2 marksShort AnswerSection A

(i) How do you calculate the possible valency of an element from the electronic configuration of its atoms?

(ii) Calculate the valency of an element X whose atomic number is 9.

Metals and Non-metals

Valency and Electronic Configuration

Official Answer

(i) The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom:

If the number of valence electrons is $1, 2, 3,$ or $4$ , the valency is equal to the number of valence electrons.
If the number of valence electrons is $5, 6, 7,$ or $8$ , the valency is calculated as $8 - \text{number of valence electrons}$ .

(ii) For element X with atomic number $9$ :

Electronic configuration: $2, 7$
Number of valence electrons = $7$
Valency = $8 - 7 = 1$

valence electronsoutermost shellelectronic configurationvalencyoctet

Marking Scheme

1Stating the rule for calculating valency when valence electrons are $\le 4$ and $> 4$ . (1 mark)
2Writing the electronic configuration of element X ( $2, 7$ ) and calculating its valency as $1$ . (1 mark)

Hint

Write the electronic configuration of the element with atomic number $9$ by distributing electrons in K and L shells, then subtract the outermost electrons from $8$ .

Quick Oral Answer

The valency of an element with atomic number $9$ is $1$ because its electronic configuration is $2, 7$ , and it needs only $1$ electron to complete its octet.

Analysis & Explanation

Valency represents the combining capacity of an atom to achieve a stable octet configuration. For elements with $1$ to $4$ valence electrons, it is easier to lose these electrons to achieve stability, so their valency equals their valence electron count. For elements with $5$ to $7$ valence electrons, it is easier to gain electrons ( $8 - \text{valence electrons}$ ) to complete their octet.

Common Mistakes

1Students often confuse valence electrons with valency and write the valency of element X as $7$ instead of $1$ .
2Incorrect distribution of electrons in shells.

Interesting Facts

Element X with atomic number $9$ is Fluorine, which is the most electronegative and reactive non-metal in the periodic table.

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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 "Metals and Non-metals" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Valency and Electronic Configuration" 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.