What is Valency of number and the difference when a number is written above a symbol (superscript) or below?

Firstly, to write a formula we need to know the valency of elements. Valency is the number of electrons the element can either accept or donate. Below is a list of elements and their valencies.
$\begin{array}{ccccccccc}Elements& Symbol& Atomic& No.of& Distribution& of& electron& & Valency\\ & & Number& electron& K& L& M& N& \\ Hydrogen& H& 1& 1& 1& & & & 1\\ Helium& He& 2& 2& 2& & & & 0\\ Lithium& Li& 3& 3& 2& 1& & & 1\\ Beryllium& Be& 4& 4& 2& 2& & & 2\\ Boron& B& 5& 5& 2& 3& & & 3\\ Carbon& C& 6& 6& 2& 4& & & 4\\ Nitrogen& N& 7& 7& 2& 5& & & 3\\ Oxygen& O& 8& 8& 2& 6& & & 2\\ Fluorin& F& 9& 9& 2& 7& & & 1\\ Neon& Ne& 10& 10& 2& 8& & & 0\\ Sodium& Na& 11& 11& 2& 8& 1& & 1\\ Magnesium& Mg& 12& 12& 2& 8& 2& & 2\\ Aluminium& Al& 13& 13& 2& 8& 3& & 3\\ Silicon& Si& 14& 14& 2& 8& 4& & 4\\ Phosphorous& P& 15& 15& 2& 8& 5& & 3\\ Sulphur& S& 16& 16& 2& 8& 6& & 2\\ Chlorine& Cl& 17& 17& 2& 8& 7& & 1\\ Argon& Ar& 18& 18& 2& 8& 8& & 0\\ Potassium& K& 19& 19& 2& 8& 8& 1& 1\\ Calcium& Ca& 20& 20& 2& 8& 8& 2& 2\end{array}$
Using the valencies we can write the ions they can from. Now, ion is nothing but the atom with a positive or negative charge. If a complete molecule has charge on it we call it radical.
Example- $C{l}^{-}$ is an ion, $N{O}_3^{-}$ is a radical
The positive charge comes when the atom loses an electron. +1 means loss of 1 electron, +2 means loss of 2 electrons and so on. The charge is written on top as superscript.
The negative charge comes when the atom accepts electrons. -1 means the atom has accepted 1 electron, -2 means 2 electrons and so on.
So, now we are clear with charge, ions, and radicals. Below is a list of some common positive and negative ions.

Now, to write a formula of the compound we need to understand that positive and negative ions combine together to form a compound. Also, net charge in the compound should be zero. (addition of positive and negative ions)
Suppose we have , $M{g}^{2+}$ ion and $C{l}^{-}$ ion. Now Mg has +2 charge and Cl has -1 charge. As they are oppositely charged, they can make a compound. Now, as mentioned above net charge has to be zero.So, one $M{g}^{2+}$ ion has to combine with two Cl- ions.That makes net charge = +2-1-1 =0
Hence, formula should be $Mg(Cl{)}^2$ {2 will be in subscript as it shows the number of atoms of that element}
To write a formula, valency is very important. Another quick method is to simply cross multiply valencies of the elements.
Mg has valency of 2, Cl has valency of 1.We write the valency of both on top.{this shows charge}
2 1
Mg Cl
Cross multiply the valency and bring down to subscript
Mg(Cl)
1 2
1 is generally omitted and taken as default so we have formula, $Mg(Cl{)}^2$