When two or more elements bond together in specific proportions, is formed. a compound an ion a metal an atom

a compound

Explanation:

Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Copper

Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass. It can be decomposed into simpler constituents using chemical reactions. Example: Water

Ion is a specie formed when a neutral atom gains or lose electrons.

Metal is an element which lose electrons to attain stability. Example:

Atom is the smallest unit of matter which is composed of electrons, protons and neutrons.

Thus when two or more elements bond together in specific proportions, a compound is formed.  

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