Metals and non-metals can be told apart by their physical properties. Metals are usually shiny, strong and solid at room temperature, with high melting points. They are good conductors of heat and electricity, and they are malleable, which means they can be hammered into shape without breaking. Most solid non-metals are dull and brittle, so they shatter when hit. They have lower melting points than metals and are poor conductors, so they are insulators. Many non-metals, such as oxygen and chlorine, are gases at room temperature. Graphite, a form of carbon, is an exception because it conducts electricity.
They also differ in their chemical properties. When a metal atom reacts it loses electrons and forms a positive ion. Non-metal atoms gain electrons to form negative ions, or they share electrons with other atoms.
Metals and non-metals both react with oxygen to form oxides. Magnesium burns in oxygen to make magnesium oxide, a metal oxide. Metal oxides are basic, and the ones that dissolve in water make alkaline solutions. Sulfur burns in oxygen to make sulfur dioxide, a non-metal oxide. Non-metal oxides are acidic, and they dissolve in water to make acidic solutions.