Class 10 - Chemistry - Periodic Classification of Elements

Question 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.

Answer:

Correct Option: - (c) The atoms lose their electrons more easily.

As we move from left to right across the periods of the periodic table, the non-metallic character increases.

Therefore tendency to lose an electron decreases.

Question 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) AI                (d) Si

Answer:

Correct Option: - (b) Magnesium (Mg)

Element X forms a chloride with the formula XCl2, so its valency is 2.

This shows X belongs to group 2 as it has 2 electrons in its valence shell. So X is Mg.

 

Question 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?

Answer:

  • Neon is the element whose both of shells are completely filled with electrons.

Electronic configuration of Neon is (2, 8) and the atomic number is 10.

Therefore it is Neon as both its K and L shells are completely filled. Neon is an inert gas.

  • Magnesium Mg has the electronic configuration of 2, 8, and 2. This shows total number of electrons is 12.
  • Also Atomic number of atom = total number of electrons present in the atom.
  • Therefore, atomic number of given element is equal to 12. And element having atomic number equal to 12 is Magnesium.
  • Thus, given element is Magnesium.
  • Silicon is the element. Silicon has a total of three shells, with four electrons in its valence shell
  • (2 electrons in K shell, 8 electrons in L shell and 4 electrons in M shell).
  • Boron is the element. As it has 2 shells, this means K and L shells are present. 2 electrons in the valence shells.
  • Electronic configuration = 2, 5. Atomic number = 5. The element which has atomic number 5 is Boron.
  • Carbon has twice as many electrons in its second shell as in its first shell (2 electrons in K shell and 4 electrons in L shell). Electronic configuration: 2, 4

This, means total number of electrons = 6

Also Atomic number of atom = total number of electrons present in the atom

Thus, atomic number of given element = 6

The element having atomic number equal to 6 (five) is Carbon.

 

Question 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?

Answer:

  • All the elements in the same column as boron have the same number of valence electrons (3).
  • Hence, they all have valency equal to 3.
  • They all have similar number of valence electrons (7).
  • They all are highly electronegative elements and all are non-metals.

 

 

Question 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)

Answer:

  • Atomic number of this element = 17.

Also Atomic number of atom = total number of electrons present in the atom.

Thus, atomic number of given element is equal to 17. And element is Chlorine.

  • It would be chemically similar to F (9) with configuration as 2, 7.

Electronic configuration of Ni (7) = 2, 5

Thus, valence electron = 5

Electronic configuration of F (9) = 2, 7

Thus, valence electron = 7

Electronic configuration of P (15) = 2, 8, 5

Thus, valence electron = 5

Electronic configuration of Ar (18) = 2, 8, 8

Thus, valence electrons = 8 = 0 (outermost shell is completely filled).

Electronic configuration of given element = 2, 8, 7

  • Valence electrons = 7.

As valence electrons of F (9) are equal to 7 therefore it will have similar chemical property to the given element Cl (7).

 

Question 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 is 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. As A lies in 17th group therefore it is a non-metal. All the non-metals belong to 17th group.

(b)C is less reactive than A, as reactivity decreases down the group in halogens. As we move down the group the number of shells increases.

As C has more number of shells so the distance between the valence electrons and nucleus is more. This shows element C is less electronegativity than A.

(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.

 

 Question 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?

Answer:

Electronic Configuration of Nitrogen (7) = 2, 5

Electronic configuration of Phosphorous (15) = 2, 8, 5

Nitrogen is more electronegative than that of phosphorous because electronegative character of an element

decreases by moving down the group and nitrogen belong to 2nd period and phosphorous belong to 3rd period.

This means phosphorous is one step down in the group than that of nitrogen.

 

Question 8:

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

Answer:

Electronic configuration tells about the number of valence electrons and number of shells present in the atom of an element.

Number of valence electrons decides the group number.

Number of shells decides the period number of an element in the Modern Periodic Table.

For Example:

Electronic configuration of Mg (12) = 2, 8, 2

Valence electron = 2

Number of shells = 3

Thus, magnesium lies in second group and third period in periodic table.

Question 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?

Answer:

Elements with atomic number 12 and 38 have physical and chemical properties same as calcium as they all belong to same group i.e. second group.

Electronic configuration of Calcium (20) = 2, 8, 8, 2

  • valence electrons = 2, Group they belong = second

Electronic configuration of element having atomic number 12 = 2, 8, 2

  • valence electrons = 2, Group = Second

 Electronic configuration of element with atomic number 19 = 2, 8, 8, 1

  • Valence electron = 1, Group = First

Electronic configuration of element with atomic number 21 = 2, 8, 8, 3

  • Valence electron = 3, Group = Third

Electronic configuration of element having atomic number 38 = [Kr], 2

  • Valence electron = 2, Group = Second

Since, elements having atomic number 12 and 38 belong to same group i.e. second same as calcium,

therefore they all have similar physical and chemical properties.

 

Question 10:

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

Answer:

Comparison between Mendeleev’s Periodic Table and the Modern Periodic Table.

           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.  Position of hydrogen not explained

3. Position of hydrogen explained clearly

4.  Elements having similar properties were placed directly under one another.

4. 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.

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.

 

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