CH₂O = C + 2.H + O
= 12.01 + 2 x 1.008 + 16
= 30.026 amu = 30.026 g/mol
Its electronic confuguration is 2.8.6 so it occupies 3 shells.
To balance Ca + Cl2 = CaCl2 you'll need to be sure to count all of atoms on each side of the chemical equation.
Once you know how many of each type of atom you can only change the coefficients (the numbers in front of atoms or compounds) to balance the equation for Calcium + Chlorine gas.
