Why is M more reactive than N even though N is further down the periodic table?  I don't understand, although M only has to loose one electron N is further down the periodic table. So how would I determine which one is more reactive, if a similar question is asked in an exam?  

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By comparing the image with periodic table of elements, we can see that M is sodium (Na) and N is (strontium). And using the reactivity series (refer to the attached link), we can see that sodium is placed higher on the series as compared to strontium, that is, sodium is more reactive than strontium. In other words, M is more reactive than N.

When going down a group, the reactivity generally increases (for metals), since we are adding an extra layer of electrons and it becomes easier to lose them. When going across a period, since we are adding extra electrons, it becomes difficult to donate them and hence reactivity decreases. In the given question, N is placed to the right and one group down of M. That is, N has 1 more electron to lose (which decreases reactivity) as compared to M (which has only 1 electron) and N has 1 extra layer of electrons (which increases reactivity). A combined effect of these two factors causes M to be more reactive. It could be because the extra electron would be more difficult to lose, even though it is from an extra layer. 

Note that V and W are noble gases here and are inert. T and U are halides and they need to gain electrons to complete the octet. As we go down a group, non-metals find it hard to attract and gain an electron and hence T is more reactive. 

Hope this helps. 

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