Ais a group of atoms that remains bonded together and behaves as a single atom in a chemical reaction. functional group radical hydrocarbon hydroxide

radical

Explanation:

A radical is group of atoms. Some common examples of radicals are the OH- (hydroxide ion), NO₃⁻ (nitrate ion) and more.  

Cu + 2HNO₃ →Cu(NO₃)₂  + H₂

In this single replacement reaction, the nitrate ion (NO₃⁻) which is the radical is replaced from nitric acid to form copper nitrate.  

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"the periodic table is arranged in order of atomic number."