2 marksShort AnswerSection A

How does the valency of elements vary (i) in going down a group, and (ii) in going from left to right in a period of the periodic table ?

Chemical Reactions and Equations

Valency and Periodic Trends

Official Answer

The variation of valency in the periodic table is as follows:

(i) In going down a group: The valency remains the same. This is because all elements in a particular group have the same number of valence electrons in their outermost shell.

(ii) In going from left to right in a period: The valency first increases and then decreases. Specifically, it increases from $1$ to $4$ (for groups 1, 2, 13, and 14) and then decreases from $3$ to $0$ (for groups 15 to 18).

Valence electronsOutermost shellRemains sameIncreases then decreasesCombining capacity

Marking Scheme

1Correct trend for a group (remains same) with reason. (1 mark)
2Correct trend for a period (increases then decreases) with reason. (1 mark)

Hint

Recall that valency depends on the number of electrons in the outermost shell.

Quick Oral Answer

Down a group, valency stays the same because valence electrons are the same. Across a period, it goes from 1 to 4 and then down to 0 as the shells get closer to being full.

Analysis & Explanation

Valency is determined by the number of valence electrons required to complete an octet. In a group, the electronic configuration of the outermost shell is identical (e.g., Group 1 elements all have $1$ valence electron), so valency is constant. In a period, the number of valence electrons increases from $1$ to $8$ . Elements with $1-4$ valence electrons lose or share them (valency $1-4$ ), while those with $5-7$ electrons gain or share electrons to reach $8$ (valency $8 - \text{valence electrons}$ ).

Common Mistakes

1Confusing valency with the atomic number.
2Stating that valency only increases across a period without mentioning the subsequent decrease.

Interesting Facts

Noble gases have a valency of zero because their outermost shells are completely filled, making them chemically inert under standard conditions.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2011?

This question carries 2 marks in the CBSE Class 10 Science 2011 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Valency and Periodic Trends" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2011 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.