All metals form basic oxides. Metallic oxides are basic in character.

Polonium is a metal, so it should form basic oxides but it forms acidic oxides. Why are its oxides acidic in character?

  • $\begingroup$ Polonium is actually a metalloid, and not strictly speaking classified as a metal… $\endgroup$ – F'x Oct 5 '12 at 7:34

All metals form basic oxides.

That's true. I cannot remember metal, that does not form basic oxide

Metallic oxides are basic in character.

Not necessary. Polonium is not the only one metal, forming acidic oxides. Mn, Cr and some others do it as well.

Oxide is considered acidic, when it forms hydroxide, that is acidic (well, it is simplification, but let's stick with it for a while). Hydroxide is acidic, when it can lose proton, giving stable anion. Acidity of hydroxide and metallicity of element formed it does not connect any way. Acidity depends of ability of remaining part of molecule to stabilize negative charge, which usually correlate with $\ce{O/OH}$ ratio and electronegativity of central atom. So, $\ce{H6TeO6}$ is quite weak acid and $\ce{H2CrO4}$ or $\ce{HMnO4}$ are quite quite strong acids.

  • $\begingroup$ Isn't O^2- extremely unstable? I mean I can understand why O would want 2 more electrons but it doesn't really want to be ionic and for example Mg^2+ O^2- would be an ionic oxide(an oxide since O is more electronegative than Mg and ionic since the 2 have opposite charges) and oxygen wants to form covalent bonds like it does in Iron Oxide, not ionic bonds like a chloride would so ionic bonds to oxygen anions or cations are extremely unstable $\endgroup$ – Caters Jul 28 '14 at 20:47
  • $\begingroup$ @caters It isn't related to the topic, but in general there is no purely ionic or covalent compounds. Some oxides, like $\ce{MgO}$, have significant ionic bonding, but just as well there is no chloride that does not have significant covalent bonding. In addition, there is so known lattice energy, an energy, freed as result of bonding of ions into lattice. It is quite big and may be enough to stabilize some unstable ions. $\endgroup$ – permeakra Jul 29 '14 at 5:50
  • $\begingroup$ MgCl2 has ionic bonding, not covalent. Same with NaCl. These are all ionic chlorides. $\endgroup$ – Caters Jul 31 '14 at 11:47
  • $\begingroup$ @caters CsCl, most ionic of all chlorides, has only 75% of ionic bonding and 25% of covalent bonding. $\endgroup$ – permeakra Jul 31 '14 at 12:44

Another way of looking at this is to understand that the structure is always


where $\ce{M}$ is the metal atom. If the $\ce{O-H}$ bond is weaker than the $\ce{M-O}$ bond, the material acts as an acid. Otherwise it acts as a base.

What determines the relative strength is a bit complex. Suffice it to say that it depends on oxidation state of the $\ce{M}$ atom as well as the constituents in the rest of the molecule and the environment all play a role.

Some materials are amphoteric. That is they can split either way. A typical example is aluminum hydroxide, $\ce{Al(OH)_3}$, which has three $\ce{O-H}$ bonds. It is usually a base, but under the right conditions it breaks the other way and is a weak acid called aluminic acid. The formula for that is usually written as $\ce{H3AlO_3}$, but the structure is the same.

  • 1
    $\begingroup$ not all metal oxides are this way. For example iron oxides have this structure: FeOx where x is the number of oxygen atoms. No hydrogens here. and yes oxides do tend to be basic When there are hydrogens in the oxide. Otherwise they tend to be acidic(which is how come CO2 acidifies water instead of making it more basic) $\endgroup$ – Caters Jul 28 '14 at 20:51

While virtually all metals from a basic oxide or amphoteric (reacts with both bases and acids) oxide like aluminium and both Tin(II) and Tin(IV), some also form acidic oxides like Chromium(VI) oxide, Iron(II) oxide. With oxides its better to describe they according to their reactivity with acids and bases as you are not trying to convert them to the hydrate.

Its not incredible for Polonium to form only an acid oxide.


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