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I have been learning chemistry for a year or two but I still can't understand this. According to maths we should place the plus or minus sign in front of a number like -2. However, In chemistry we always put the sign behind a number when writing ions like $\ce{Ba^2+}$ ,$\ce{Cu^2+}$, $\ce{O^2-}$ etc. So, I want to know why do we write it that way?

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Chemistry has an organization called the International Union of Pure and Applied Chemistry (IUPAC), which sets standards for nomenclature (among other things). Chemists usually follow its recommendations.

IUPAC says

Ionic charge is indicated by means of a right upper index, as in $\ce{A^n+}$ or $\ce{A^n-}$ (not $\ce{A^$+n$}$ or $\ce{A^$-n$}$).

and

The iconic charge number is denoted by a right superscript, by the sign alone when the charge number is equal to plus one or minus one.
...
$\ce{Al^3+}$ is commonly used in chemistry and recommended by [74 = see quote above]. The forms $\ce{Al^$+3$}$ and $\ce{S^$-2$}$, although widely used, are obsolete [74], as well as the old notations $\ce{Al^{+++}}$, $\ce{S^=}$, and $\ce{S^{--}}$.

No reason is given, but you'll find a historical account of sign conventions for charge and a defense of IUPAC's choice in this article. He says,

[IUPAC's convention] avoids confusion with the conventional symbolism for inherently positive and negative numbers and maintains consistency in how we count physical entities. Thus, in counting apples, we say two apples, three apples, etc., not apples two, apples three – that is, the number always precedes the name of the entity being counted. Likewise, when counting charges, we should say two positive charges or three negative charges, not positive charges two or negative charges three. The IUPAC ruling was intended to make the charge number symbolism consistent with this verbal convention.

Personally, I think the charge should be written like any other signed number. Formulas typed from the keyboard without superscripting can be ambiguous; is X2+ to be interpreted as $\rm X^{2+}$ or $\rm X_2^+$? (You should type X(2+) or X2(+), but few people actually do that.) But it's pretty clear what X+2 would mean. Putting the sign first also makes it trivial to parse the charge from a formula typed this way without the need for parentheses.

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  • $\begingroup$ A previous version was quoting IUPAC talking about charges in written long names. I modified the answer with better quotes. $\endgroup$ – mhchem Nov 28 '19 at 14:30
  • $\begingroup$ I understand Jensen's defense of IUPAC, but don't agree. Of course, this is all arbitrary, but here's my view: If the Ca ion were Ca w/ 2 pos. chgs., it would make sense to write "2+" (like "2 apples"). However, that's not the case— both the ion and the neutral atom have 20 pos. chgs. Rather, the Ca ion is in a net +2 state. And to indicate this state, we first indicate what's most important (the sign)—and then, within that big category, the subcategory: the magnitude. I.e., the proper convention to apply here is what we use to indicate the value of numbers: e.g., "–3", not "3–". $\endgroup$ – theorist Nov 28 '19 at 20:55
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$\ce{Ba^{2+}}$ means that you have a Barium ion with two positive charges. so the 2 counts the positive charges and the + is not the symbol to add something, the + means only a positive charge.

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I have 2 cars, 5 bikes and 3 cats. I don't say I have cars 2, bikes 5, and cats 3.

Barium ions have 2 positive (+) charges, oxides 2 negative (-) charges. Don't say Barium has + charges 2.

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    $\begingroup$ History based customs beat logic in many areas, from chemistry to English. $\endgroup$ – Poutnik Nov 28 '19 at 20:49
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I'm adding this answer as maybe reasoning of why IUPAC proposed putting the charge symbol after the number. Consider it a supplement to the @fred-senese's answer of 'cause IUPAC says so, and the linked article.

We put the charge symbol (+ or -) after the number because charges are entities, not numbers. Placing the number before the symbol helps to keep the idea that charges (more specifically multiples of the charge on an electron) are things we are counting clear.

It is the same convention as when there is multiple of an entity in algebra. If you had two $a$'s, that is $a+a$, it is written as $2a$, not $a2$. What is confusing (and unfortunate) is that the symbols chosen, for units of charge are the same as those for positive and negative (signed) numbers; + and -.

Even though we use the positive and negative symbols for charge, they are not positive and negative numbers. In the context of charges, 2+ means two positive things, 3- means three negative things. (In the case of ions, the charge represents the difference from a net zero, neutral, charge, but that discussion is getting away from the point of the question of "why does the charge symbol come last in the subscript?".)

To get philosophical, we, historically, could have chosen any other two names for charges: red and green, apple and orange, foo and bar, Romeo and Juliet, the article linked in Fred Senese's answer proposed primes and dots as the symbols! In the end we chose positive and negative, and use the associated symbols from positive and negative numbers, + and -. We can use the arithmetic of signed numbers to do computations involving charge, balancing ions, redox, conservation of charge and so on; but, this is because the properties of charge mirror that of signed numbers, not because charges are positive and negative numbers.

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