Metal
A metal is an element that is typically shiny, malleable, ductile, and a good conductor of heat and electricity; metals lose electrons to form positive ions.
Definition
A metal is a shiny element that bends without breaking and lets heat and electricity flow through it, and most elements on the periodic table are metals. They sit on the left and center of the table, have luster, conduct heat and electricity well, and are malleable (can be hammered into sheets) and ductile (can be drawn into wires); they have few valence electrons and lose them easily to form positive ions, so they react with nonmetals to form ionic compounds and most react with acids to release hydrogen. These properties come from metallic bonding, in which valence electrons are delocalized in a "sea" shared among a lattice of cations: the mobile electrons explain conductivity, the luster (free electrons absorb and re-emit light), and the malleability (layers of cations slide without breaking directional bonds). Metals have low ionization energies and low electronegativities and act as reducing agents in reactions.
Example
Copper wires carry electricity, aluminum foil bends around food, and iron makes strong beams, and all three are metals. Zinc reacts with hydrochloric acid, $Zn + 2HCl \rightarrow ZnCl_2 + H_2$, and iron rusts with oxygen and water to form iron(III) oxide, $Fe_2O_3$, while gold is so unreactive it stays shiny for centuries. Copper conducts electricity at about $5.96 \times 10^7\text{ S/m}$, second only to silver, because each copper atom contributes one $4s$ electron to the conduction band, yet its reactivity is low: copper does not react with dilute $HCl$ because its standard reduction potential ($+0.34\text{ V}$) is above that of hydrogen.
Key Insight
Not every metal is hard; sodium can be cut with a butter knife, and mercury is a liquid at room temperature. About three quarters of all elements are metals, but only a few are common in everyday life, and iron, aluminum, and copper dominate because they are abundant and cheap to extract. The activity series ranks metals by how readily they lose electrons, from potassium and sodium at the top to gold and platinum at the bottom, and a metal higher in the series displaces one below it from solution, which is why iron nails plate with copper in copper sulfate.