Which element is NOT classified as a trace element?

Oxygen is classified as a major element rather than a trace element because it is essential in far larger amounts for biological processes. In living organisms, it plays a critical role in cellular respiration and is a key component of water, which is vital for all biochemical reactions.

Trace elements, on the other hand, are required by organisms in very small quantities, typically less than 0.01% of body weight. They include minerals such as iron, which is crucial for oxygen transport in the blood, and magnesium and nickel, which also play important roles in various enzymatic reactions. Trace elements are still essential, but their need is significantly less when compared to major elements like oxygen, carbon, nitrogen, and hydrogen.

Given this distinction, oxygen stands out as not being a trace element, as it is abundant and needed in much larger quantities for sustaining life.