Which element is NOT part of the essential elements needed for life?

Chlorine is not considered one of the essential elements needed for life in the same way that carbon, oxygen, and nitrogen are. Essential elements are those that are critical for the biological processes necessary for life.

Carbon is the backbone of organic molecules and forms the structure of carbohydrates, proteins, lipids, and nucleic acids. Oxygen is vital for cellular respiration, a process through which organisms convert nutrients into energy. Nitrogen is a key component of amino acids and nucleotides, which are essential for proteins and nucleic acids, respectively.

Chlorine, while it has some roles, such as in maintaining fluid balance in organisms and in the synthesis of some compounds, is not universally required for all forms of life and is not a fundamental building block in biological macromolecules. Thus, its role does not equate to the necessity of carbon, oxygen, and nitrogen in the fundamental processes of life.