Chemistry: chemical formulas?

The number outside is called a subscript. It tells you the number of that element's atoms contained in the molecule. Molecules always contains atoms. Atoms do not contain molecules.

Let me work on some examples.

You want to balance out the equation given always. The Law of Conservation of Mass and Atoms, and charge.

You have Mg and Cl.
Mg is 2+
Cl 1-

You will have MgCl2 to balance the equation, having a net charge neutral.

For NaOH = 1 Na atom, 1 O atom, 1 H atom
Na(OH)₂ = 1 Na atom, 2 O atoms, 2 H atoms
It's exactly like mathematics. Multiply through everything within the bracket.

If an ion contains 2 or more atoms, it is known as a polyatomic ion.

In a chemical formula, if the number of polyatomic is greater than 1, brackets are needed to show its number.

For example, magnesium hydroxide contains 1 Mg²⁺ ion and 2 OH⁻ ions. OH⁻ is a polyatomic ion and its number is greater than 1, then it is enclosed in brackets and its number 2 is shown as a suffix, i.e. Mg(OH)₂.

The formula Mg(OH)₂ shows clearly that it contains 1 Mg²⁺ ion and 2 OH⁻ ions. If the formula were written as MgOH₂, it would be confusion.

However, if the number of the polyatomic ion is equal to 1, brackets are not needed. For example, the formula NaOH shows clearly that it contains 1 Na⁺ ion and 1 OH⁻ ion. Besides, brackets are not needed for any monatomic ions.

More example to show whether brackets are needed or not for polyatomic ions :
KNO₃ Ca(NO₃)₂
MgSO₄, Al₂(SO₄)₃
NH₄Cl, (NH₄)₂CO₃

p.s. no such formula as Na(OH)2 only NaOH

formula is written according to valencies of two ions
e.g. if A has valency a& B has valency b then the formula for their compound will be ---
Ab Ba
e.g. Mg has 2 valency & OH has 1 valency so formula is
Mg1(OH)2 or Mg(OH)2
now the (OH) bracket is used as [ OH-1 ] is hydroxide ion has two letter O& H so we have use
bracket
other examples ---
formula for aluminium sulphate is Al2 (SO4)3
so when the latter ion has two or more letter then we use bracket for latter
as calcium bicarbonate has formula as
Ca(HCO3)2 [ bicarbonate has formula HCO3- its valency is one & that of Ca is 2.

in this case OH is considered as one unit(anion), and brackets are used to prevent confusion between OH2 and (OH)2 in first one we have one Oxygen and two Hydrogen but in next one we have one Oxygen and one Hydrogen in one anion

It is written as Mg(OH)2 because two OH ions combine with one Mg atom to make one molecule of Mg(OH)2

NaOH is written without brackets because only one OH ion combines one Na atom.

Yes