Which group of elements has seven valence electrons?
a) the carbon family b) the halogens c) the halogens d) the noble gases

The answer is -

Halogens, the elements in the family are fluorine, chlorine, bromine, iodine, and astatine.

They each have 7 valence electrons hence the reason they're the most active non-metals

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"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"