Question

An element X forms a unipositive ion with the electronic structure 2, 8, 8. What is the atomic number of X?
A. 16
B. 17
C. 18
D. 19

Answer 1

Hint: the elements in the periodic table are arranged according to the atomic numbers. Atomic number of any element is the total number of electrons which are equal to the total number of protons in that atom. The electronic structure or configuration of elements consist of filling of electrons in the shells K, L, M, and N as 2, 8, 18 and 32 maximum electrons respectively.

Complete answer:
Atomic number of any element is represented as ‘Z’ which is the total number of protons also equal to the total number of electrons in that atom. The electronic structure of atoms has a filling of electrons in K, L, M and N shells as 2, 8, 18 and 32 maximum electrons; this also defines the atomic number.
We have been given a unipositive ion X with electron structure 2, 8, 8. We have to find its atomic number. A unipositive ion means an atom having a positive 1 charge that is a cation with +1 charge. This means it has donated one of its electrons to form a unipositive cation. Therefore, it must have only one electron in its outer shell and the given configuration is its noble gas configuration. So, it must have an electronic structure as 2, 8, 8, 1. This makes the total electrons to be 19.
Hence, the atomic number of element X that forms a unipositive ion with the electronic structure 2, 8, 8 is 19.

So, option D is correct.

Note:
The atoms that have 1 electron in their valence shell are the metals. They are highly reactive due to 1 or 2 electrons in their valence shell. Metal with atomic number 19 is potassium (K). It loses one electron to achieve the nearest noble gas stable configuration and forms a cation as ${{K}^{+}}$. It is a highly reactive metal.