Hydrogen is both a metal an a non-metal. It says so on the specially produced coffee cups produced by my company (and sold by my company) but not on any other periodic table i’ve seen. That’s a shame for at least two reason. First, on a physiochemical level, while hydrogen is a metal in the sense that it combines with non-metals like chlorine and oxygen to form HCl and H2O, it’s not a metal in how it looks (not very shiny, malleable, etc.). Hydrogen acts like a chemical non-metal in the sense that it reacts with most metals to form metal hydrides like NaH CaH2 and YH3 (my company sells metal hydride getters, and metal membranes that use this property), and it also looks like a non-metal; it’s a gas like non-metallic chlorine, fluorine, and oxygen.

REB Research, Periodic table coffee cup

Most middle schoolers and high schoolers learn to differentiate metals and nonmetals by where they sit on the periodic tables they are given, and by general appearance and feel, that is by entirely non-scientific methods. Most of the elements on the left side of their periodic tables are shiny and conduct electricity reasonably well, so students come to believe that these are fundamental properties of metals without noting that boron and iodine (on the right side) are both shiny and conduct electricity, while hydrogen (presumably the first metal) does not. Students note that many metals are ductile without being told that calcium and chromium are brittle, while boron and tin (non-metals) are ductile. And what’s with the jagged dividing line: some borderline cases, like aluminum, look awfully metallic by normal standards.

The actual distinction, and the basis for the line, has nothing to do with the descriptions taught in middle school, but everything to do with water. When an element is oxidized to its most common oxide and dissolved in water the solution will be either acidic or basic. This is the basis of the key distinction: we call something a metal if the metal oxide solution is basic. We call something a non-metal if the oxide solution is an acid. To make sulfuric acid or nitric acid: you dissolve the oxides of sulfur or nitrogen respectively, in water. That’s why nitrogen and sulfur are nonmetals. Similarly, since you make boric acid by dissolving boron oxide in water boron is a non-metal. Calcium is a metal because calcium oxide is lime, a strong base. Aluminum and antimony are near borderline cases, because their oxides are nearly neutral.

And now we return to hydrogen and my cup. hydrogen is the only element listed as both a metal and a non-metal because hydrogen oxide is water. It is entirely neutral. When water dissolves in water the pH is 7; by definition, hydrogen is the only real borderline case. It is not generally shown that way, but it is shown as a metal and a non metal is on a cup produced by my company.