Question

How many total atoms are in the formula ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{(C}}{{\text{O}}_{\text{3}}}{\text{)}}_{\text{3}}}$ ?

Answer 1

In the above question, it is asked to find out the total number of atoms present in ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{(C}}{{\text{O}}_{\text{3}}}{\text{)}}_{\text{3}}}$ . In other words, we have to find out the atomicity of the molecule. For this, we just have to count the number of atoms of each element to get the required answer.

Complete step by step solution
In ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{(C}}{{\text{O}}_{\text{3}}}{\text{)}}_{\text{3}}}$ , 2 atoms of Al , 3 atoms of C and 9 atoms of O is present and hence, atomicity of ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{(C}}{{\text{O}}_{\text{3}}}{\text{)}}_{\text{3}}}$ is equal to ${\text{2 + 3 + 9 = 14}}$
Therefore, the total number of atoms in ${\text{A}}{{\text{l}}_{\text{2}}}{{\text{(C}}{{\text{O}}_{\text{3}}}{\text{)}}_{\text{3}}}$ is 14.

Additional Information
Let us take the example of oxygen, since each molecule of oxygen ( ${{\text{O}}_{\text{2}}}$ ) is composed of two oxygen atoms, so its atomicity is 2.
On the basis of atomicity, molecules can be classified as monatomic, diatomic, triatomic and polyatomic.
Monoatomic are molecules which are composed of 1 atom only like He, Ne, Ar (all noble gases)
Diatomic are molecules which are composed of 2 atoms like ${{\text{H}}_{\text{2}}}{\text{,}}{{\text{N}}_{\text{2}}}$ .
Triatomic are molecules which are composed of 3 atoms like ${{\text{O}}_{\text{3}}}$
Polyatomic are molecules which are composed of more than 3 atoms like ${{\text{S}}_{\text{8}}}$
All metals and some other elements, such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together. Their atomicity cannot be determined and is usually considered as 1.
Atomicity may vary in different allotropes of the same element.

Note
Most elements of the periodic table have atomicity either 1 or 2 except Sulphur whose atomicity is 8 as it can form a network lattice. Atomicity of boron varies since it has too many allotropes.