#### Question

The atomic numbers of three elements A, B and C are given below:

Element |
Atomic number |

A | 5 |

B | 7 |

C | 10 |

(i) Which element belongs to group 18?

(ii) Which element belongs to group 15?

(iii) Which element belongs to group 13?

(iv) To which period/periods do these elements belong?

#### Solution

(i) Element C belongs to group 18.

Electronic configuration of element C (10) = 2,8

Therefore, group number = valence shell + 10

= 8+10 = 18.

(ii) Element B belongs to group 15.

Electronic configuration of element B (15) = 2,8,5

Therefore, group number = valence shell + 10

= 5+10 = 15.

(iii) Element A belongs to group 13.

Electronic configuration of element A (5) = 2,3

Therefore, group number = valence shell + 10

= 3+10 = 13.

(iv) The elements A, B and C all belong to period 2 because all three have the same valence shell, *i.e,* L.