Why do oxides go from basic to acidic across a period, and why are CO and NO called neutral oxides?

I colour-coded the oxide trend for period 3 in my notes: blue for basic (Na2O, MgO), green for amphoteric (Al2O3), red for acidic (SiO2, P4O10, SO3, Cl2O7). NCERT states the trend but I want to understand the reason, because I forget lists.

Things I am stuck on:

Why does the nature change from basic to acidic as we go left to right? Is it something to do with metallic character, or the type of bond to oxygen?
Al2O3 is called amphoteric. I know it means it reacts with both acids and bases, but I can't write the equations. My brain just says Al is a metal so the oxide should be basic.
CO, NO and N2O are neutral oxides, but CO is made from carbon and dissolves nowhere. Why is "neutral" a separate category and not just "weak acidic"?
Does the trend reverse when I go down a group? I saw somewhere that B2O3 is acidic but Tl2O is basic.

Also NCERT says chemical reactivity is high at the two extremes of a period and low in the middle. Does that mean silicon is the least reactive element of period 3? That doesn't feel right.

7 answers & replies

[+]Siddharth RaoDropperSample· 1mo ago

The reason is the bond type. On the left you have metals with a big electronegativity difference from oxygen, so the oxide is ionic and contains $O^{2-}$ . That ion grabs a proton from water and gives hydroxide: $Na_2O + H_2O \rightarrow 2NaOH$ . So basic.

On the right the element is a non-metal, the oxide is covalent, and it reacts with water to give an acid: $SO_3 + H_2O \rightarrow H_2SO_4$ , $P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4$ .

In the middle the bond is in between and the oxide can behave either way, which is the amphoteric case. Metallic character decreasing along the period is the same story, just said differently. Down a group metallic character increases so oxides become more basic. Your B2O3 vs Tl2O example is correct.

[+]Ishaan Bose11thSample· 1mo ago

honestly al2o3 is basic na. aluminium is a metal, metals make basic oxides, why are we making it complicated 😭 chemistry just loves exceptions

[+]Farhan ShaikhDropperSample· 1mo ago

Ishaan, it is not an exception, it is the definition. Amphoteric means it reacts with both. Equations:

$Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O$ (acts as a base) $Al_2O_3 + 2NaOH + 3H_2O \rightarrow 2Na[Al(OH)_4]$ (acts as an acid)

Learn these two lines and you are done. ZnO and BeO are also amphoteric. And a lot of people forget that Al2O3 is amphoteric only. It is not "weakly basic", it genuinely dissolves in NaOH.

[+][–]Ananya Reddy11thSample· 1mo agoAuthor
okay writing both equations in green. so amphoteric = acts as base with acid AND acts as acid with base. thank you!!
[+][–]Ishaan Bose11thSample· 1mo ago
lol okay chemistry wins again. it dissolves in naoh, noted

[+]Lakshmi TripathiDropperSample· 1mo ago

Neutral oxides do not react with water to form acid or base and do not react with acids or alkalies either. CO, NO and N2O are the standard three in NCERT (H2O is sometimes listed as well). They are formed by non-metals, but the non-metal (C or N) is in a low oxidation state and the oxide doesn't give an acid with water, so they stay out of the acidic group. It is a classification by behaviour, not by which element it is.

[+]Harsh Gupta12thSample· 1mo ago

The reactivity thing is a U shape. Sodium is the drama queen on the left, chlorine is the drama queen on the right, and silicon in the middle is the chill friend who does nothing and has no opinions.

Serious part: reactivity is highest at both extremes (alkali metals lose electrons easily, halogens gain them easily) and lowest in the middle, where the element is neither a good electron donor nor a good acceptor. Don't say silicon is totally unreactive though, it just needs heat.

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