Elements of the same subgroup have the same number of electrons on the outer layer. Am i right?

Can anyone (a-level) a student was analysing a carbonate compound, mco3, containing an unknown group 2 metal, m. the student carried out thermal decomposition on 0.730g of the carbonate and measured the volume of gas produced. mco3 (s) mo (s) + co2 (g) the student collected and measured 120cm3 of carbon dioxide. 1 mol of carbon dioxide occupies 24 000 cm3 under these conditions. calculate the molar mass of the group 2 carbonate and hence deduce the identity of the group 2 metal, m.

A+b→2c when the reaction begins, the researcher records that the rate of reaction is such that 1 mole of a is consumed per minute. after making changes to the reaction, the researcher notes that 2 moles of a are consumed per minute. what change could the researcher have made to effect this change?

Significant figures are digits read directly from the measuring instrument plus one more digit, which is __ by the observer.