic element present in the third period other than sulphur and chlorine is 1. oxygen 2. Fluorine 3. Nitrogen 4. Phosphorus Ans: 4 Question (5): At the end of each period the valence shell is. 1. incomplete 2. half filled 3. singly occupied 4. completely filled Ans: 4 Question (6): The family of elements having seven electrons in the outermost shell is. 1. alkali metals 2. alkaline earth metals 3. Halogens 4. noble gases Ans: 3 Question (7): Which of the following factors does not affect the metallic character of an element? 1. Atomic size 2. Ionisation potential 3. Electronegativity 4. Atomic radius
Ans: 3 Question (8): The family of elements to which potassium belongs is. 1. alkali metals 2. alkaline earth metals 3. Halogens 4. noble gases Ans: 1 Question (9): The modern periodic table is given by 1. Mendeleev 2. Einstein 3. Bohr 4. Mosley Ans: 4 Question (10): Elements belonging to groups 1 to 17 are called. 1. noble gases 2. normal elements 3. transition elements 4. inner transition elements Ans: 2 Question (11): A liquid non-metal is. 1. phosphorous 2. Mercury 3. Bromine 4. nitrogen Ans: 3 Question (12): The first alkali metal is. 1. hydrogen 2. Lithium 3. Sodium 4. francium Ans: 2 Question (13): A purple coloured solid halogen is. 1. chlorine 2. Bromine 3. Iodine 4. astatine Ans: 3 Question (14): Lanthanides and actinides are also called. 1. normal elements 2. transition elements 3. noble gases 4. inner transition elements Ans: 4 Question (15): The family of elements to which calcium belongs is. 1. alkali metals 2. alkaline earth metals 3. Halogens 4. noble gases Ans: 2 Question (16): The least reactive element in group 17 is. 1. fluorine 2. Chlorine 3. Bromine 4. iodine Ans: 4 Question (17): The valency of chlorine with respect to oxygen is. 1. 1 2. 3 3. 5 4. 7 Ans: 4 Question (18): The number of shells in the elements of 3rd period is. 1. 1 2. 2 3. 3 4. 0 Ans: 3 Question (19): Four elements along a period have atomic number (11, 13, 16 and 17). The most metallic among these has an atomic number of. 1. 11 2. 12 3. 16 4. 17 Ans: 1
Question (20): Six elements A, B, C, D, E and F have the following atomic numbers (A = 12, B = 17, C = 18, D = 7, E = 9 and F = 11). Among these elements, the element, which belongs to the 3rd period and has the highest ionization potential, is? 1. A 2. B 3. C 4. F Ans: 3 Question (21): A factor that affects the ionization potential of an element is. 1. atomic size 2. electron affinity 3. electro-negativity 4. neutrons Ans: 1 Question (22): The element, which has the highest electron affinity in the 3rd period is. 1. Na 2. Mg 3. Si 4. Cl Ans: 4 Question (23): The element, which has zero electron affinity in the 3rd period is. 1. Al 2. P 3. Ar 4. S Ans: 3 Question (24): The statement that is not true about electron affinity is 1. It causes energy to be released 2. It causes energy to be absorbed 3. It is expressed in electron volts 4. It involves formation of an anion Ans: 2 Question (25): Down a group, the electron affinity. 1. increases 2. Decreases 3. remains same 4. increases and then decreases Ans: 2