If helium does not obey the octet rule, then why do its atoms not react?

When we discuss helium's electron configuration, it’s essential to first recognize that helium is a unique case in the periodic table of elements. Normally, the octet rule guides us to understand that atoms prefer having eight electrons in their valence, or outermost, shell to achieve stability similar to that of noble gases. However, helium is already a noble gas, and an exceptional one at that. Its electron configuration is represented as 1s², which indicates that it has two electrons in the only shell it possesses, also known as the 1s shell.

This configuration is significant because helium's first shell is its valence shell, and for this shell, the maximum number of electrons it can hold is two—not eight. By having both these electrons, helium effectively achieves a full valence shell and is in its most stable form. This stability is the reason why helium does not seek additional electrons or need to share them. It’s essentially content with its pair, adhering to what we could consider the 'duet rule' instead of the octet rule.

The periodic table is a profound tool in chemistry that organizes elements in a systematic fashion based on their atomic number, electronic properties, and reactivity. It segments elements into groups that share similar properties and periods that indicate the number of shells in an atom. Helium is positioned at the top of the noble gases group, which is notorious for low reactivity due to their filled valence shells.

You might wonder why helium, with just two electrons, is placed among elements that typically have eight valence electrons. The placement signifies that helium, despite having only two electrons, has achieved a stable electron configuration and does not need to engage in reactions to alter this stability. If you look across the periodic table, you will notice that stability correlates with a lack of reactivity, and this is a trend that holds true, especially for the noble gases.

Delving into the reactivity of helium, one can see that this element is fundamentally non-reactive, or inert. To grasp this concept, consider that atoms interact to reach a state of equilibrium where their outer shells are filled to capacity, mimicking noble gas configurations. Helium is naturally in this desired state with its two electrons.

This stability precludes the need for helium atoms to form bonds with other atoms. In its elemental form, helium is a gas that does not form compounds under normal conditions. It’s this lack of reactivity that makes helium exceedingly useful in applications where chemical inertness is valued, such as in balloons, blimps, and as a protective atmosphere for arc welding. Truly, the reactivity of helium, or the lack thereof, is a testament to how atomic structure dictates chemical properties and behaviors.