2 marksShort AnswerSection A

(a) Determine the number of valence electrons and the valency of ‘X’. (b) Molecular formula of the compound when ‘X’ reacts with hydrogen and write its electron dot structure. (c) Name the element ‘X’ and state whether it is metallic or non-metallic. An element ‘X’ has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of ‘X’. Also write group number, period number and valency of ‘X’.

Q: (a) Determine the number of valence electrons and the valency of ‘X’. (b) Molecular formula of the compound when ‘X’ reacts with hydrogen and write its electron dot structure. (c) Name the element ‘X’ and state whether it is metallic or non-metallic. An element ‘X’ has mass number 35 and number of neutrons 18. Write atomic number and electronic configuration of ‘X’. Also write group number, period number and valency of ‘X’.

(a) **Valence electrons and Valency of 'X'**: - Number of valence electrons = $7$ - Valency of 'X' = $1$ (b) **Reaction with Hydrogen**: - Molecular formula of the compound: $\text{HCl}$ (Hydrogen chloride) - Electron dot structure: ```diagram {"xrange": [63.2, 236.8], "yrange": [-124.0, -31.0], "points": [{"x": 80.0, "y": -80.0, "label": ""}, {"x": 180.0, "y": -80.0, "label": ""}, {"x": 130.0, "y": -72.0, "label": ""}, {"x": 150.0, "y": -78.0, "label": ""}, {"x": 185.0, "y": -40.0, "label": ""}, {"x": 185.0, "y": -115.0, "label": ""}, {"x": 220.0, "y": -78.0, "label": ""}]} ``` (c) **Identification of 'X'**: - Name of the element 'X': **Chlorine** ($\text{Cl}$) - Nature: **Non-metallic** - Atomic number: $17$ - Electronic configuration: $2, 8, 7$ - Group number: $17$ - Period number: $3$ - Valency: $1$

Metals and Non-metals

Chemical Properties of Non-metals and Electronic Configuration

Official Answer

(a) Valence electrons and Valency of 'X':

Number of valence electrons = $7$
Valency of 'X' = $1$

(b) Reaction with Hydrogen:

Molecular formula of the compound: $\text{HCl}$ (Hydrogen chloride)
Electron dot structure:

Name of the element 'X': Chlorine ( $\text{Cl}$ )
Nature: Non-metallic
Atomic number: $17$
Electronic configuration: $2, 8, 7$
Group number: $17$
Period number: $3$
Valency: $1$

Chlorinenon-metallic2, 8, 7valency 1group 17period 3HClelectron dot structure

Marking Scheme

1Calculation of atomic number ( $17$ ) and electronic configuration ( $2, 8, 7$ ): 0.5 Marks
2Determination of valence electrons ( $7$ ), valency ( $1$ ), group ( $17$ ), and period ( $3$ ): 0.5 Marks
3Identification of element 'X' as Chlorine and its non-metallic nature: 0.5 Marks
4Molecular formula ( $\text{HCl}$ ) and correct electron dot structure: 0.5 Marks

Hint

Recall that Atomic Number = Mass Number - Number of Neutrons. Use the atomic number to write the electronic configuration ( $2, 8, \dots$ ) and find the valence electrons. Group number for elements with 3 or more valence electrons is $10 + \text{valence electrons}$ .

Quick Oral Answer

Element X is Chlorine. It has an atomic number of 17, electronic configuration of 2, 8, 7, and a valency of 1. It is a non-metal because it gains or shares one electron to complete its octet. When it reacts with hydrogen, it forms hydrogen chloride (HCl), which is a covalent compound.

Analysis & Explanation

This question tests the fundamental understanding of atomic structure, periodic classification, and chemical bonding. By calculating the atomic number from the mass number and neutron count, students can deduce the electronic configuration. This configuration directly reveals the position of the element in the periodic table (period and group) as well as its chemical nature (metallic vs. non-metallic). Non-metals typically have 5, 6, or 7 valence electrons and form covalent compounds with hydrogen by sharing electrons, which is represented using Lewis electron dot structures.

Common Mistakes

1Confusing valence electrons (7) with valency (1).
2Writing the group number as 7 instead of 17.
3Forgetting to show the lone pairs on Chlorine in the electron dot structure.
4Incorrectly identifying Chlorine as a metal.

Interesting Facts

Chlorine was first discovered in 1774 by Carl Wilhelm Scheele, who mistakenly thought it contained oxygen. It was Humphry Davy in 1810 who insisted it was a novel element and named it after 'chloros', the Greek word for pale green.

Despite being a highly toxic gas in its elemental form, chlorine in the form of chloride ions ( $\text{Cl}^-$ ) is essential for life and is a key component of common table salt ( $\text{NaCl}$ ).

Spotted a mistake or something unclear?

Tell us — we fix reported answers fast.

Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2016?

This question carries 2 marks in the CBSE Class 10 Science 2016 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 "Chemical Properties of Non-metals and Electronic Configuration" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2016 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2016 paper.