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An ion is formed when an atom gains or loses an electron. An atom which has lost one or more electrons has a positive charge. The protons remain the same

what is an ion, hydrogen ions
[Image source: Jkwchui – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12617370 Retrieved from https://en.wikipedia.org/wiki/Ion July 6, 2017]

in number but are no longer balanced by a full complement of electrons. The atom has become a positively charged ion. If, on the other hand, an atom has gained an excess of electrons, it has become a negatively charged ion.

Metal atoms tend to lose an electron (or electrons) to become positive ions. Non-metals tend to gain an electron (or electrons) to become negatively-charged ions.

Ions with opposite charges attract each other, and ions of the same charge repel each other. Because they are charged, ions which travel through a magnetic field are deflected by it.

Ions can occur in gases and liquids, but tend to rapidly join with ions of the opposite charge and neutralize each other.

Metal Ions

ions & free electrons in a piece of metal
Metal ions in a sea of electrons. [Image source: Jkwchui – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=12617370 Retrieved from https://en.wikipedia.org/wiki/Ion July 6, 2017]

Metals are a different story. As shown in the accompanying diagram, ions form bonds with each other in a geometric pattern, called a “lattice.” The ions are shown in blue with a central black nucleus. The outermost electrons of each atom are not tightly bonded within their nuclei. The electrons fly loose and form a jittering sea of electrons not bonded with any particular ion. This leaves the blue/black ions with a positive charge.

The loose electrons are called “free electrons” or “conductivity electrons.” They are responsible for the good heat and electrical conductivity of metals. They also play a key role in magnetism.

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