The atomic structure of atoms is a nucleus made of neutrons and protons around which electrons orbit. Electrons occupy different energy levels and when an atom has an electronic configuration such that the energy levels are full the atom does not need to bond with other atoms and can exist as independent atoms. Only a few elements have these electronic configurations and fall in the periodic table under the category noble gases. Atoms of these elements exist as independent bodies and as the inter-atomic forces are very small are found as gases. Noble gases in the Earth's crust are present as pockets of gases that have been trapped in igneous rocks as they formed. Some are produced during radioactive disintegration of elements. The occurrence of these gases is primarily in the atmosphere from where they are extracted.
In light of this, it is correct to say that it is unlikely to find single atoms not bonded with others in the Earth's crust.
Alkali metals are highly reactive and exist in compounds. The reactivity increases as you move down the period (Fr being the most reactive).
Also, there are some nonmetals that do not exist as single elements in nature. A useful mnemonic depicts several elements that are diatomic or polyatomic.
HONCl BrIF P4S8
Hydrogen, oxygen, nitrogen, chlorine, bromine, iodine, and fluorine all exist as diatomic elements. (ex. `"H"_2` ).
Phosphorus exists with the subscript 4.
Sulfur exists with the subscript 8.