Periodic Classification of Elements - Class 10th Science

NCERT Activity Solution2

Activity: 5.6.

Question:1. How do you calculate the valency of an element from its electronic configuration?

Answer: After writing electronic configuration, valence electrons of an element will be calculated.

And valency of an element = valence electrons of the element.

Explanation

Valency is the combining capacity.

Every atom is trying to complete its octate in its outermost orbit to make it stable.

And atoms complete their octate by either losing electron or by the gain of electrons depend upon situation.

Atoms which have less than 4 electrons in their outermost orbit, generally complete their octate by loosing of electrons. And atoms having more than 4 electrons in their outermost orbit generally have the tendency to complete their octate by the gain of electrons.

The number of electrons required to complete the octate of an atom is called its valency.

But atoms of element fall in 1st and 2nd group of periodic table get stable after making duet, that means 2 electrons in their outermost orbit.

Example

(1) The atomic number of carbon is 6.

Hence electronic configuration of carbon is 1s² 2s² 2p²

Or, electronic configuration of Carbon = 2, 4

Thus, number of electrons in outermost orbit in carbon atom = 4

Thus, valence electron of carbon = 4

Thus, carbon requires 4 more electrons to complete its octate.

Thus, valency of carbon = 4

(2) The atomic number of Oxygen is 8

Hence electronic configuration of Oxygen is 1s² 2s² 2p⁴

Or, electronic configuration of Oxygen = 2, 6

Thus, number of electrons in outermost orbit in Oxygen atom = 6

Thus, valence electron of Oxygen = 6

Thus, Oxygen requires 2 more electrons to complete its octate.

Thus, valency of Oxygen = 2

Question:2. What is the valency of magnesium with atomic number 12 and sulphur with atomic number 16?

Answer:

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

Valence electron of Magnesium = 2

Thus, valency of Magnesium = 2

Electronic configuration of Sulphur (16) = 2, 8, 6

Valence electrons of Sulphur (S) = 6

Thus, valency of sulphur = 2

Since suphur requires 2 more electrons to complete its octate

Question:3. Similary find out the valency of first twenty elements.

Answer:

(1) Electronic configuration of Hydrogen (Atomic number: 1) = 1

Valence electrons of Hydrogen = 1

Thus, Valence of hydrogen = 1

(2) Electronic configuration of Helium (Atomic number: 2) = 2

Valence electrons of Helium (He) = 0

[Because it has completely filled outermost orbit]

Thus, valency of Helium (He) = 0

(3) Electronic configuration of Lithium (Atomic number: 3) = 2, 1

Valence electrons of Lithium (Li) = 1

Thus, valency of Lithium = 1

(4) Electronic configuration of Beryllium (Atomic number: 4) = 2, 2

Valence electrons of Beryllium (Be) = 2

Thus, valency of Beryllium (Be) = 2

(5) Electronic configuration of Boron (Atomic number: 5) = 2, 3

Valence Electrons of Boron (B) = 3

Thus, valency of Boron (B) = 3

(6) Electronic configuration of Carbon (Atomic number: 6) = 2, 4

Valence Electrons of Carbon (C) = 4

Thus, valency of Carbon (C) = 4

(7) Electronic configuration of Nitrogen (Atomic number: 7) = 2, 5

Valence Electrons of Nitrogen = 5

Thus, valency of nitrogen = 5 or 3

[Because nitrogen is a electronegative element, thus it can accept 3 electrons, hence valency of nitrogen may be 5 or 3. Many elements have variable valency about which you will study in higher classes.]

(8) Electronic configuration of Oxygen (Atomic number: 8) = 2, 6

Valence Electrons of of Oxygen(O) = 6

Since oxygen is an electronegative element thus, it completes its octet by gains of electrons. Since oxygen has two electron less to complete its octet.

Thus, valency of oxygen is equal to 2 (two)

(9) Electronic configuration of Fluorine (Atomic number: 9) = 2, 7

Valence electrons of fluorine = 7

Since Fluorine is an electronegative element thus, it completes its octet by gains of electrons. Since, fluorine has one electron less to complete its octet.

Thus, valency of fluorine is equal to 1 (one)

(10) Electronic configuration of Neon (Atomic number: 10) = 2, 8

Valence Electrons of Neon (Ne) = 0

[Since, neon has outermost orbit completely filled, thus its valence electron will become equal to zero]

Thus, valency of Neon (Ne) = 0

(11) Electronic configuration of Sodium (Na: Atomic number: 11) = 2, 8, 1

Valence electrons of Sodium (Na) = 1

Thus, valency of Sodium (Na) = 1

(12) Electronic configuration of Magnesium (Mg: Atomic number = 12) = 2, 8, 2

Valence electrons of Magnesium (Mg) = 2

Thus, valency of Magnesium = 2

(13) Electronic configuration of Aluminium (Al: Atomic number = 13) = 2, 8, 3

Valence electrons of Aluminium (Al) = 3

Thus, valency of Aluminium = 3

(14) Electronic configuration of Silicon (Si) [Atomic number = 14] = 2, 8, 4

Valence electrons of Silicon (Si) = 4

Thus, valency of silicon = 4

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

Valence electrons of Phosphorous (P) = 5

Since phosphorous can gain three electrons to complete its octet.

Thus, valency of Phosphorous (P) = 5 or 3 (Variable valency)

(16) Electronic configuration of Sulphur (Atomic number = 16) = 2, 8, 6

Valence electrons of Sulphur (S) = 6

Since, sulphur completes it octet by accepting two electrons,

Thus, valency of Sulphur (S) = 2

(17) Electronic configuration of Chlorine (Atomic number = 17) = 2, 8, 7

Valence electrons of Chlorine (Cl) = 7

Chlorine completes its octet by gain of one electrons as it is an electronegative element.

Thus, valency of Chlorine (Cl) = 1

(18) Electronic configuration of Argon (Atomic number = 18) = 2, 8, 8

Number of electrons in outermost orbit = 8

Since, it has completely filled outermost orbit, and elements having completely filled outermost orbit have valence electrons equal to zero.

Thus, valence electrons of Argon (Ar) = 0

Thus, valency of Argon (Ar) = 0

(19) Electronic configuration of Potassium (Atomic number = 19) = 2, 8, 8, 1

Valence electrons of Potassium (K) = 1

Thus, valency of Potassium (K) = 1

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

Valence electrons of Calcium (Ca) = 2

Thus, valency of Calcium (Ca) = 2

Question:4. How does the valency vary in a period on going from left right?

Answer: Valency first increases upto 4 and then decreases on going from left to right in a period.

Question:5. How does valency vary in going down a group?

Answer: On going down in a group valency of an element remains same.

Activity: 5.7.

Question:1. Atomic radii of the elements of the second period are given below:

Arrange them in decreasing order of their atomic radii.

Answer: Elements are arranged in the decreasing order of their atomic radii in the table below:

Question:2. Are the elements now arranged in the pattern of a period in the Periodic Table?

Answer: Yes

Question:3. Which elements have the largest and the smallest atoms?

Answer: Lithium has the largest atom and oxygen has the smallest atom.

Question:4. How does the atomic radius changes as you go from left to right in a period?

Answer: Atomic radius decreases on going from left to right in a period.

Activity: 5.8.

Question:1. Study the variation in the atomic radii of first group elements given below and arrange them in an increasing order.

Answer: Elements are arranged in the increasing order of their atomic radii in the table below:

Question:2. Name the elements which have the smallest and the largest atoms.

Answer: Caesium (Cs) has the largest atom and Lithium (Li) has the smallest atom.

Question:3. How does the atomic size vary as you go down a group?

Answer: Atomic size increases as going down a group.

Activity: 5.9

Question:1. Examine elements of the third period and classify them as metals and non-metals.

Answer: Elements of third period are Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorous (P), Sulphur (S), Chlorine (Cl), Argon(Ar)

Metals: Sodium (Na), Magnesium (Mg), Aluminium (Al)

Metalloid: Silicon (Si)

Non-metal: Phosphorous (P), Sulphur (S), Chlorine (Cl), Argon(Ar)

Out of which Argon (Ar) is an inert gas although it is classified as non-metal.

Question:2. On which side of the Periodic Table do you find the metals?

Answer: On the left side of Periodic Table metals are found.

Question:3. On which side of the Periodic Table do you find the non-metals?

Answer: On the right side of Periodic Table non-metals are found.

Activity: 5.10.

Question:1. How do you think the tendency to lose electrons will change in a group?

Answer: By moving from top to bottom the tendency to lose electrons by an atom increases.

Question:2. How will this tendency change in a period?

Answer: On going from left to right the tendency to loses electron by an atom decreases.

Activity: 5.11.

Question:1. How would the tendency to gain electrons change as you go from left to right across a period?

Answer: The tendency to gain electron increases by going from left to right across a period.

Question:2. How would the tendency to gain electrons change as you go down a group?

Answer: The tendency to gain electrons decreases on going down in a group.

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