This is a displacement reaction. Aluminum, being higher in the electrochemical series (higher electrode potential) displaces copper from its aqueous salt to give an aluminum salt and copper metal.
The equation is balanced with the formula of the products in mind. Since the products must be Al2(SO4)3 and 3Cu, then the stoichiometric coefficient of aluminum must be 2 on the reactant side and that of CuSO4 must be 3 on the reactants side.
This fulfils the general principle for balancing reaction equations which states that, the number of atoms of each element on the right hand side of the reaction equation, must be the same as the number of atoms of the same element on the left hand side of the reaction equation.
If you had more than one atom chemically bonded together, then regardless of the types of atoms that are bonded, you're going to have a molecule regardless.
A flame test is an analytical procedure used in chemistry to detect the presence of certain elements, primarily metal ions, based on each element's characteristic emission spectrum. The color of flames in general also depends on temperature; see flame color.
