Many sources define Qi as "vital energy", but I think that explanation is too simplistic. According to Chinese Acupuncture and Moxibustion edited by Cheng Xinnong, a common textbook for acupuncture students, Qi is a fundamental substance that helps, "maintain the normal vital activities of the human body." Additionally, this text further states that, "according to ancient Chinese thought, Qi was the fundamental substance constituting the universe, and all phenomena were produced by changes and movement of Qi."
I remember a eureka moment one of my classmates had during my second year of acupuncture school, which was about 18 years ago. During a lecture, my classmate raised her hand and the instructor called on her. We had been discussing how blood transports Qi throughout the body, to which she asked, "so does that mean Qi could be oxygen?"
It was a stunning moment for me because it was the first time we were overlapping Eastern and Western medical terminology. The instructor acknowledged that Qi could be oxygen but wouldn't go so far as to say that all Qi was oxygen.
Then years ago I read articles and heard discussions among the acupuncture community about whether or not Qi is nitric oxide. Some made very strong arguments, especially given that nitric oxide is an essential substance of the body much like Qi, but even in those arguments nobody was willing to definitively make that statement.
Perhaps the reason for the hesitancy to settle in on a specific, tangible substance to label Qi comes from the fact that there's also different kinds of Qi. There's Yuan Qi, also known as primary Qi, which is rooted in the Kidneys and spreads throughout the body to promote the fundamental activities of the organs. Zong Qi is formed by combining Qing Qi, which is inhaled from the lungs, and Qi from food. Ying Qi comes from the Qi from food and its function is to produce blood and circulate with it. Wei Qi, also known as defensive or protective Qi, also comes from the Qi from food but circulates outside of the vessels to protect the somatic parts of the body.
If your head is spinning from reading that last paragraph, I think you understand the dilemma regarding translating Qi into Western terms. It's simply not that simple.
However, I think it's relatively safe to state that Qi is molecules and atoms, or perhaps specifically ions. Atoms frequently gain or lose electrons, which turns them into ions and causes them to be positively or negatively charged. Molecules are made up of atoms bound together because of these charges, but these bonds form and break all the time. For those who enjoy organic chemistry, diving into the different kinds of Qi and what molecules and atoms may be involved may be a fun project.
From my lens as a practicing acupuncturist, I like viewing Qi as Qi was explained in Chinese medicine using Chinese medical terminology, but I can't help but think back to my classmate from 18 years ago who asked if Qi is oxygen. I wonder about these things when I have a patient experiencing chronic muscle cramps and if their body is a little deficient in either calcium or potassium....are these ions Wei Qi? As far as I understand, that remains to be confirmed.