A copper atom has 29 electrons around its nucleus. There are 2 in the K shell, 8 in the L shell, 18 in the M shell and 1 in the N shell.
Copper mainly forms the doubly charged positive ion \mathrm{Cu}^{2+}, but it also forms compounds in which it has a charge of +1, such as copper(I) oxide, \mathrm{Cu_2O}.
Copper does not form ions readily, so it does not dissolve in hydrochloric acid or dilute sulfuric acid, but it does dissolve in nitric acid and in hot concentrated sulfuric acid.
Solutions containing \mathrm{Cu}^{2+} are blue. The blue crystals of copper(II) sulfate, \mathrm{CuSO_4 \cdot 5H_2O}, turn white when heated and lose their water, and turn blue again when water is added.
When a little ammonia solution is added to a solution containing \mathrm{Cu}^{2+}, pale blue copper(II) hydroxide is precipitated. When more ammonia solution is added, the precipitate dissolves to give a deep blue solution containing the tetraamminecopper(II) ion, [\mathrm{Cu(NH_3)_4}]^{2+}.
Copper is the best conductor of electricity and heat among the metals after silver. It is cheaper than silver, so it is widely used for electric wires, the wiring of electronic parts, pans and more.
It is also soft and easily drawn out into thin wires.
The copper made from ore is called crude copper, and it contains impurities.
When electrolysis is carried out in copper(II) sulfate solution with crude copper as the anode and pure copper as the cathode, the copper of the anode dissolves as \mathrm{Cu}^{2+}, and pure copper is deposited on the cathode. Purifying a metal by electrolysis in this way is called electrolytic refining, and it gives copper that is at least 99.99 % pure.
Gold and silver in the crude copper form ions less readily than copper, so they do not dissolve; they collect below the anode as anode slime.
Copper is also used in alloys, mixed with other metals. Bronze, an alloy of copper and tin, is used for Japan's 10-yen coin and for statues; brass, an alloy of copper and zinc, for the 5-yen coin and for brass instruments; and cupronickel, an alloy of copper and nickel, for the 50-yen and 100-yen coins.
Over many years, a blue-green layer of corrosion called verdigris forms on the surface of copper.