Question

An element X has 7 electrons in its L shell. What is true about the element?
(I) It belongs to period 9 of a modern periodic table
(II) Its atom contains 9 protons.
(III) It has a valency of 7.
(IV) Its atoms can accept one electron to acquire noble gas configuration.
A. (I) and (II)
B. (II) and (III)
C. (III) and (IV)
D. (II) and (IV)

Answer 1

Hint: If an element has 7 electrons in its L shell, it clearly says that it has 2 electrons in its K shell. These electrons together form an atomic number which is equal to the number of protons. L shell can attain a maximum of eight electrons.

Complete step-by-step answer:
A periodic table consists of groups and periods. Groups are vertical columns and periods are horizontal rows. Number of protons increases both ways. The number of electrons an atom has can be indicated by the energy levels present KLMN.
K denotes the first shell or energy level which can accommodate only two electrons. L shell can have a maximum of eight electrons. So, if an element has 7 electrons in its L shell, it definitely has two electrons already present in its K shell. Therefore, it has a total of 9 electrons.
Since there are the same number of electrons and protons present in a neutral atom, the element X has 9 protons in its nucleus. Thus, (II) is correct.
Element having 9 electrons in total has an electronic configuration of $$1s^{2}2s^{2}2p^5$$. It can accept one electron to acquire noble gas configuration of neon and gain extra stability. Thus, (IV) is correct.
Fluorine has atomic number 9. It can form bonds by losing maximum five electrons or gaining one electron as it is a highly electronegative atom. So, it cannot have 7 as its valency. Its valency is 1. Thus. (III) is incorrect. Fluorine belongs to the halogen family, group 17 and period 2. So, (I) is also incorrect.

Hence, the correct option is (D).

Note: Elements in periodic table are characterised by their unique group and period. Every electron has four quantum numbers to describe it. These are: principal quantum number, azimuthal quantum number, magnetic quantum number and spin quantum number.