An ion with mass number 37 possesses one unit of negative charge.

Solution:

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)

$\therefore$ Number of protons in the neutral atom $=x-1$

Given,

Mass number of the ion = 37

$\therefore(x-1)+1.111 x=37$

$2.111 x=38 x$

= 18

:The symbol of the ion is ${ }_{17}^{37} \mathrm{Cl}^{-}$.