Question

Question 2.43 An ion with mass number 37 possesses one unit of negative charge. If the ion contains 11.1% more neutrons than the electrons, find the symbol of the ion.

Class XI Structure of Atom Page 67

Answer 1

Let the number of electrons in the ion carrying a negative charge be x.

Then,

Number of neutrons present

= x + 11.1% of x

= x + 0.111 x

= 1.111 x

Number of electrons in the neutral atom = (x – 1)

(When an ion carries a negative charge, it carries an extra electron)

∴ Number of protons in the neutral atom = x – 1

Therefore 37 = 1.111x + x -1

Or 2.111 x = 38

Or x = 18

Therefore no of protons = atomic no = x -1 = 18 – 1 = 17

Answer 2

Let the number of electrons in an ion = x
so, the number of electron in neutral atom = x -1 [ because ion have one negative charge ]
because in neutral atom ,
number of protons = number of electrons
so, number of protons in neutral atom = x -1

now, A/C to question,
number of neutrons = number of electrons + 11.1 % of number of electrons
= x + 111x/1000 = 1.111x

we know,
mass number = number of protons + number of neutrons
37 = x -1 + 1.111x
37 + 1 = 2.111x
38/1.111 = x = 18.0009 ≈ 18 ( approx )
Number of protons = x -1 = 18 -1 = 17
hence, atomic number = 17 , it means Cl.
therefore , symbol of the ion is ³⁷₁₇Cl⁻.