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Oxygen is more reactive than Bromine because it has fewer shells than Bromine thus can get more electrons than Bromine

Lithium is more reactive than calcium because it can lose 1 electron while calcium has to lose 2 electrons 
Remember Na also loses 1 electrons therefore it is placed at the top in activity series of Metals
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Oxygen and lithium is more reactive than others because in first and second horizontal and vertical row and column are very reactive in nature.