Question

(c) A part of the periodic table is shown below with one element missing: Based on the above table, answer the following questions: (i) Name the element that has duplet structure. (ii) Name the lightest alkali metal. H He Li Be B C N O F Ne Na Mg Al Si – S Cl Ar(iii) Name the halogen of period 2. (iv) Identify the missing element​

Answer 1

How are elements arranged in the periodic table?

• In modern periodic table, elements are arranged in increasing order of atomic number, based on periodicity in property.
• Elements are arranged in horizontal rows called Periods and vertical columns called Groups.

What are Alkali metals and Halogens?

• Alkali metals refer to Group 1 of the periodic table, which all have valence electrons = 1. They include elements like H, Li, Na and K.
• Halogens refer to Group 17 (or group 7A) of the periodic table which all have valence electrons=7. They include elements like Cl, Br and I.

We can observe for this question,

1. Duplet structure will be shown by a single Noble gas called Helium, which has electronic configuration as 2. Rest all other noble gases have octet configuration.
2. Lithium will be the lightest alkali metal (in Group 1) since Hydrogen is not a metal. Lithium occurs as the first metal in Group 1
3. In period 2, we have Fluorine (F) as the halogen with atomic number 9. It has a configuration of $2,7$ and lies in Group 17.
4. Phosphorus (P) occurs in between Silicon (Si) and Sulphur (S) so it is the missing element here.