I have learned about metal and their reactivity series. However, why metals have different reactivity? What are the factors of different reactivity rate for metal? All I can think of is the number of electrons which causing it.
I suppose that with reactivity you mean a metal's tendency to become oxidized. There are several reasons and it depends on the metals you are analyzing.
If you are referring to alkaline or alkaline earth metals, reactivity increases as you go down the group. They all have a large oxidation potential since it's relatively easy to remove an electron (or two) of its orbital. As you move down the group, atoms are larger and the valence electrons will be less atracted by the nucleus. Therefore, they are easier to remove and the metal will be more reactive.
For d-block transition metals (most of them are considered "transition" metals) it's a whole different story. There are many factors that contribute to its reactivity and it's harder to specify general trends. Many of them (especially the ones on the second and third transition rows) can be oxidized to different oxidation states depending on the conditions. You need to look at their electron configuration and reduction potentials to start rationalizing its properties. Depending upon the electron configuration, they will be able to lose (or even gain) less or more electrons and with different ease.
I don't know your background with chemistry, so I won't write a more detailed explanation but if you clarify that you have a more concrete interest in any explanation, just ask.
There are different factors that affect the re-activity of any element. These are as follows:
- The atomic radius, or the distance of the outermost shell from the nucleus.
- The nuclear charge (atomic number), because the greater the number of protons, the greater the pull on the outermost shell.
- The electron shielding due to the repulsion of electrons from the inner shells, pushing the outermost shells away from the nucleus.
Some factors can 'overpower' others depending on the position in the Periodic Table.
The atomic radius or distance of the out most shell to the nucleus The atomic number. The greater the number of protons, the greater the pull on the out most shell. The electron shielding due to the repulsing of electrons from the inner shells, pushing the outemost shells away from the radius.