Question

1. An isotope of atom X has a mass number of 18. If the most stable ion form MgX. How many neutrons does X have.

A.) 19    B.) 10      C.) 11   D.) 18    E.) 9

2. Element Y has 17 protons. ³⁶ Y^- is isoelectronic with which atom below

            S, Al, Ar, Cl, P

3. An element with the ground state has two unpaired electrons in the 2p orbital. The element react with fluorine to form a neutral covalent compound with 20 valence electrons. Which f the following is the electron?

            S, O, Si, C, F

Answer 1

Ans 1: 10

If the stable ion of X forms MgX , then the oxidation state is -2 for X.

Since the 2 electrons are required to fill the octet , the valence electrons must be 6 , and hence the atomic number ( no. of protons) will be 8

Now mass number is 18 , so number of neutrons :

= mass number - atomic number

= 18 - 8

=10

Ans 2 :Ar

The atomic number of Y is 17 , so its neutral atom will have 17 electrons. The monovalent anion Y^- will have 18 electrons. So it will be isoelectronic with the noble gas Argon (Ar).

Ans 3 : O

The compound OF2 has one O atom and two F atoms. So the valence electrons : 6 + 7 + 7 = 20 electrons