Periodic Classification of Elements

NCERT Exercise Solution

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: (c) The atoms lose their electrons more easily

Explanation: When going from left to right across the periods of Periodic Table the tendency to lose electrons by atoms decreases. This happens because of increase in nuclear charge which tends to pull the electrons closer to the nucleus and because of increase in nuclear charge the tendency to lose electrons by an atom decreasing.

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

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

Answer: (b) Mg

Explanation: Since, element X forms a chloride with the formula XCl₂. This means the valency of the element is 2 (two), i.e. it has valence electron equal to 2(two) and is a metal.

And Mg (Magnesium) is a metal having valence electron equal to 2(two).

Thus, given element X is placed in the same group as Mg of the Periodic Table.

Question: 3. Which element has

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

Answer: Neon

Explanation: First shell of an atom is K and second shell is L.

The maximum number of K shell can have equal to 2(two). And the maximum number of L shell can have equal to 8.

Thus, total number of electrons = 2 + 8 =10

We know that, Total number of electrons= Atomic number

Thus, given element has atomic number equal to 10.

Thus, the given element is neon and neon has two shells which are completely filled. Since neon has completely filled outer shell thus, it is a inert gas.

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

Answer: Magnesium (Mg)

Explanation: Since electronic configuration is 2, 8. 2. This means total number of electrons is equal to 12.

We know that Atomic number of atom = total number of electrons present in the atom

Thus, atomic number of given element is equal to 12. And element having atomic number equal to 12 is Magnesium.

Thus, given element is Magnesium.

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

Answer: Silicon

Explanation: Since given element have total three shells, this means K, L and M shells are present.

And given there are four (4) electrons in valence shell.

This means electronic configuration: 2, 8, 4

This, means total number of electrons = 14

We know that Atomic number of atom = total number of electrons present in the atom

Thus, atomic number of given element = 14

The element having atomic number equal to 14 (fourtreen) is Silicon.

Thus, given element is Silicon.

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

Answer: Boron

Explanation: Since given element have total two shells, this means K and L shells are present.

And given there are two (2) electrons in valence shell.

This means electronic configuration: 2, 3

This, means total number of electrons = 5

We know that Atomic number of atom = total number of electrons present in the atom

Thus, atomic number of given element = 5

The element having atomic number equal to 5 (five) is Boron.

Thus, given element is Boron.

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

Explanation: Since given element have total two shells, this means K and L shells are present.

The total number of electrons K shell (1^st shell) can have equal to 2

And as given, second shell has twice as many electrons in its first shell.

Therefore, number of electrons in second shell = 2 x number of electrons in first shell

This means total number of electrons in second shell = 2 x 2 = 4

This means electronic configuration: 2, 4

This, means total number of electrons = 6

We know that 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.

Thus, given element is Carbon.

Question: 4.

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

Answer: They all have same number of valence electrons.

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

Answer:

(a) They all have similar number of valence electrpns.

(b) They all are highly electronegative elements

(c) They all are non-metals.

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

(a) What is the atomic number of this element

Answer: Atomic number of element = 17

Explanation:

Here total number of electrons = 17

We know that 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

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

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

Answer: F(9)

Explanation:

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 configuarion of Ar(18) = 2, 8, 8

Thus, valence electrons = 8 = 0

[If outermost orbit is completely filled, then valence electrons become equal to zero]

Now, electronic configuration of given element = 2, 8, 7

Thus, valence electrons = 7

Here, since valence electron of F(9) is also equal to 7 which equal to the given element, thus F(9) have similar chemical property to the given element Cl(17).

Question: 6. The position of three elements A, B and C in the Periodic Table are show below:

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

Answer: A is a non metal.

Explanation: Since, given element A lies in group 17^th thus it is non metal because elements belong to group 17^th are non metals.

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

Answer: C is less reactive than A

Explanation: C is down in the group than A. By going down in a group number of shell increases. This means C has more number of shells than A. Since, C has more number of shells than A this means valence electrons of C is farther than A from the nucleus. And due to the more distance of valence electrons; element C has less electonnegativity than A.

Thus, A is more reactive than C.

Explanation: 2. Electronegative and electropositive character of an element decreases by going down in a group.

Since, C has two steps down position in given group 17, than A, thus C is less reactive than A.

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

Answer: C is smaller in size than B.

Explanation: Atomic size decreases by moving from left to right in a period. Since C is one position right in period, thus C is smaller in size than B.

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

Answer: Anion is formed by A.

Explanation: Since A is in the group 17, thus it has tendency to lose one electron. And after losing an electron it will form A^–, which is an anion (negatively charged ion).

This is as similar to chlorine (17), which forms Cl^– by losing one electron.

Question: 7. Nitrogen (atomic number 7) and phosphorous (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 and why? 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 2^nd period and phosphorous belong to 3^rd 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 gives the number of valence electrons and number of shells present in the atom of an element.

Number of valence electrons decides the group number and number of shells decides the period number of an element in the Modern Periodic Table.

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 number 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 resembling calcium as they all belong to same group i.e. 2^nd.

Explanation:

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

Thus, valence electrons = 2

∴ group number = second (2^nd)

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

Thus, valence electrons = 2

∴ group number = second (2^nd)

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

Thus, Valence electron = 1

∴ group number = first (1^st)

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

Thus, valence electron = 3

∴ group number = Third (3^rd)

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

Valence electron = 2

∴ group number = second (2^nd)

Since, elements having atomic number 12 and 38 belong to same group, i.e. second group as calcium belong, thus they all have similar physical and chemical properties.

Question: 10. Compare and contrast the arrangement of elements in Mendeleev's Periodic Table and Modern Periodic Table.

Answer:

Contrast comparison of Mendeleev's Periodic Table and Modern Periodic Table