Here's why:

* Natural Elements: These are elements found in nature, either in their pure form or as part of compounds. They were formed through various processes in stars and supernovae.

* Synthetic Elements: These are elements created artificially in laboratories through nuclear reactions. They are not found naturally on Earth because they are either too unstable to exist for long periods or were not created in the early universe.

Examples of synthetic elements include:

* Technetium (Tc): Used in medical imaging.

* Plutonium (Pu): Used in nuclear weapons and nuclear power.

* Americium (Am): Used in smoke detectors.

Keep in mind that some elements, like uranium, are considered "naturally occurring" but have also been synthesized artificially. This is because the natural uranium found on Earth is a mixture of isotopes, some of which are unstable and decay over time.