Final answer:
Neon and helium are noble gases and exist as individual atoms in nature due to their full outer electron shell, which makes them chemically inert. They do not form molecules because they do not need to interact with other atoms to achieve stability. This is unlike reactive halogens such as fluorine, which do not exist freely in nature.
Step-by-step explanation:
The question asks about substances that do not exist as molecules in nature. Neon and helium, being noble gases, are examples of elements that exist as individual atoms rather than molecules. Noble gases are unique as they possess a complete outer electron shell, which makes them chemically inert and unlikely to form compounds or molecules under normal conditions. In contrast, other elements like halogens are very reactive and do not exist freely in nature due to their tendency to form compounds with other elements, such as salts.
Neon and helium are found in their atomic form in nature, not bonded to other atoms. This is because noble gases have a full outer shell of electrons, providing them with a stable electronic configuration, which minimizes their reactivity. Unlike halogens, which are highly reactive and interact violently with other substances, noble gases do not readily form molecules or compounds, thus existing in their pure atomic form. This fundamental concept is crucial in understanding the nature of chemical bonds and molecular formation.