Question

Question:A certain element has only two naturally occurring isotopes: one with 18 neutrons and the otherwith 20 neutrons. The element forms 1-charged ions when in ionic compounds. Predict theidentity of the element. What number of electrons does the 1- charged ion have?

Short answer

Answer

The element name is chlorine. There are 18 electrons in negatively charged chloride ions.

Step-by-step solution

Definition of Isotopes and Mass number

The element has same atomic number but different mass number called isotopes. The sum of atomic number and neutron number equal to mass number. Mass number and atomic number can be represented as A and Z. It can be represented as,

_Z^AX

7A group elements with mass number

These group elements can form negatively charged ions with mass and atomic numbers.

Element	Atomic Number	Mass Number
F	9	19
Cl	17	35
Br	35	80
I	53	127

Adding atomic and neutron number for each element to find the element name and electrons

The elements having two isotopes. On adding atomic and neutron number to the formula,

$$\begin{gathered} \text{Mass} \text{Number} = \text{Atomic} \text{Number} + \text{No.} \text{of} \text{neutrons} \\ \text{Mass} \text{Number} \text{of} \text{F}=9+18=27 \\ \text{Mass} \text{Number} \text{of} \text{Cl}=17+18=35 \\ \text{Mass} \text{Number} \text{of} \text{Br}=35+18=53 \\ \text{Mass} \text{Number} \text{of} \text{I}=53+18=71 \end{gathered}$$

Here, only chlorine has the correct mass number. Thus, the element is chlorine and there are 18 electrons in negatively charged chloride ions.