Chapter-5

Periodic Classification of Elements

1. Overview: There are 118 elements are known at present. All have different properties. Out of them, only 94 are naturally occurring. As there is a chance that different elements may be discovered in future and scientist tries to gather more and more properties of elements. It is required to organise all elements in a well-defined manner so that one can obtain maximum information at once. Earlier, there was no way to organise or tabulate them altogether in informatics way. Hence, classification of elements in proper and scientific way was needed.
   
2. Early attempts at the classification of elements: The earliest attempt to classify elements resulted in grouping them into two parts as Metals & Non-Metals. Later, further classifications were also tried out. Some of the attempted ways of classification are given below:
   
3. Dobereiner’s triads: In 1817, Johann Wolfgang Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups. He identified these groups having three elements each. Accordingly, he named these groups ‘Triads’.
   
4. He showed that when the three elements in a group triad were written in the order of increasing atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. The same can be understand by the following tabulation.
   

1. This Triads is not successful to most of the elements discovered at that time. Hence, this way of classification was not found to be useful.
   
2. Newlands’ Law of Octaves: In the year 1866, an English scientist named John Newlands tried to arranged the then known elements in the order of increasing atomic masses. He started with the element having the lowest atomic mass (Hydrogen) & ended with the element having the last element. He found that every eight elements among them having similar properties to that of the first so arranged. He named this arrangement as ‘Octave’ in the tune of musical notes.
   
3. Later, it was found that the Law of Octaves was applicable only upto Calcium (Ca). After that, every eight elements did not possess similar property to that of the first of the Octaves. Further, he assumed only 56 existed and no more elements would be discovered in future. In order to fit elements into his table, he adjusted two elements in the same slot, and in this manner he also put two unlike elements under the same note. Hence, Octaves arrangement was not successful.
   
4. Questions (Page: 81)
   
   1. Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
      

Ans: Dobereiner’s triads did exist in the columns of Newlands’ Octaves; For example, the elements like Lithium (Li), Potassium (K) and Sodium

(Na) constitute a Dobereiner’s Triad but are also found in the second column of Newland’s Octaves.

Ans: the limitations of Debereiner’s classification are as follows:

• They were not applicable for very low mass or very high mass elements.
  
• All the elements couldn’t fit into Dobereiner’s triads.
  
• As the methods to calculate atomic mass improved, Dobereiner’s triads validity began to decrease. Example: In the triad of F, Cl and Br, the arithmetic mean of atomic masses of F and Br are not equal to the atomic mass of Cl.
  
  3. What were the limitations of Newlands’ Law of Octaves?
  

  Ans: Following are the limitations of Newlands’ law of octaves
  

• Limitations of Newlands’ Law of Octaves are as follows
  
• Newlands’ Law of Octaves applicable to elements up to Calcium
  
• Newland assumed there are 56 elements in the nature and no more elements would be discovered in the future.
  
• To fit elements into table Newland put two elements into one slot. Newland introduced unlike elements with different properties into one column.
  
• Iron(Fe) was placed away from elements that resembles in properties . Ex: Nickel and Cobalt
  
  5.1 Mendeleev’s Periodic Table: With the concept of Newlands’ law of Octaves, further scientists begun to find way to classify the elements on the basis of Atomic Mass. A Russian chemist named Dmitri Ivanovich Mendeleev, gave out his classification of element principle. His principle is:
  

  “The physical & Chemical properties of an element is periodic function of their Atomic Mass”.
  

  When Mendeleev started his work, only 63 elements were known. He examined the physical and chemical properties of the elements on the basis of their Atomic Mass. He arranged the elements in ascending order of their atomic mass and constructed a table of elements which is known as periodic table. Mendeleev’s periodic table consisted of 8 vertical columns called groups & 7 horizontal rows called period. However, a new group called group ‘O’ was added to this period table after the discovery of inert gases.

5.2 While developing the periodic table, there were a few instances where Mendeleev had to place an element having slightly greater atomic mass before an element with a slightly lower atomic mass. The sequence was introduced so that elements with similar properties could be grouped together. Further, Mendeleev left some gapes in his periodic table, instead of looking upon these gaps as defects, Mendeleev boldly predicted the existence of some elements that were not discovered at that time. It was success of Mendeleev prediction that led chemists not only to accept his periodic table but also recognise him as the originator of the concept on which it is based. Noble gases (inert gages) like Helium (He), Neon (Ne) & Argon (Ar) have been mentioned in may a context before this. These gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. The strength of Mendeleev’s periodic table was that when inert gases were discovered they could be placed in a new group without disturbing the existing order so arranged.

5.3    Limitations of Mendeleev’s Periodic classification:

(a) He could not assign a correct position of hydrogen in his periodic table, as the properties of hydrogen resembles both with alkali metals as well as with halogens.

(b) The isotopes of the same element will be given different position if atomic number is taken as basis, which will disturb the symmetry of the periodic table.

(c) The atomic masses do not increase in a regular manner in going from one elements to the next.

As a result, it was not possible to predict how many elements could be discovered between two elements.

5.4    Questions (Page: 85)

Q-1: Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements: K, C, Al, Si, Ba.

Ans: The oxides of the above elements are given below:

K: K₂O, C: C₂O₄ or CO₂ Al: Al2O₃ Si: Si₂O₄  or SiO_2, Ba₂O₂

Q-2: Besides Gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (Any two)

Ans: Germanium and Scandium are the element that are left by Mendeleev in his Periodic Table since its discovery.

Q-3: What were the criteria used by Mendeleev in creating his Periodic Table?

Ans: Mendeléev concentrated on various compounds formed by the elements with Hydrogen and Oxygen. Among physical properties, he observed the relationship between the atomic masses of various elements while creating his periodic table.

Q-4: Why do you think the noble gases are placed in a separate group?

Ans: Noble gases are placed in a separate group because of their stable nature and low concentration in atmosphere. They are kept in a separate group called Zero group so that they don’t disturb the existing order.

6.1    Modern long form of periodic table: The modern long form of periodic table is classified by Henery Moseley in 1913. He gave away the principle of modern long form of periodic table. The H. Moseley’s principle is:

“The physical & chemical properties of an element are period function of their Atomic

Number”.

6.2    H. Mosley arranged the elements according to Atomic Number in ascending order. This periodic table takes care of all three limitations of Mendleev’s periodic table. The modern long form of periodic table has 18 vertical columns called groups & 7 horizontal rows called periods. The elements are arranged in such a informative way to know their maximum properties and its make simple to predict the exact position of any element.

6.3    Process to find out the position of an element in Modern long form of periodic table: The elements are placed on the basis of their electronic configuration. Total number of orbits exhibit number of period and valence electron exhibit number of group. This may be understand by the following example:

Position of Chlorine (Cl): Electronic configuration of CL is 2,8,7. Here total number of orbit is 3 & valence electron is 7. Hence, CL is placed in 7^th group of 3^rd period.

7.1     Properties of Modern Periodic Table:

(a) Valency: The valency of any element can be known by number of group it is placed in and also the valence electron can easily be known.

(b) Atomic Size: the atomic size is known as it’s radius which is defined as the distance between the nucleus & the outermost orbit of the element. The atomic size down the periodic table increases as new orbits are added whereas it decreases as number of orbit is same but valence electron increases and hence effective nuclear charge increases and radius decreases.

(c) Metallic & Non-metallic properties: The metallic properties decreases from left to right and non-metallic properties increases. The metallic character decreases across a period and non-metallic character increases.

The Modern Periodic table at a glance

Questions: (Page-90)

Q-1: How could the Modern Periodic Table remove various anomalies of Mendeléev’s Periodic Table?

Ans: The modern periodic table removes various anomalies of Mendeleev’s period table in following ways:

(a) In the Modern Periodic table elements are arranged in the increasing order of their atomic number. This removes the anomaly regarding certain pairs of elements in Mendeleev’s periodic table.

(b) Atomic number of Cobalt is 27and Nickel is 28. Hence Cobalt will come before Nickel even though its atomic mass is greater.

(c) All Isotopes of the same elements have different atomic masses, but same atomic number, therefore they are placed in the same position in the modern periodic table.

Q-2: Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?

Ans: The two elements are Calcium and Beryllium which are similar to Magnesium because all the three elements belong to the same group and have 2 valence electrons in their outer shell.

Q-3: Name

(a) three elements that have a single electron in their outermost shells.

(b) two elements that have two electrons in their outermost shells.

(c) three elements with filled outermost shells

Ans: (a) Lithium(Li), Sodium (Na) and Potassium(K) have single electron in their outermost shells.

(b) Magnesium(Mg) and Calcium (Ca) have two electrons in their outermost shells.

(c) Neon(Ne), Argon(Ar) and Xenon (Xe) fill outermost shells.

Q-4:     (a) Lithium, sodium, potassium are all metals that react with water to

liberate hydrogen gas. Is there any similarity in the atoms of these

elements?

(b) Helium is an unreactive gas and neon is a gas of extremely low

reactivity. What, if anything, do their atoms have in common?

Ans: (a) They have one electron in their outermost shells hence, they are very unstable. So, they readily react with water to liberate hydrogen. They are also called Alkali metals.

(b) Their outermost shells are full leading to high stability. They react only in extreme circumstances and hence are called noble gases.

Q-5: In the Modern Periodic Table, which are the metals among the first ten elements?

Ans: Lithium and Beryllium are the metals among the first ten elements in Modern Periodic Table.

Q-6: By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic? Ga Ge As Se Be

Ans: Among the elements listed in the question. Be and Ga are expected to be most metallic. Out of Be and Ga, Ga is bigger in size and hence has greater tendency to lose electrons than Be. Therefore, Ga is more metallic than Be.

Exercise (Page: 91)

Q-1: Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.

(a) The elements become less metallic in nature.

(b) The number of valence electrons increases.

(c) The atoms lose their electrons more easily.

(d) The oxides become more acidic

Ans: The correct answer is (c). The atoms lose their electrons more easily.

The atoms lose their electrons more easily is a wrong statement because as we move from left to right across the periods of the periodic table, the non-metallic character increases. Hence, tendency to lose an electron decreases.

Q-2: Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as

(a) Na         (b) Mg        (c) Al        (d) Si

Ans: The correct answer is Magnesium (Mg) because Mg has the valency 2 which is same as the group (a) Na (b) Mg (c) AI (d) Si. Also Mg when combines Chloride forms MgCl₂.

Q-3: Which element has

(a) two shells, both of which are completely filled with electrons?

(b) the electronic configuration 2, 8, 2?

(c) a total of three shells, with four electrons in its valence shell?

(d) a total of two shells, with three electrons in its valence shell?

(e) twice as many electrons in its second shell as in its first shell?

Ans:

(a) Neon has two shells which are completely filled.

(b) Silicon has the electronic configuration 2, 8, 2

(c) Carbon has a total of three shells, with four electrons in its valence shell

(d) Boron a total of two shells, with three electrons in its valence shell

(e) Magnesium has twice as many electrons in its second shell as in its first shell

Q-4: (a) What property do all elements in the same column of the Periodic Table as boron have in common?

(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?

Ans: (a)All the elements which lie in me same column as that of boron belong to group 13. Therefore, they have three electrons in their respective valence shells. Except, boron which is a non-metal, all other elements (i.e., Aluminum, Gallium, Indium and Thallium) in this group are metals.

(b) All elements in the same column of the Periodic Table as fluorine have in common three electrons in their valence shell and they all are belong to group thirteen.

Q-5: An atom has electronic configuration 2, 8, 7.

(a) What is the atomic number of this element?

(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)

N(7) F(9) P(15) Ar(18)

Ans:

(a)The element with electronic configuration (2+8+7) 17 is chlorine.

The No. of Atomic Number = No. of electrons. Hence, atomic number is 17.

(b) An atom with electronic configuration 2, 8, 7 would be chemically similar to F(9)

Q-6: The position of three elements A, B and C in the Periodic Table are shown below –

Group 16                           Group 17

–                                          –

–                                          A

                                             –                                          –

B                                          C

(a) State whether A is a metal or non-metal.

(b) State whether C is more reactive or less reactive than A.

(c) Will C be larger or smaller in size than B?

(d) Which type of ion, cation or anion, will be formed by element A?

Ans:

1. Element A is a non-metal.
   
2. Element C is less reactive than Element A.
   
3. C is smaller in size than B.
   
4. A will form anion.
   

Q-7: Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?

Ans: The Atomic number of Nitrogen is 7 hence Electronic configuration of Nitrogen is 1s² 2s² 2p3.

The Atomic number of Nitrogen is 15 hence Electronic configuration of Phosphorous is 1s²2s²2p⁶3s²3p^3.
On moving down a group in the periodic table, the number of shell increases. Because of which valence electrons move away from the electrons and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electro negativity decreases. Because of all these reasons Nitrogen is more electronegative than phosphorus.

Q-8: How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?

Ans: The number of valence electrons decides an atom’s position in the periodic table while the electronic configuration decides the number of valence electrons.

Q-9: In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these have physical and chemical properties resembling calcium?

Ans: Calcium has an atomic number of 20, and thus has an electronic configuration of 2, 8, 8, 2. Thus, calcium has 2 valence electrons. The electronic configuration of the element having atomic number 12 is 2, 8.2. Thus, this element with 2 valence electrons resemble calcium the most.

10. Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

Ans: The comparison is tabulated below:

—