Question

Which elements are non-reactive?
A. Oxygen
B. Hydrogen
C. Helium
D. Carbon

Answer 1

Hint: The reactivity of an element is decided by the count of the valence electrons in an atom. The electrons occupying the valence shell of an atom are termed valence electrons.

Complete Step by Step Solution:
Let's first understand about valence shells. The shell that is the outermost one is termed the valence shell.

To check the reactivity of the given elements, we have to first write the valence shell configuration.

The element Oxygen possesses an atomic number of 8. Its configuration is 2,6. So, it can take two electrons to achieve octet and attain stability. So, Oxygen is a reactive element.
The element Hydrogen possesses the atomic number of 1. And its configuration is 1. So, it can take one electron to achieve stability. Therefore, Hydrogen is a reactive element.
The element Helium possesses an atomic number of 2. And its configuration is 2. It is already in a stable state. So, it does not undergo any reaction.

The element Carbon possesses an atomic number of 6. And its configuration is 2,4. So, it can take four electrons to achieve octet and attain stability. Therefore, it is a reactive element.
Hence, the correct answer is option C, that is, Helium.

Note: It is to be noted that all elements belonging to the 18th group of the modern periodic table such as Helium, Neon, Argon, Xenon etc. are called inert gas. They do not undergo reaction as they have completely filled the valence shell.