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.

Do you understand the necessity of this project?

That information is unnecessary.

That information is completely unnecessary.

He thinks that war is a necessary evil.

Fish use their respiratory organs to take in oxygen underwater.

He was taken to the hospital because of dyspnea.

He hyperventilated due to tension.