Answer
$^{35}_{17}Cl^{+}$
Work Step by Step
An element can be represented as $\,^{A}_{Z}X$ where $A$ is its mass number, $Z$ is the atomic number and $X$ is its symbol.
$Z$= number of protons=17
$A$= number of protons+ number of neutrons=17+18=35
The symbol of the element with Z=17 is Cl (chlorine).
In the neutral state, the number of protons and electrons are equal. But here one electron is lost to become a positive ion. Therefore, we can represent the element as
$^{35}_{17}Cl^{+}$
