2 marksShort AnswerSection A

Consider the following elements: $4\text{Be}, 9\text{F}, 19\text{K}, 20\text{Ca}$ . Choose from the following: (i) The element having one electron in the outermost shell. (ii) Two elements of the same group.

Metals and Non-metals

Electronic Configuration and Periodic Trends

Official Answer

(i) The element having one electron in the outermost shell is Potassium ( $19\text{K}$ ).

(ii) The two elements belonging to the same group are Beryllium ( $4\text{Be}$ ) and Calcium ( $20\text{Ca}$ ).

Potassium19KBeryllium4BeCalcium20Caoutermost shellsame groupvalence electrons

Marking Scheme

11 mark
21 mark

Hint

Write down the electronic configuration of each element based on its atomic number. Elements with the same number of valence electrons belong to the same group.

Quick Oral Answer

Potassium has one electron in its outermost shell, and Beryllium and Calcium belong to the same group because both have two valence electrons.

Analysis & Explanation

The chemical properties and group placement of elements are determined by their valence electrons. Potassium ( $19\text{K}$ ) is an alkali metal (as mentioned in the text) and has a single valence electron, making it highly reactive. Beryllium ( $4\text{Be}$ ) and Calcium ( $20\text{Ca}$ ) both have two valence electrons, placing them in Group 2 (alkaline earth metals). Fluorine ( $9\text{F}$ ) has seven valence electrons and belongs to Group 17 (halogens).

Common Mistakes

1Confusing the atomic number with mass number.
2Writing incorrect electronic configurations, especially for Potassium ( $19\text{K}$ ) by putting $9$ electrons in the third shell instead of $8, 1$ .

Interesting Facts

Potassium is so reactive that it is stored under kerosene oil to prevent it from reacting with atmospheric moisture and oxygen, as described in the NCERT text.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 2 marks in the CBSE Class 10 Science 2012 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 "Electronic Configuration and Periodic Trends" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.