CBSE - Periodic Classification of Elements

Only one triad of Dobereiner's triads exists in the columns of Newlands' octaves. The triad formed by the elements Li, Na, and K of Dobereiner's triads also occurred in the columns of Newlands' octaves.

Dobereiner's triads

Li Na K

Ca Sr Br

Cl Br I

Limitation of Dobereiner’s classification:
All known elements could not be classified into groups of triads on the basis of their properties.

Limitations of Newlands’ law of octaves:

→ It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.
→ Those elements that were discovered after Newlands’ octaves did not follow the law of octaves.
→ The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.
→ Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.

K is in group 1. Therefore, the oxide will be K₂O.
C is in group 4. Therefore, the oxide will be CO₂.
Al is in group 3. Therefore, the oxide will be Al₂O₃.
Si is in group 4. Therefore, the oxide will be SiO₂.
Ba is in group 2. Therefore, the oxide will be BaO.

Scandium and germanium.

Mendeleev used atomic mass of the elements as the unique criteria of the elements. He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.

oble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.

Various anomalies of Mendeleev’s Periodic Table removed as follows in the Modern Periodic Table:

→ Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.
→ In Modern Periodic Table there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.

→ Elements having same valence electron are kept in same group.
→ Elements having same number of shells were put under the same period.
→ Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.

Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements and since chemical properties are due to valence electrons, they show same chemical reactions.

(a) Lithium (Li), sodium (Na), and potassium (K) have a 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) have filled outermost shells.

(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.

(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.

Among the first ten elements, lithium (Li) and beryllium (Be) are metals.

Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.

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(c) The atoms lose their electrons more easily.

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 (b) Mg

(a) Neon
(b) Magnesium
(c) Silicon
(d) Boron
(e) Carbon

(a) Valency equal to 3.
(b) Valency equal to 1.

(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2, 7.

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

(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? 

Answer

(a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.

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?

Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases.

In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of valence electrons.

The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).

	Mendeleev's periodic table		Modern periodic table
1.	Elements are arranged in the increasing order of their atomic masses.	1.	Elements are arranged in the increasing order of their atomic numbers.
2.	There are a total of 7 groups (columns) and 6 periods (rows).	2.	There are a total of 18 groups (columns) and 7 periods (rows).
3.	Elements having similar properties were placed directly under one another.	3.	Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group.
4.	The position of hydrogen could not be explained.	4.	Hydrogen is placed above alkali metals.
5.	No distinguishing positions for metals and non-metals.	5.	Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side.

1. Upto which element, the Law of Octaves was found to be applicable?
(a) Oxygen
(b) Calcium
(c) Cobalt
(d) Potassium
Ans. (b) Calcium

2. According to Mendeleev's ′ Periodic Law, the elements were arranged in the periodic table in the order of
(a) increasing atomic number
(b) decreasing atomic number
(c) increasing atomic masses
(d) decreasing atomic masses
Ans. (c) Increasing atomic masses

3. In Mendeleev′s Periodic Table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the periodic table later?
(a) Germanium
(b) Chlorine
(c) Oxygen
(d) Silicon
Ans. (a) Germanium
Explanation: Mandeleev named the undiscovered elements as Eka-boron, Ekaaluminium and Eka-silicon. These were replaced with scandium, gallium and germanium respectively in due course of time.

4. Which of the following statement (s) about the Modern Periodic Table are incorrect?
(i) The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number
(ii) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses
(iii) Isotopes are placed in adjoining group (s) in the Periodic Table
(iv) The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number
(a) (i) only
(b) (i), (ii) and (iii)
(c) (i), (ii) and (iv)
(d) (iv) only
Ans. (b) (i), (ii) and (iii).

Explanation: Atomic mass is not a criterion in Modern Periodic Table. Hence, option (ii) is incorrect. Isotopes were given the same position in Periodic Table, hence option (iii) is incorrect.

5. Which of the following statements about the Modern Periodic Table is correct:
(a) It has 18 horizontal rows known as Periods
(b) It has 7 vertical columns known as Periods
(c) It has 18 vertical columns known as Groups
(d) It has 7 horizontal rows known as Groups
Ans. (c) It has 18 vertical columns known as groups
Explanation: Modern Periodic Table has 18 Groups and 7 periods. Columns are called groups, while rows are called periods

6. Which of the given elements A, B, C, D and E with atomic number 2, 3, 7, 10 and 30 respectively belong to the same period?
(a) A, B, C
(b) B, C, D
(c) A, D, E
(d) B, D, E
Ans. (b) B, C, D
Explanation: Element with atomic number 2 belongs to the first period. Elements from 3 to 10 belong to the second period.

7. The elements A, B, C, D and E have atomic number 9, 11, 17, 12 and 13 respectively. Which pair of elements belong to the same group?
(a) A and B
(b) B and D
(c) A and C
(d) D and E
Ans. (c) A and C
Explanation: Fluorine (9) and Chlorine (17) belong to the same group.

8. Where would you locate the element with electronic configuration 2, 8 in the Modern Periodic Table?
(a) Group 8
(b) Group 2
(c) Group 18
(d) Group 10
Ans. (c) Group 18
Explanation: In this element, the outermost shell is completely filled. Hence, this belongs to Group 18.

9. An element which is an essential constituent of all organic compounds belongs to
(a) Group 1
(b) Group 14
(c) Group 15
(d) Group 16
Ans. (b) Group 14
Explanation: Carbon belongs to group 14

10. Which of the following is the outermost shell for elements of period 2?
(a) K shell
(b) L shell
(c) M shell
(d) N shell
Ans. (b) L shell
Explanation: In period 2, there are 2 shells, i.e. K and L
11. Which one of the following elements exhibit maximum number of valence
electrons?
(a) Na
(b) Al
(c) Si
(d) P
Ans. (d) P
Explanation: Electronic configuration of Na: 2, 8, 1
Al: 2,8,3 Si: 2,8,4 and P: 2,8,5
Phosphorus has 5 valence electrons, while others have less number of valence electrons.

12. Which of the following gives the correct increasing order of the atomic radii of O, F and N?
(a) O, F, N
(b) N, F, O
(c) O, N, F
(d) F, O, N
Ans. (d) F, O, N
Explanation: Atomic radius decreases as we move from left to right across a period. N(7), O(8) and F(9) are in ascending order of atomic numbers.

13. Which among the following elements has the largest atomic radii?
(a) Na
(b) Mg
(c) K
(d) Ca
Ans. (d) Ca
Explanation: Na K and Ca belong to the same group, i.e. Group I. Out of these, calcium is at the bottom and hence has the largest atomic radius.
 

14. Which of the following elements would lose an electron easily?
(a) Mg
(b) Na
(c) K
(d) Ca
Ans. (b) Na
Explanation: Na, K and Ca belong to the same group, i.e. Group I. But Na is at the top among them. Hence, sodium would lose an electron more easily compared to others. Mg belongs to groups II and hence would not lose electrons as easily as Na.

15. Which of the following elements does not lose an electron easily?
(a) Na
(b) F
(c) Mg
(d) Al
Ans. (b) F
Explanation: Sodium has 1, Mg has 2 and Al has 3 electrons in outermost orbits. But F has 7 electrons in its outermost orbit. Hence, F does not lose an electron easily.

16. Which of the following are the characteristics of isotopes of an element?
(i) Isotopes of an element have same atomic masses.
(ii) Isotopes of an element have same atomic number.
(iii) Isotopes of an element show same physical properties.
(iv) Isotopes of an element show same chemical properties.
(a) (i), (iii) and (iv)
(b) (ii), (iii) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: Isotopes of an element have same atomic number but different atomic masses. They show same chemical properties but different physical properties.

17. Arrange the following elements in the order of their decreasing metallic character
Na, Si, Cl, Mg, Al
(a) Cl > Si >Al > Mg >Na
(b) Na >Mg >Al >Si > Cl
(c) Na > Al > Mg > Cl > Si
(d) Al > Na> Si > Ca> Mg
Ans. (b) Na>Mg>Al>Si>Cl
Explanation: Sodium has 1, Mg has 2 and Al and 3, Si has 4 electrons in outermost orbits. But Cl has 7 electrons in its outermost orbit. Hence, sodium shows the maximum character of metallic element and chlorine shows the character of non-metallic element.
It is important to remember that electropositive elements are metallic in character.

18. Arrange the following elements in the order of their increasing non-metallic character Li, O, C, Be, F
(a) F < O < C < Be < Li
(b) Li < Be < C < O< F
(c) F < O < C < Be < Li
(d) F < O < Be < C < Li
Ans. (b) Li < Be < C < O < F
Explanation: Li is at the extreme left of 2nd period; followed by Be. Fluorine is at extreme right of the second period; just before Neon. Hence, F shows the maximum character of a non-metal. Li and Be are metals.

19. What type of oxide would Eka– aluminium form?
(a) EO3
(b) E3O2
(c) E2O3
(d) EO
Ans. (c) E2O3
Explanation: Eka-aluminium is in the same group as Aluminum, i.e. Group 13. Hence, there are 3 electrons in its outermost orbit. So, it can lost 3 electrons to form a bond. Oxygen has 6 electrons in its outermost orbit which means it can gain 2 electrons to form a bond.

20. Three elements B, Si and Ge are
(a) metals
(b) non-metals
(c) metalloids
(d) metal, non-metal and metalloid respectively
Ans. (c) Metalloids
21. Which of the following elements will form an acidic oxide?
(a) An element with atomic number 7
(b) An element with atomic number 3
(c) An element with atomic number 12
(d) An element with atomic number 19
Ans. (a) An element with atomic number 7
Explanation: Element with atomic number 7 has following electronic configuration: 2,5. This means, it can gain 3 electrons to make a bond and this is an electronegative element. So, this a non-metal. Non-metals make acidic oxide.

22. The element with atomic number 14 is hard and forms acidic oxide and a covalent Halide. To which of the following categories does the element belong?
(a) Metal
(b) Metalloid
(c) Non-metal
(d) Left-hand side element
Ans. (b) Metalloid
23. Which one of the following depicts the correct representation of atomic radius(r) of an atom?

(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Ans. (b) (ii) and (iii)
Explanation: The distance between nucleus and outermost orbit is called atomic radius.In fig (iii), it is quite clear. In fig (ii), only the first orbit contains electron and hence this fig shows correct depiction of atomic radius.

24. Which one of the following does not increase while moving down the group of the
periodic table?
(a) Atomic radius
(b) Metallic character
(c) Valence
(d) Number of shells in an element
Ans. (c) Valence
Explanation: Valence of an element remains the same in a group.

25. On moving from left to right in a period in the periodic table, the size of the atom.
(a) increases
(b) decreases
(c) does not change appreciably
(d) first decreases and then increases
Ans. (b) Decreases
Explanation: Atomic radius decreases when we move from left to right in a period. Hence, size of atom also decreases.

26. Which of the following set of elements is written in order of their increasing metallic character?
(a) Be Mg Ca
(b) Na Li K
(c) Mg Al Si
(d) C O N
Ans. (a) Be Mg Ca
Explanation: These elements are in same group but Be is at the top, while Ca is at
bottom. Metallic character increases as we move down the group.

Such an arrangement of elements is called as triad. Triad of elements was proposed by Dobereiner. He arranged elements in triads. In a triad, the atomic mass of the middle element was equal to the average of atomic massed of the other two elements. Lithium, Sodium and Potassium make a triad. Atomic massed of Li, Na and K are 6.9, 23.0 and 39.0 respectively. Average of masses of Li and K is approximately 23.0 which is equal to the atomic mass of Na

Elements have been arranged in the following sequence on the basis of their increasing atomic masses. F, Na, Mg, Al, Si, P, S, Cl, Ar, K
(a) Pick two sets of elements which have similar properties.
 Ans. Na, Mg, Al and K are metals, while F and Cl are halogens. These make two sets of
elements.
(b) The given sequence represents which law of classification of elements?
 Ans. Mendeleev’s Law of Periodicity 

Can the following groups of elements be classified as Dobereiner's triad?
(a) Na, Si, Cl
 Ans. No, because all these elements do not have similar properties although the atomic mass of silicon is average of atomic masses of sodium (Na) and chlorine (Cl).
(b) Be, Mg, Ca
Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40. Explain by giving reason.
Ans. Yes, because they have similar properties and the mass of magnesium (Mg) is roughly the average of the atomic mass of Be and Ca

There are instances in Mendeleev’s Periodic Table when an element with higher atomic mass is placed before and element with lower atomic mass. This was done to ensure that elements with similar chemical properties were put in the same group. Due to this, cobalt was placed before nickel in spite of the fact that cobalt has higher atomic mass than nickel.

Hydrogen occupies a unique position in Modern periodic Table; because of various reasons. From the context of atomic number, hydrogen occupies the correct position in Modern Periodic Table. However, hydrogen shows properties similar to elements of Group 1, IV and VII. Hydrogen shows the behavior of both electropositive and electronegative elements. Hence, it can be said that the position of hydrogen is still not decided in the Periodic Table.

Chloride of Eka-silicon: ECl₄
 Chloride of Eka-aluminium: ECl₃ 

A belongs to Group 13, B belongs to Group 14 and C belongs to Group 2. Valency of A is 3, that of B is 4 and that of C is 2.

If an element is placed in group 14, then it has 4 electrons in its outermost orbit. Hence, the formula of its chloride is ECl₄. Since the element can make compound by sharing of electrons, so its compound will have covalent bond.

Compare the radii of two species X and Y. Give reasons for your answer.
(a) X has 12 protons and 12 electrons
(b) Y has 12 protons and 10 electrons
Ans. The atomic radius of X is greater than that of Y. While, X is the element with equal number of protons and electrons, Y is its ion after the loss of two electrons. For the same element, atomic radius of ion is less than that of the electroneutral element. 

Arrange the following elements in increasing order of their atomic radii.
(a) Li, Be, F, N
 Ans. Li >Be>F>N
 These elements are in same period and are arranged from left to right.
(b) Cl, At, Br, I
 Ans. Cl<Br<I<At
 These elements are in same group and are arranged from top to bottom. 

Identify and name the metals out of the following elements whose electronic
configurations are given below.
(a) 2, 8, 2
 Ans. Magnesium
(b) 2, 8, 1
 Ans. Sodium
(c) 2, 8, 7
 Ans. Non-metal
 (d) 2, 1
 Ans. Lithium 

The formula of product formed by A and B is AB.
 Electronic configuration of A: 2, 8, 8, 1
 Electronic configuration of B; 2, 8, 7
 They would make compound by forming ionic bond. Electronic dot structure is as follows: 

Ge<Ga<Mg<Ca<K 

(a) An element which is a soft and reactive metal
Ans. Sodium
(b) The metal which is an important constituent of limestone
 Ans. Calcium
(c) The metal which exists in liquid state at room temperature
 Ans. Mercury 

(a) A soft metal stored under kerosene
 Ans. Sodium Group 1, Period 3
(b) An element with variable (more than one) valency stored under water.
 Ans. Phosphorus Group 15, Period 3
(c) An element which is tetravalent and forms the basis of organic chemistry
 Ans. Carbon Group 14, Period 2
(d) An element which is an inert gas with atomic number 2.
 Ans. Helium Group 18, Period 1
(e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of “anodising”
 Ans. Aluminum Group 13, Period 3 

(a) Identify the element
 Ans. Magnesium
(b) Write the electronic configuration
 Ans. 2, 8, 2
(c) Write the balanced equation when it burns in the presence of air
 Ans. 2 Mg +  O₂ → 2 MgO
(d) Write a balanced equation when this oxide is dissolved in water
 Ans. MgO + H₂O → Mg(OH)₂
(e) Draw the electron dot structure for the formation of this oxide
 Ans. Electron dot structure for the formation of magnesium oxide. 

(a) Where in the periodic table are elements X and Y placed?
Ans. X is in Group 17, period 3 and Y is in group 2, period 4
(b) Classify X and Y as metal (s), non-metal (s) or metalloid (s)
 Ans. X is a non-metal, While Y is a metal.
(c) What will be the nature of oxide of element Y? Identify the nature of bonding in the
compound formed
 Ans. Oxide of element Y would be basic. This compound is formed by ionic bond.
(d) Draw the electron dot structure of the divalent halide
 Ans.

(a) Identify the elements
 Ans. Neon (10), calcium (20), Nitrogen (7) and Silicon (14)
(b) Identify the Group number of these elements in the Periodic Table
 Ans. Neon belongs to group 18, calcium belongs to group 2, nitrogen belongs to group 7 and silicon belongs to group 14.
(c) Identify the Periods of these elements in the Periodic Table
 Ans. Nitrogen and Neon belong to period 2. Calcium and Silicon belong to period 3.
(d) What would be the electronic configuration for each of these elements?
 Ans. Ne: 2, 8
 Ca: 2, 8, 8, 2
 N: 2, 5
 Si: 2, 8, 4
(e) Determine the valency of these elements
 Ans. Ne: O, Ca: 2, N: 3 and Si: 4 

Across:
(1) An element with atomic number 12.
 Ans. Magnesium
 (3) Metal used in making cans and member of Group 14.
 Ans. Tin
(4) A lustrous non-metal which has 7 electrons in its outermost shell.
 Ans. Iodine.
 
Down:
(2) Highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene
 Ans. Sodium
(5) The first element of second Period
 Ans. Lithium 
 (6) An element which is used in making fluorescent bulbs and is second member of Group 18 in the Modern Periodic Table
 Ans. Neon
(7) A radioactive element which is the last member of halogen family.
 Ans. Astatine
(8) Metal which is an important constituent of steel and forms rust when exposed to moist air.
 Ans. Iron
(9) The first metalloid in Modern Periodic Table whose fibres are used in making bulletproof vests
 Ans. Boron 

(a) In this ladder (Figure 5.2) symbols of elements are jumbled up. Rearrange these symbols of elements in the increasing order of their atomic number in the Periodic Table.

 Ans. H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca
 
(b) Arrange them in the order of their group also. Fig. 5.2
 Ans. Group 1: H, Li, Na< k
 Group 2: Be, Mg, Ca
 Group 13: B, Al
 Group 14: C, Si
 Group 15: N, P
 Group 16: O, S
 Group 17: F, Cl
 Group 18: He, Ne, Ar 

(a) Name the elements which have taken the place of these elements
 Ans. Eka-silicon was replaced by Germanium and Eka-aluminium was replaced by Gallium.
(b) Mention the group and the period of these elements in the Modern Periodic Table.
 Ans. Gallium: Group 13 Period 5
 Germanium: Group 14 period 5 
(c) Classify these elements as metals, non-metals or metalloids
 Ans. Gallium is metal, while Germanium is metalloid
(d) How many valence electrons are present in each one of them?
 Ans. Gallium has 4 valence electrons, while Germanium has 5. 

(a) Electropositive nature of the element(s) increases down the group and decreases across the period
(b) Electronegativity of the element decreases down the group and increases across the period
(c) Atomic size increases down the group and decreases across a period (left to right)
(d) Metallic character increases down the group and decreases across a period.

On the basis of the above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9.

(a) Name the most electropositive element among them
Ans. Element with 3 as atomic number, i.e. Lithium

(b) Name the most electronegative element
 Ans. Element with 9 as atomic number, i.e. Fluorine
 
(c) Name the element with smallest atomic size
 Ans. Fluorine
 
(d) Name the element which is a metalloid
 Ans. Boron (5)
 
(e) Name the element which shows maximum valency.
 Ans. Carbon, valency 4

An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants.
(a) Identify the element X
 Ans. Sulphur
 
(b) Write the electronic configuration of X
 Ans. 2, 8, 6
 
(c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals?
 Ans.2FeSO₄ →  Fe₂O₃ +SO₂ + SO₃ 
 
(d) What would be the nature (acidic/ basic) of oxides formed?
 Ans. Oxides of Sulphur are acidic in nature.
 
(e) Locate the position of the element in the Modern Periodic Table.
 Ans. Group 16, Period 3 

 (a) Identify the element X. How many valence electrons does it have?
Ans. Nitrogen, valence electrons=5

(b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it?
Ans. Electronic dot structure is as follows. Covalent bond is formed in it.

(c) Draw the electron dot structure for ammonia and what type of bond is formed in it?
Ans. Covalent bond is formed

Inert gases could be placed in Mendeleev’s Periodic Table without disturbing the original order.

 Noble gases; like Helium, Neon and Argon had been mentioned by various scientists even before Mendeleev. But these gases could be grouped together as noble gases much later than Mendeleev’s discovery. Mendeleev used 63 elements till his periodic table because these were the only known elements till his time. However, he was bold enough to leave gaps in his periodic table. He had predicted that that more elements would be discovered in times to come.
 
 Mendeleev’s Periodic Table was the first comprehensive attempt to classify elements on the basis of their chemical properties. This is the main reason that new elements could be easily placed in Mendeleev’s Periodic Table without disturbing the original order.

During Mendeleev’s time, only 63 elements were known. Mendeleev examined the correlation between chemical properties and Atomic masses of elements. For this, he concentrated on compounds which were formed by the elements with hydrogen and oxygen. He selected hydrogen and oxygen, because these elements make compound with most of the other elements.

 Mendeleev made 63 cards and wrote the name of an element on each card; along with the chemical properties of a particular element. Then, he pinned those cards on a wall. He could observe that most of the elements could be arranged in periodic table in increasing order of their atomic masses. That is how Mendeleev arrived at “Periodic Law”.
 
Mendeleev’s Periodic Law states that properties of elements are periodic function of their atomic masses.