CHAPTER-3
PERIODIC CLASSIFICATION OF ELEMENTS
Text Book Questions
I. Four alternatives are given to each of the following incomplete statement / question. Select the most appropriate alternative:
1. In triads of ‘A’, ‘B’, ‘C’ elements, if the atomic mass of ‘B’ and ‘C’ are 150 and 200 respectively, the atomic mass of ‘A’ is.
(a) 100 (b) 125 (c) 250 (d) 350
Answer: (a) 100
2. In the periodic law the following repeat at regular intervals.
(a) Atomic number. (b) General properties. (c) Atomic mass. (d) Isotopes.
Answer: (b) General properties.
3. In the periodic table the elements of 18th group in general.
(a) Form ionic bond. (b) Form covalent bond. (c) Form hydrogen bond. (d) Do not form bond.
Answer: (d) Do not form bond.
4. The following is an example for transitional element
(a) Sodium. (b) Chlorine. (c) Argon. (d) Uranium.
Answer: (d) Uranium.
II. Fill in the blanks
1. The group number of the element neon is 18th.
2. The period number of the element aluminium is 3.
3. Horizontal similarity can be observed in the period number six and seven.
4. The first element of 14th group is carbon which is a non-metal. The metallic members of this group are tin and lead.
III. Give reasons for the following.
1. ‘f’ block elements are kept separately in the periodic table.
As they form inner transition elements and those two periods are long and there is more horizontal similarity.
2. Atomic size decreases across the period.
Because along a period there will not be any change in the number of shells but more electrons are added to the same shell and the nucleus exerts greater inward pull in the electrons.
3. In the first period hydrogen and helium are not placed adjacent to each other.
As helium has an identical outer shell electronic configuration to that of zero group elements.
4. In the construction of periodic table the periodic law is broken, in some places.
Due to the presence of lanthanides and actinides these are placed separately.
IV. Answer the following
1. What are periods and groups in the periodic table?
The horizontal rows in the periodic table are known as periods and the vertical columns are known as groups.
2. State the modern periodic law.
The modern periodic law states that “The properties of elements are the periodic functions of their atomic number”
3. Atomic mass is not useful to construct the periodic table. Give reasons with an example.
Atomic mass is not useful to construct the periodic table as it cannot solve the problem of isotopes.
Eg: carbon has four isotopes namely 6c11, 6c12, 6c13, 6c14. If atomic mass is used to classify the elements, carbon has to be placed in four places but when atomic number is used carbon is placed in one place.
4. In which part of the periodic table do we find mostly non-metals?
Non-metals are found in p block mostly between 13th group to 18th group.
5. What are transitional elements?
Elements where electrons of their atoms occupy ultimate shells leaving penultimate shells partially vacant are called transitional elements. This is applicable to d block and f block elements.
6. What are metalloids? Give an example.
Elements which are not distinctively metals and possess same physical properties of non-metals are called metalloids eg: Arsenic
Additional points, question and answers.
1. What is atomic number?
The number of protons present in the nucleus of an atom is called atomic number
2. What is atomic number?
The ratio between the mass of one atom of that element and one-twelfth the mass of one atom of carbon-12 is called atomic number
3. What is mass number?
The total number of protons and neutrons present in the nucleus of an atom is called mass number.
4. Who suggested that chemical elements could be classified on the basis of their atomic mass?
William Prout suggested in 1815 that chemical elements could be classified on the basis of their atomic mass.
5. 'Who was the first to classify chemical elements on the basis of their atomic mass?
Dobereiner was the first scientist to attempt the classification of chemical elements on the basis of their atomic mass
6. State Dobereiner’s law of triads with an example.
Dobereiner formed a group of three elements called triads having similar properties and arranged them in the increasing order of their atomic masses.
Dobereiner’s law of triads state-Among the like triads, the atomic mass of the middle element was found to be approximately equal to the average mass number of the other two.
Eg;
Elements Cl Br I
Atomic mass 35.5 80 127
Atomic mass of Br = (35.5 + 127)/2 = 81
7. Why the system of classification of elements into triads was not found suitable?
The classification of elements into triads was not found suitable for the following reasons: All known elements could not be classified into groups of triads on the basis of their properties. Secondly, not all groups obeyed the Law of Triads. For example, nitrogen family does not obey the Law of Triads. Thirdly, Dobereiner could identify only three triads from the elements known at that time.
8. State Newlands law of octaves. Give an example.
When the elements are arranged in the ascending order of their atomic masses, the eight element resembles the first element in its properties.
Eg: Li resembles Na, H resembles F, Be resembles Mg, B resembles Al, C resembles Si, N resembles P, O resembles S, F resembles Cl, Na resembles K, Mg resembles Ca.
9. What is meant by periodicity of properties of chemical elements?
It was found that the properties of chemical elements repeated after a definite interval when the chemical elements were arranged in a certain order. This repetition of similar properties of chemical elements after a definite interval is known as periodicity.
10. What were the limitations of Newlands' law of octaves?
All the elements discovered at that time could not be classified into octaves. It was not possible to extend Newlands’ law of octaves beyond seventeen elements.
11. State Mendeleev’s periodic law.
Mendeleev’s periodic law is stated as “The properties of elements are periodic functions of their atomic mass”
12. What is a Periodic Table of elements?
A Periodic Table of elements is a chart of elements prepared in such a way that the elements having similar properties occur in the same vertical column i.e., same group.
13. Which two criteria did Mendeleev use to classify elements in his Periodic Table?
Mendeleev used the following two criteria to classify elements: Atomic mass and similarity in chemical properties of elements.
14. How did Mendeleev classify chemical elements? Explain,
During the time of Mendeleev, only 63 elements were known. He arranged these elements in the increasing order of their atomic masses. He found that the properties of elements showed periodicity at regular intervals. He found that every eighth element had properties similar to that of the first. He used this periodicity to arrange the elements in periods and groups in the table. He left some gaps in the Periodic Table and indicated that some elements that should take these positions are yet to be discovered,
15. List the main features of Mendeleev’s Periodic Table
The main features of Mendeleev’s Periodic Table are:
a. The horizontal rows present in the Periodic Table are called period. There are seven periods in the table.
b. Properties of elements in a particular period show regular gradation from left to right.
c. The vertical columns present in it are called group? These are nine in number and are numbered from I to VIII and Zero
d. Groups I to VII are subdivided into A and B subgroups. Groups Zero and VIII do not have any subgroups.
e. All the elements in a particular group are chemically similar in nature. They show regular gradation in their physical properties and chemical reactivity
16. State the limitations of Mendeleev's Periodic Table.
Mendeleev's Periodic Table could not provide place to noble gases which were discovered later. Although Mendeleev arranged the elements in the increasing order of their atomic masses, this could not be followed strictly in all cases. There was no place for isotopes although they differed in their atomic masses.
17. Who suggested that atomic number and not atomic mass is the fundamental property for classification of elements?
An English physicist by name Henry Moseley suggested that atomic number and not atomic mass is the fundamental property for classification of elements.
18. How are the elements arranged in the Modern Periodic Table?
In the Modem Periodic Table, the elements are arranged in the increasing order of their atomic numbers based on the modem periodic law which states that The properties of chemical elements are periodic functions of their atomic number’.
19. When was the Modern Periodic Table adopted?
The Modern Periodic Table was adopted by the International Union of Pure and Applied Chemists (IUPAC) in 1984.
20. What are isotopes? Give an example.
Atoms of the same element with different mass numbers are called isotopes.
For example, carbon has three known isotopes. They are carbon-12„ carbon-13 and carbon-14. All these atoms have the same atomic number and hence occupy the same place in the Modern Periodic Table.
21. Why are the isotopes of an element given the same place in the Modern Periodic Table?
Isotopes of an element are atoms with the same atomic number but different atomic mass. In the Modem Periodic Table, the elements are arranged in the increasing order of their atomic numbers in accordance with the modem periodic law. Since isotopes of an element have the same atomic number, they are given the same place in the periodic table.
22. Define the following terms: Atomic size, valence electrons, ionisation potential, electron affinity, metal, non-metal, metalloids.
Atomic size: The radius of the atom is called its atomic size. It is the distance between the centre of the nucleus and the outermost shell of an isolated atom.
Valence electrons: The number of electrons present in the outermost orbit of an atom are called valence electrons.
Ionisation potential: The energy required to remove completely an electron from its atom is called ionisation potential.
Electron affinity: The ability of an atom or molecule to form a negative ion is called electron affinity. It is expressed as the energy released, when an electron is attached to it.
Metal: A chemical element which is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity is called a metal.
Non-metal: A chemical element which is soft, neither malleable nor ductile and poor conductor of heat and electricity is called a non-metal.
Metalloids: Elements which have some properties of both metals and non-metals are called metalloids.
23. Explain the main features of the Modern Periodic Table.
i) Elements are arranged in the increasing order of their atomic numbers.
ii) Elements are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
iii) There are short periods, long periods and incomplete periods in the table.
iv) Elements with the same number of valence electrons and similar properties are placed in the same group.
v) The Periodic Table is divided into four blocks based on their electronic configuration: s-block elements, p-block elements, d-block elements and f-block elements, d-block elements are called outer transition elements and the f-block elements are called inner transition elements.
vi) The noble gases with valency zero are placed in the 18th group.
vii) f-block elements are placed separately in two rows at the bottom of the Periodic Table.
24. Explain the terms ‘periods’ and ‘group’ as they are applicable to modern periodic table.
In the periodic table there are 7 horizontal rows called periods and 18 vertical columns called groups. Based on energy levels, the periods are written as 1 to 7. Based on electronic configuration the groups are written 1 to 18.
25. Why group number 18 is called zero group.
Group number 18 is called zero group because the valency of these elements is usually zero. They do not form chemical bond on their own with other elements. They are called noble gases instead of inert gases.
26. What is octet structure? Name the elements whose atoms have octet structure.
The arrangement of having eight electrons in the outermost shell of an atom is called octet structure. The elements in the Periodic Table which have octet structure are neon, argon, krypton, xenon and radon.
27. What is electronic configuration of an element?
The arrangement of electrons in the orbitals of a neutral atom of an element is called its electronic configuration
28. What is a block in a Periodic Table? How are they named?
A block in the Periodic Table refers to a set of elements located in adjacent element groups. Element blocks are named after their characteristic orbital, which is determined by the highest energy electrons present in their atom
29. Into how many blocks are the elements in the Periodic Table classified? Name them.
The elements of the Periodic Table are classified into four blocks. The four blocks are s-block, p-block, d-block and f-block,
30. What is the other name given to d-block elements?
The elements of the d-block of the Periodic Table are also called outer transitional elements.
31. What is the other name given to f-block elements?
The elements of the f-block of the Periodic Table are also called inner transitional elements. They are also called rare earth elements.
32. On which side of the Periodic Table do we find (a) metals (b) non-metals, and (c) metalloids?
a) Metals are found on the left side of the Periodic Table.
b) Non-metals are found on the right side of the Periodic Table.
c) Metalloids are found in the middle of the Periodic Table between metals and non-metals
33. How do the atomic number, atomic size, metallic character and nan-metallic character of elements vary as we move from left to right across a period and top to down in a group?
| Property | In a period [from left to right] | In a group [from top to down) |
| Atomic number | increases | increases |
| Atomic size | decreases | increases |
| Metallic character | decreases | increases |
| Non-metal lie character | increases | decreases |
| Ionization energy | increases | decreases |
34. What are transition elements? Give examples.
Elements whose atoms have partially filled penultimate shells are called transition elements.
For example, any of the metallic elements within Groups 3 through 12 in the Periodic Table are transition elements. All the transition elements have two electrons in their outermost shell, and all but zinc, cadmium, and mercury have an incompletely filled inner shell. Such elements are transition elements.
35. What are metalloids? Give an example.
Chemical elements which have some properties of both metals and non-metals are called metalloids. E.g. Germanium, Silicon etc.
36. Why is the periodic law broken in some places in the construction of the periodic table?
Periodic law is broken in some places in the construction of the periodic table for reasons of convenience. For instance, the inner transition elements are placed at the bottom separately in two rows. They do not fit into the elements of the main table as there is more horizontal similarity rather than vertical similarity in the case of these elements.
37. What are lanthanides and actinides?
The inner transition elements are two series of elements known as the lanthanides and the actinides. They are usually shown below all the other elements in the standard view of the periodic table, but they really belong to periods 6 and 7. The lanthanide series consists of the 14 elements cerium through lutetium (atomic numbers 58-71), which immediately follow lanthanum. Likewise, the actinide series consists of the 14 elements thorium through lawrencium (atomic numbers 90-103), which immediately follow actinium.
38. f- block elements are kept separately in the Periodic Table. Give reason.
‘f ‘- block elements belong to Group 3 of the Periodic Table. They are kept separately at the bottom of the main body of the Periodic Table because they are larger in number, and showing them in the main body of the table will distort its shape and hence a separate f- block.
39. What is periodicity?
The recurrence of similar properties of the elements, when they are arranged in the order of increasing atomic number after certain regular interval is called periodicity.
40. Define “ionisation energy”
Ionisation energy is the energy required to remove the electron from the outermost shell from the free and isolated atom of an element.
41. Explain electro chemical nature.
Atoms which give up electrons easily are called electropositive. For example: Sodium atom.
Atoms which receive electrons easily are called electro negative. For example: Chlorine.
Electro positivity decreases along a period and increases down the group. Similarly Electronegativity increases along a period and decreases down the group.
42. Explain metallic nature.
Metallic nature decreases along a period and increases down the group. For example in 14th group there are elements namely carbon, silicon, germanium, tin and lead. Carbon and silicon are non-metals, tin and lead are metals. Germanium has characteristics of non-metal and metal.
43. Distribution of electron in shells and orbitals
44. Write the advantages of the periodic table.
1. Study of chemistry is simplified and organised leading to easy access of data of the elements.
2. It was possible to predict the atomic mass, some of the properties and other details of elements which are yet to be discovered. Many elements were discovered during the life time of Mendeleev.
3. There is striking similarity between the periodic table constructed empirically by Mendeleev and the modern periodic table based on the electronic configuration and nature of chemical bond. Common-sense plays an important role in scientific research.
4. Interpretation of trends and periodicities of properties of elements provided a clear understanding and proof for the atomic behaviour.
5. It is possible to predict the properties of elements by considering the position of elements in the periodic table. On the other hand properties of an element help us to predict the position of an element in the periodic table.
· Law of triads-Dobereiner
· Law of octaves-Newlands
· Mendaleleev’s law-Mendeleev
· Modern periodic law-Moseley
· ‘d’ block elements are called outer transitional elements.
· ‘f’ block elements are called inner transitional elements.
· Elements having similar properties are kept in the same group.
· Mendeleev helped in the discovery of germanium element.
· Based on electronic configuration all the elements in the periodic table can be grouped into four general categories. They are ‘s’ block, ‘p’ block, ‘d’ block and ‘f’ block.
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