Astudent records the mass of a piece of iron. the iron is then left outside until it rusts. what type of reaction occurs, and how does the mass of the object after rusting compare with its original mass?

a. decomposition; the mass of the rusted iron is greater

b. synthesis; the mass of the rusted iron is greater

c. decomposition; the mass of the rusted iron is less

d. synthesis; the mass of the rusted iron is less

The Iron combines with Oxygen to form Rust ( Iron Oxide) This is an Oxidation reaction. It is also an Oxidation Reduction reaction. The Iron Oxide has more mass than the original Iron. (This can also be considered a synthesis reaction.

Iron atomic number 26 is not stable in its ground state of 
1s22s22p63s23p64s23d6 
The valance electrons 4s23d6 can become somewhat stable by losing the two 4s2 electrons resulting in a plus two charge(+2) 
or by losing the two 4s2 electrons and 1 of the 3d electrons leaving a half filled subshell of 3d5 electrons. resulting in a plus three charge ( +3)

Oxygen is a strong oxidizer it will take the electrons from the Iron resulting in Oxygen having a negative two charge (-2)

One equation for the Oxidation of Iron is below ( there are more than this possibility)

4Fe+3O22Fe2O3

the three Oxygen atoms that have been adding for every two Iron atoms add 48 grams to the compound. This increases the mass of the compound over the original mass of the Iron.In a short term,Your answer is B.

Hope this helps.

pie chart because it is in percents ( % )