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Theory:

The chemical formula is the easy notation of a molecule (compound).
It shows the exact number of atoms of each element present in a molecule of a substance.
The following steps are used to write down chemical formula:
Step 1: Place the positive radical on the left and the negative radical on the right while writing the symbols of elements/ions side by side.
  
Step 2: In superscript, write the valencies of the two radicals to the right of their symbols. (The ions'+' and '-' signs were omitted).
  
Step 3: The valencies of the elements/ions are interchanged. These numbers should be written as subscripts.

However, the number \(1\) on the symbol's superscript is omitted.
Example:
(\(i\)). Calcium chloride (\(CaCl_2\)).
 
1st step: Place the calcium and chlorine symbols side by side.
\( Ca\)     \( Cl\)
 
2nd step: Write the valencies of calcium and chlorine to the right of their symbols.
\( Ca^2\)     \( Cl^1\)
 
3rd step: Interchange the valencies of elements.
\(CaCl_2\)
  
The chemical formula of Calcium chloride is (\(CaCl_2\)).  
 
(\(ii\)). Magnesium chloride (\(MgCl_2\))
  
1st step: Place the magnesium and chlorine symbols side by side.
\( Mg\)     \( Cl\)
 
2nd step: Write the valencies of magnesium and chlorine to the right of their symbols.
\( Mg^2 \)     \( Cl^1 \)
 
3rd step: Interchange the valencies of elements.
\(MgCl_2\)
  
The chemical formula of Magnesium chloride is (\(MgCl_2\)).  
  
(\(iii\)). Sodium hydroxide (\(NaOH\))
  
1st step: Place the sodium and hydroxide symbols side by side.
\( Na\)     \( OH\)
 
2nd step: Write the valencies of sodium and hydroxide to the right of their symbols.
\( Na^1 \)     \( OH^1 \)
 
3rd step: Interchange the valencies of elements.
\(NaOH\)
 
The chemical formula of Sodium hydroxide is (\(NaOH\)).
 
(\(iv\)).  Calcium phosphate (\(Ca_3(PO_4){_2}\))
  
1st step: Place the calcium and phosphate symbols side by side.
\(Ca\)   \( PO_4\)
 
2nd step: Write the valencies of calcium and phosphate to the right of their symbols.
\( Ca^2 \)     \( (PO_4){_3}\)
 
3rd step: Interchange the valencies of elements.
\(Ca_3(PO_4){_2}\)
 
The chemical formula of Calcium phosphate is (\(Ca_3(PO_4){_2}\)).
Chemical or Molecular formula