Fluorine naturally exists in the Earth's atmosphere.

Fluorine is an important element for dental health.

Foot baseball is a very popular sport among children.

The associative law is one of the basic laws of mathematics.

Metallic bonds are formed by ions floating in a sea of electrons.

An isotope refers to elements with the same atomic number but different numbers of neutrons.

Rare earth elements are essential materials for the manufacture of electronic devices.

Transition elements play an important role in chemical reactions.

Transitive verbs require an object that is directly moved by the subject.

His body showed a rejection reaction to the transplanted organ.