Why is hydrogen considered a non-metal?

Every source I’ve seen classifies hydrogen as a non-metal. The best reason I can think of for this is its incredibly low melting (-259°C) and boiling (-253°C) points - metals tend to be solid around room temperature. Of course, there are exceptions to this rule - mercury (MP -39°C) and francium (MP 27°C) are both considered metals. Why isn’t hydrogen also an exception?

Hydrogen usually forms positive ions, which is what metals do (it can also form negative “hydride” ions, but if I remember correctly, these are a lot less common than H+ ions). Solid hydrogen exhibits metallic properties (such as conducting electricity). It is also in Group 1 of the periodic table, along with a whole bunch of metals.

So what I’m asking is this: Apart from the low melting/boiling point, what reasons are there for hydrogen to be considered a non-metal?

I’m a bit rusty on the chemistry front, but I think metals are usually described as a lattice of positive ions in a sea of delocalised electrons. This accounts for the metal’s ability to conduct electricity.

Hydrogen forms hydrogen molecules - distinct entities which do not have a sea of delocalised electrons flying around and therefore are not very electrically conductive.

Sorry, think I misread the OP in my haste.

But even so- historically speaking my comments would seem to make sense. When these things were being classified initially, producing solid hydrogen wouldn’t have been an option since it requires some pretty extreme temperatures to make it solidify. It would be therefore natural to look at the gaseous, molecular form.

Many things will form positive ions in ionic compounds, which does not therefore make them metals. To cite the extreme case, when oxygen atoms are introduced into a fluorine atmosphere, they function as positive ions in forming the compound oxygen fluoride.

I defer to others with a more thorough knowledge of physical chemistry on this, but ISTM that the characteristic of a metal is to take on a “metallic” structure in the matter that Mr Creosote defined as the pure element or in alloy-type “compounds” with other substances of similar electronegativity.

Under conditions of appropriate pressure/temperature range, hydrogen will behave as a metal; some planetary scientists believe that the core of Jupiter is comprised largely of metallic hydrogen. However, the solid for hydrogen under cryogenic temperatures and more-or-less normal pressures is a non-metallic allotrope. (Antimony is an element which has metallic and non-metallic allotropes at quasi-normal STP ranges.)

IIRC, the liquid interior of Jupiter is believed to be metallic hydrogen, crystallized under great pressure.

I was under the impression that hydrogen had both metallic and non-metallic properties, but that since you only notice the metallic properties in the middle of jupiter, tradition lumps it on the other side. </paraphrase> But take that with a grain of salt.

It might also be explained by convention, rather than hard-and-fast definitions of what makes a metal “metallic.” That is, elements in a certain section of the periodic table are called “metals.” Those in another region are considered “non-metals.” Since the periodic table is arranged acording to atomic structure and not by physical properties, you end up with some non-intuitive groupings.

I disagree, the atomic properties that are grouped together in the periodic table are what give physical properties. Low pressures available on earth are why we don’t see metallic properties of hydrogen.