An element with atomic number 18 is a noble gas. Into which family you shall place elements with atomic number 17 and 19 and why?

Explanation:

An element with atomic number 18 is a noble gas.

The element with atomic number 19 will have one extra electron so it can donate one electron or shed one electron to attain noble gas configuration .

It tends to shed one electron to become positively charged ion . Hence they are electro-positive elements .  

Similarly the element with atomic number 17 will have one short  electron so it can gain  one electron  to attain noble gas configuration .

They  tend to gain  one electron to become negatively charged ion . Hence they are electro-negative  elements . They have high  electronegativity .

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.