Question 20 - Past Year - 3 Mark Questions - Chapter 5 Class 10 - Periodic Classification of Elements

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’.

Answer

Mass number of X=35

Number of neutrons=18

• Atomic number= mass number-number of neutrons = 35 - 18 = 17
• Therefore, the electronic configuration of X: 2, 8, 7
• Number of valence electrons = 7.

         Group number = 10 + Number of electrons in outermost shell(valence electrons) = 10 + 7 = 17

• Since there are 3 shells, it belongs to the third period.
• The valency of X will be 8-7= 1