Question 13 Marks
Halogens or group 17 elements.
Answer
View full question & answer→(i) The second last column in the periodic table is group 17 (VII – A) which contains halogens.
(ii) The members of this group from top to bottom are as given in the following table:
(iii) The valence shell of a halogen contains seven electrons.
(iv) Thus, it needs one electron to complete its octet.
(v) The halogen complete their octet and attain the stable inert gas configuration by gaining one electron.
(vi) Hence, the valency of halogens is one, i.e. they are monovalent
(vii) All of them have general formula $X_2$. Fluorine $\left( F _2\right)$ and Chlorine $\left( Cl _2\right)$ are gases. Bromine $\left( Br _2\right)$ is liquid while Iodine $\left(I_2\right)$ is solid.
(ii) The members of this group from top to bottom are as given in the following table:
| Group 17 elements | Symbol | Atomic Number (Z) | Electronic configuration | Valency |
| Fluorine | F | 9 | 2, 7 | 1 |
| Chlorine | Cl | 17 | 2, 8, 7 | 1 |
| Bromine | Br | 35 | 2, 8, 18, 7 | 1 |
| Iodine | I | 53 | 2, 8, 18, 18, 7 | 1 |
| Astatine | At | 85 | 2, 8, 18, 32, 18, 7 | 1 |
(iv) Thus, it needs one electron to complete its octet.
(v) The halogen complete their octet and attain the stable inert gas configuration by gaining one electron.
(vi) Hence, the valency of halogens is one, i.e. they are monovalent
(vii) All of them have general formula $X_2$. Fluorine $\left( F _2\right)$ and Chlorine $\left( Cl _2\right)$ are gases. Bromine $\left( Br _2\right)$ is liquid while Iodine $\left(I_2\right)$ is solid.